Theory and Simulation

Group Leader: Pablo Ordejón

Publications

2024

Charge density waves and electron-hole instabilities of the hidden-nesting materials P4⁢W12⁢O44, ????- and ????−Mo4⁢O11
Bogdan Guster; Miguel Pruneda; Pablo Ordejón; Enric Cañadell; Jean‐Paul Pouget Physical Review. B./Physical Review. b; 110 (9) 2024. 10.1103/physrevb.110.094103.
Density functional Bogoliubov-de Gennes theory for superconductors implemented in the SIESTA code
Reho, R; Wittemeier, N; Kole, A H; Ordejon, P; Zanolli, Z Physical Review b; 110 (13): 134505. 2024. 10.1103/PhysRevB.110.134505. IF: 3.200
Inhibitory behaviour and adsorption stability of benzothiazole derivatives as corrosion inhibitors towards galvanised steel
Deng, QS; Castillo-Robles, JM; Martins, ED; Ordejon, P; Gorges, JN; Eiden, P; Chen, XB; Keil, P; Cole, I Molecular Systems Design & Engineering; 9 (1): 29 - 45. 2024. 10.1039/d3me00153a. IF: 3.200
Microscopic understanding of the in-plane thermal transport properties of 2H transition metal dichalcogenides
Roberta Farris; Olle Hellman; Zeila Zanolli; David Saleta Reig; Sebin Varghese; Pablo Ordejón; Klaas‐Jan Tielrooij; Matthieu J. Verstraete Physical Review. B./Physical Review. b; 109 (12) 2024. 10.1103/physrevb.109.125422.
Molecular modeling applied to corrosion inhibition: a critical review
Castillo-Robles, Jose Maria; Martins, Ernane de Freitas; Ordejon, Pablo; Cole, Ivan Npj Materials Degradation; 8 (1): 72. 2024. 10.1038/s41529-024-00478-2. IF: 6.600
TDEP: Temperature Dependent Effective Potentials
Florian Knoop; Nina Shulumba; Aloïs Castellano; José Batista; Roberta Farris; Matthieu J. Verstraete; Matthew Heine; David Broido; Dennis Kim; Johan Klarbring; Igor A. Abrikosov; Sergei I. Simak; Olle Hellman Journal Of Open Source Software; 9 (94): 6150 - 6150. 2024. 10.21105/joss.06150.
Tungsten wall cratering under high-velocity dust impacts: Influence of impact angle and temperature
Dwivedi, P; Fraile, A; Polcar, T Journal Of Nuclear Materials; 600: 155289. 2024. 10.1016/j.jnucmat.2024.155289. IF: 2.800
Twisting Between Topological Phases in 1D Conjugated Polymers via a Multiradical Transition State
Alcon, Isaac; Canonico, Luis Manuel; Papior, Nick; Garcia, Jose-Hugo; Cummings, Aron W; Tremblay, Jean-Christophe; Pruneda, Miguel; Brandbyge, Mads; Paulus, Beate; Roche, Stephan Advanced Functional Materials; 2024. 10.1002/adfm.202409174.

2023

A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films
Varghese, S; Mehew, JD; Block, A; Reig, DS; Wozniak, P; Farris, R; Zanolli, Z; Ordejon, P; Verstraete, MJ; van Hulst, NF; Tielrooij, KJ Review Of Scientific Instruments; 94 (3): 34903. 2023. 10.1063/5.0102855. IF: 1.600
Device-to-Materials Pathway for Electron Traps Detection in Amorphous GeSe-Based Selectors
Slassi, A; Medondjio, LS; Padovani, A; Tavanti, F; He, X; Clima, S; Garbin, D; Kaczer, B; Larcher, L; Ordejon, P; Calzolari, A Advanced Electronic Materials; 9 (4) 2023. 10.1002/aelm.202201224. IF: 6.200
Independent and coherent transitions between antiferromagnetic states of few-molecule systems
Besson, C; Stegmann, P; Schnee, M; Zanolli, Z; Achilli, S; Wittemeier, N; Vierck, A; Frielinghaus, R; Kögerler, P; Maultzsch, J; Ordejón, P; Schneider, CM; Hucht, A; König, J; Meyer, C Physical Review b; 107 (24): 245414. 2023. 10.1103/PhysRevB.107.245414. IF: 3.700
Modular implementation of the linear- and cubic-scaling orbital minimization methods in electronic structure codes using atomic orbitals
Lebedeva, IV; Garcia, A; Artacho, E; Ordejon, P Royal Society Open Science; 10 (4): 230063. 2023. 10.1098/rsos.230063. IF: 3.500
Self-sustained deformable rotating liquid He cylinders: The pure normal fluid 3He and superfluid 4He cases
Pi, M; Ancilotto, F; Barranco, M; Butler, SL; Escartín, JM Physical Review b; 108 (5): 54524. 2023. 10.1103/PhysRevB.108.054524. IF: 3.700

2022

Atomically Sharp Lateral Superlattice Heterojunctions Built-In Nitrogen-Doped Nanoporous Graphene
Tenorio M., Moreno C., Febrer P., Castro-Esteban J., Ordejón P., Peña D., Pruneda M., Mugarza A. Advanced Materials; 34 (20, 2110099) 2022. 10.1002/adma.202110099. IF: 30.849

Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.

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Author Correction: The elphbolt ab initio solver for the coupled electron-phonon Boltzmann transport equations
Protik, Nakib H.; Li, Chunhua; Pruneda, Miguel; Broido, David; Ordejón, Pablo Npj Computational Materials; 8 (1) 2022. 10.1038/s41524-022-00746-2. IF: 12.256
Colossal phonon drag enhanced thermopower in lightly doped diamond
Li C., Protik N.H., Ordejón P., Broido D. Materials Today Physics; 27 (100740) 2022. 10.1016/j.mtphys.2022.100740.

Diamond is one of the most studied materials because of its unique combination of remarkable electrical, mechanical, thermal and optical properties. Using a fully self-consistent ab initio theory of coupled electron-phonon transport, we reveal another striking behavior: a huge drag enhancement of the thermopower of lightly doped diamond. Thermopower values of around 100,000 μV K−1 are found at 100 K, significantly exceeding the highest previously measured value in the correlated metal FeSb2, and occurring at much higher temperatures. The enormous thermopower in diamond arises primarily from exceptionally weak anharmonic phonon decay around and below 100 K that facilitates efficient momentum exchange between charge carriers and phonons through electron-phonon interactions. Exceedingly large thermoelectric power factors are also identified. This work gives insights into the physics of the coupled electron-phonon system in solids and advances our understanding of thermoelectric transport in the regime of strong drag. © 2022 Elsevier Ltd

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Competition between Ta-Ta and Te-Te bonding leading to the commensurate charge density wave in TaTe4
Guster B., Pruneda M., Ordejón P., Canadell E. Physical Review B; 105 (6, 064107) 2022. 10.1103/PhysRevB.105.064107. IF: 4.036

The origin of the charge density wave in TaTe4 is discussed on the basis of a first-principles density functional theory analysis of the Fermi surface, electron-hole response function, phonon band structure of the average structure, and structural optimization of the modulated phase. Analysis of the band structure and Fermi surface of the average structure clearly proves that despite the presence of TaTe4 chains in the crystal structure, TaTe4 is in fact a 3D material as far as the electronic structure near the Fermi level is concerned. A Fermi surface nesting mechanism is dismissed as the origin of the 2a×2a×3c structural modulation. The optimized 2a×2a×3c structure, which is found to be the more stable modulation in agreement with the experimental observations, can be obtained directly from a soft-phonon mode computed for the undistorted structure. Our results suggest that the driving force for the distortion is the maximization of Ta-Ta metal-metal bonding subject to inducing the minimum bonding decrease in the Te sublattice. © 2022 American Physical Society.

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Dynamic control of octahedral rotation in perovskites by defect engineering
Jia J., He X., Akhtar A., Herranz G., Pruneda M. Physical Review B; 105 (22, 224112) 2022. 10.1103/PhysRevB.105.224112.

Engineering oxygen octahedra rotation patterns in ABO3 perovskites is a powerful route to design functional materials. Here we propose a strategy that exploits point defects that create local electric dipoles and couple to the oxygen sublattice, enabling direct actuation on the rotational degrees of freedom. This approach, which relies on substituting an A site with a smaller ion, paves a way to couple dynamically octahedra rotations to external electric fields. A common antisite defect, AlLa, in rhombohedral LaAlO3 is taken as a prototype to validate the idea, with atomistic density functional theory calculations supported with an effective lattice model to simulate the dynamics of switching of the local rotational degrees of freedom to long distances. Our simulations provide an insight of the main parameters that govern the operation of the proposed mechanism, and allow to define guidelines for screening other systems where this approach could be used for tuning the properties of the host material. © 2022 American Physical Society.

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Electrical control of spin-polarized topological currents in monolayer WTe2
Garcia, JH; You, JX; Garcia-Mota, M; Koval, P; Ordejon, P; Cuadrado, R; Verstraete, MJ; Zanolli, Z; Roche, S Physical Review b; 106 (16) 2022. 10.1103/PhysRevB.106.L161410. IF: 3.908
Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles
Saunderson T.G., Annett J.F., Csire G., Gradhand M. Physical Review B; 105 (1, 014424) 2022. 10.1103/PhysRevB.105.014424. IF: 4.036

We have implemented the Bogoliubov-de Gennes equation in a screened Korringa-Kohn-Rostoker method for solving, self-consistently, the superconducting state for three-dimensional (3D) crystals including substitutional impurities. In this paper we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3D magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in in-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of a first-principles-based description which captures the quantitative details, making direct comparisons with experimental findings possible. © 2022 American Physical Society.

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Interference effects in one-dimensional moiré crystals
Wittemeier N., Verstraete M.J., Ordejón P., Zanolli Z. Carbon; 186: 416 - 422. 2022. 10.1016/j.carbon.2021.10.028. IF: 9.594

Interference effects in finite sections of one-dimensional moiré crystals are investigated using a Landauer-Büttiker formalism within the tight-binding approximation. We explain interlayer transport in double-wall carbon nanotubes and design a predictive model. Wave function interference is visible at the mesoscale: in the strong coupling regime, as a periodic modulation of quantum conductance and emergent localized states; in the localized-insulating regime, as a suppression of interlayer transport, and oscillations of the density of states. These results could be exploited to design quantum electronic devices. © 2021 The Authors

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Magnetic properties of coordination clusters with {Mn4} and {Co4} antiferromagnetic cores
Achilli S., Besson C., He X., Ordejón P., Meyer C., Zanolli Z. Physical Chemistry Chemical Physics; 24 (6): 3780 - 3787. 2022. 10.1039/d1cp03904k. IF: 3.676

We present a joint experimental and theoretical characterization of the magnetic properties of coordination clusters with an antiferromagnetic core of four magnetic ions. Two different compounds are analyzed, with Co and Mn ions in the core. While both molecules are antiferromagnetic, they display different sensitivities to external magnetic field, according to the different atomic magnetic moments and strength of the intra-molecular magnetic couplings. In particular, the dependence of the magnetization versus field of the two molecules switches with temperature: at low temperature the magnetization is smaller in {Mn4} than in Co4, while the opposite happens at high temperature. Through a detailed analysis of the electronic and magnetic properties of the two compounds we identify a stronger magnetic interaction between the magnetic ions in {Mn4} with respect to {Co4}. Moreover {Co4} displays not negligible spin-orbit related effects that could affect the spin lifetime in future antiferromagnetic spintronic applications. We highlight the necessity to account for these spin-orbit effects together with electronic correlation effects for a reliable description of these compounds. © the Owner Societies.

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Magnetically textured superconductivity in elemental rhenium
Csire G., Annett J.F., Quintanilla J., Újfalussy B. Physical Review B; 106 (2, L020501) 2022. 10.1103/PhysRevB.106.L020501.

Recent μSR measurements revealed remarkable signatures of spontaneous magnetism coexisting with superconductivity in elemental rhenium. Thus, pure rhenium could be the first elemental crystal where unconventional superconductivity is realized in nature. Here we provide a quantitative theory that uncovers the nature of the superconducting instability by incorporating every details of the electronic structure together with spin-orbit coupling and multiorbital physics. We show that conventional s-wave superconductivity combined with strong spin-orbit coupling is inducing even-parity odd-orbital spin triplet Cooper pairs, and in presence of a screw-axis Cooper pairs' migration between the induced equal-spin triplet component leads to an exotic magnetic state with atomic-scale texture. Our first-principles-based model contains two phenomenological parameters that characterizes the pairing interaction fixed by the experimental value of the superconducting transition temperature and the slope of the specific heat, and allows quantitative prediction of the magnetic structure. © 2022 American Physical Society.

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Manipulation of spin transport in graphene/transition metal dichalcogenide heterobilayers upon twisting
Pezo A., Zanolli Z., Wittemeier N., Ordejón P., Fazzio A., Roche S., Garcia J.H. 2D Materials; 9 (1, 015008) 2022. 10.1088/2053-1583/ac3378. IF: 7.103

Proximity effects between layered materials trigger a plethora of novel and exotic quantum transport phenomena. Besides, the capability to modulate the nature and strength of proximity effects by changing crystalline and interfacial symmetries offers a vast playground to optimize physical properties of relevance for innovative applications. In this work, we use large-scale first principles calculations to demonstrate that strain and twist-angle strongly vary the spin–orbit coupling (SOC) in graphene/transition metal dichalcogenide heterobilayers. Such a change results in a modulation of the spin relaxation times by up to two orders of magnitude. Additionally, the relative strengths of valley-Zeeman and Rashba SOC can be tailored upon twisting, which can turn the system into an ideal Dirac–Rashba regime or generate transitions between topological states of matter. These results shed new light on the debated variability of SOC and clarify how lattice deformations can be used as a knob to control spin transport. Our outcomes also suggest complex spin transport in polycrystalline materials, due to the random variation of grain orientation, which could reflect in large spatial fluctuations of SOC fields. © 2021 IOP Publishing Ltd

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Merging of superfluid helium nanodroplets with vortices
Escartín J.M., Ancilotto F., Barranco M., Pi M. Physical Review B; 105 (2, 024511) 2022. 10.1103/PhysRevB.105.024511. IF: 4.036

Within density functional theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and antivortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity. © 2022 American Physical Society.

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Simulation of Tribological Properties of a Graphene Bilayer with Twisted Layers
Minkin, AS; Lebedeva, IV; Popov, AM; Knizhnik, AA Nanobiotechnology Reports; 17 (4): 472 - 476. 2022. 10.1134/S2635167622040176.
The elphbolt ab initio solver for the coupled electron-phonon Boltzmann transport equations
Protik N.H., Li C., Pruneda M., Broido D., Ordejón P. npj Computational Materials; 8 (1, 28) 2022. 10.1038/s41524-022-00710-0. IF: 12.241

elphbolt is a modern Fortran (2018 standard) code for efficiently solving the coupled electron–phonon Boltzmann transport equations from first principles. Using results from density functional and density functional perturbation theory as inputs, it can calculate the effect of the non-equilibrium phonons on the electronic transport (phonon drag) and non-equilibrium electrons on the phononic transport (electron drag) in a fully self-consistent manner and obeying the constraints mandated by thermodynamics. It can calculate the lattice, charge, and thermoelectric transport coefficients for the temperature gradient and electric fields, and the effect of the mutual electron–phonon drag on these transport properties. The code fully exploits the symmetries of the crystal and the transport-active window to allow the sampling of extremely fine electron and phonon wave vector meshes required for accurately capturing the drag phenomena. The coarray feature of modern Fortran, which offers native and convenient support for parallelization, is utilized. The code is compact, readable, well-documented, and extensible by design. © 2022, The Author(s).

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Translational Covariance of Flexoelectricity at Ferroelectric Domain Walls
Diéguez O., Stengel M. Physical Review X; 12 (3, 031002) 2022. 10.1103/PhysRevX.12.031002.

Macroscopic descriptions of ferroelectrics have an obvious appeal in terms of efficiency and physical intuition. Their predictive power, however, has often been thwarted by the lack of a systematic procedure to extract the relevant materials parameters from the microscopics. Here we address this limitation by establishing an unambiguous two-way mapping between spatially inhomogeneous fields and discrete lattice modes. This yields a natural treatment of gradient couplings in the macroscopic regime via a long-wavelength expansion of the crystal Hamiltonian. Our analysis reveals an inherent arbitrariness in both the flexoelectric and polarization gradient coefficients, which we ascribe to a translational freedom in the definition of the polar distortion pattern. Remarkably, such arbitrariness cancels out in all physically measurable properties (relaxed atomic structure and energetics) derived from the model, pointing to a generalized translational covariance in the continuum description of inhomogeneous ferroelectric structures. We demonstrate our claims with extensive numerical tests on 180° domain walls in common ferroelectric perovskites, finding excellent agreement between the continuum model and direct first-principles calculations. © 2022 authors. Published by the American Physical Society.

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Tuning the topological band gap of bismuthene with silicon-based substrates
Wittemeier N., Ordejón P., Zanolli Z. JPhys Materials; 5 (3, 035002) 2022. 10.1088/2515-7639/ac84ad.

Some metastable polymorphs of bismuth monolayers (bismuthene) can host non-trivial topological phases. However, it remains unclear whether these polymorphs can become stable through interaction with a substrate, whether their topological properties are preserved, and how to design an optimal substrate to make the topological phase more robust. Using first-principles techniques, we demonstrate that bismuthene polymorphs can become stable over silicon carbide (SiC), silicon (Si), and silicon dioxide (SiO2) and that proximity interaction in these heterostructures has a significant effect on the electronic structure of the monolayer, even when bonding is weak. We show that van der Waals interactions and the breaking of the sublattice symmetry are the main factors driving changes in the electronic structure in non-covalently binding heterostructures. Our work demonstrates that substrate interaction can strengthen the topological properties of bismuthene polymorphs and make them accessible for experimental investigations and technological applications. © 2022 The Author(s). Published by IOP Publishing Ltd.

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Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer
Saleta Reig D., Varghese S., Farris R., Block A., Mehew J.D., Hellman O., Woźniak P., Sledzinska M., El Sachat A., Chávez-Ángel E., Valenzuela S.O., van Hulst N.F., Ordejón P., Zanolli Z., Sotomayor Torres C.M., Verstraete M.J., Tielrooij K.-J. Advanced Materials; 34 (10, 2108352) 2022. 10.1002/adma.202108352. IF: 30.849

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH

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2021

A First-Principles Investigation on the Electronic and Mechanical Properties of 1T TiSe2Multilayers for Energy Storage
Antonio J.E., Cervantes J.M., Rosas-Huerta J.L., Pilo J., Carvajal E., Escamilla R. Journal of the Electrochemical Society; 168 (3, 030531) 2021. 10.1149/1945-7111/abed29. IF: 4.316

In this work, the electronic and mechanical properties of bulk TiSe2 were studied, and the effects of confinement on the compound, into mono-, bi-, and tri-layered systems, on the electronic and mechanical properties using DFT-based calculations within the Generalized Gradient Approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Lithium atoms were placed at different adsorption sites of the TiSe2 monolayer to study the consequences on the electronic and mechanical properties and to identify the most favourable adsorption site for Li in the TiSe2 systems. Mono -, bi-, and tri-layered systems have associated a metallic behaviour, similar to the bulk material. Young's modulus for mono-, bi-, and tri-layered systems show similar behaviour to the bulk case. On the other hand, monolayers with Li are metallic when Li atoms are placed at the surface; and this behaviour could be favourable to facilitate electronic transport by the monolayer. Finally, the mechanical properties analysis supported that the better adsorption sites are those labelled as Top and Hollow. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.

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Addressing the theoretical and experimental aspects of low-dimensional-materials-based fet immunosensors: A review
Martins E.F., Pinotti L.F., Silva C.C.C., Rocha A.R. Chemosensors; 9 (7, 162) 2021. 10.3390/chemosensors9070162. IF: 3.398

Electrochemical immunosensors (EI) have been widely investigated in the last several years. Among them, immunosensors based on low-dimensional materials (LDM) stand out, as they could provide a substantial gain in fabricating point-of-care devices, paving the way for fast, precise, and sensitive diagnosis of numerous severe illnesses. The high surface area available in LDMs makes it possible to immobilize a high density of bioreceptors, improving the sensitivity in biorecognition events between antibodies and antigens. If on the one hand, many works present promising results in using LDMs as a sensing material in EIs, on the other hand, very few of them discuss the fundamental interactions involved at the interfaces. Understanding the fundamental Chemistry and Physics of the interactions between the surface of LDMs and the bioreceptors, and how the operating conditions and biorecognition events affect those interactions, is vital when proposing new devices. Here, we present a review of recent works on EIs, focusing on devices that use LDMs (1D and 2D) as the sensing substrate. To do so, we highlight both experimental and theoretical aspects, bringing to light the fundamental aspects of the main interactions occurring at the interfaces and the operating mechanisms in which the detections are based. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

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Assessing Nickel Titanium Binary Systems Using Structural Search Methods and Ab Initio Calculations
Lang L., Payne A., Valencia-Jaime I., Verstraete M.J., Bautista-Hernández A., Romero A.H. Journal of Physical Chemistry C; 125 (2): 1578 - 1591. 2021. 10.1021/acs.jpcc.0c10453. IF: 4.126

Nickel titanium, also know as nitinol, is a prototypical shape memory alloy, a property intimately linked to a phase transition in the microstructure, which allows the meso/macroscopic sample shape to be recovered after thermal cycling. Not much is known about the other alloys in this binary system, which prompted our computational investigation of other compositions. In this work, structures are found by probing the potential energy surfaces of NiTi binary systems using a minima hopping method, in combination with ab initio electronic structure calculations. We find stable structures in 34 different stoichiometries and calculate derived physical properties of the low energy phases. From the results of this analysis a new convex hull is formed that is lower in energy than those in the Materials Project and Open Quantum Materials Databases. Two previously unreported phases are discovered for the NiTi2 and Ni5Ti compositions, and two metastable states in NiTi and NiTi2 shows signs of negative linear compression and negative Poisson ratio, respectively. ©

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Atomic-scale defects restricting structural superlubricity: Ab initio study on the example of the twisted graphene bilayer
Minkin A.S., Lebedeva I.V., Popov A.M., Knizhnik A.A. Physical Review B; 104 (7, 075444) 2021. 10.1103/PhysRevB.104.075444. IF: 4.036

The potential energy surface (PES) of interlayer interaction of twisted bilayer graphene with vacancies in one of the layers is investigated via density functional theory (DFT) calculations with van der Waals corrections. These calculations give a non-negligible magnitude of PES corrugation of 28 meV per vacancy and barriers for relative sliding of the layers of 7-8 meV per vacancy for the moiré pattern with coprime indices (2,1) (twist angle 21.8). At the same time, using the semiempirical potential fitted to the DFT results, we confirm that twisted bilayer graphene without defects exhibits superlubricity for the same moiré pattern and the magnitude of PES corrugation for the infinite bilayer is below the calculation accuracy. Our results imply that atomic-scale defects restrict the superlubricity of two-dimensional layers and can determine static and dynamic tribological properties of these layers in a superlubric state. We also analyze computationally cheap approaches that can be used for modeling of tribological behavior of large-scale systems with defects. The adequacy of using state-of-the-art semiempirical potentials for interlayer interaction and approximations based on the first spatial Fourier harmonics for the description of interaction between graphene layers with defects is discussed. © 2021 American Physical Society.

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Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3and monoclinic-TaS3
Guster B., Pruneda M., Ordejón P., Canadell E., Pouget J.-P. Journal of Physics Condensed Matter; 33 (48, 485401) 2021. 10.1088/1361-648X/ac238a. IF: 2.333

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions at TP1 and TP2 associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron-hole response function. Because of the stronger inter-chain interactions, the FS and electron-hole response function are considerably more complex for NbSe3 than m-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron-hole response for NbSe3 are found with the (0a∗, 0c∗) and (1/2a∗, 1/2c∗) transverse components at TP1 and TP2, respectively, whereas the second maximum is not observed for m-TaS3 at TP2. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse a∗ components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling along c∗ in NbSe3 and along both a∗ and c∗ for m-TaS3. Phonon spectrum calculations reveal the formation of a giant 2kF Kohn anomaly for m-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides. © 2021 IOP Publishing Ltd.

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Coexistence of vortex arrays and surface capillary waves in spinning prolate superfluid He 4 nanodroplets
Pi M., Escartín J.M., Ancilotto F., Barranco M. Physical Review B; 104 (9, 094509) 2021. 10.1103/PhysRevB.104.094509. IF: 4.036

Within density functional theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate He4 droplets made of several thousand helium atoms. Surface capillary waves are ubiquitous in prolate superfluid He4 droplets, and depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium configuration of small prolate droplets is vortex free, evolving towards vortex hosting as the droplet size increases. This result is in agreement with a recent experiment [O'Connell, Phys. Rev. Lett. 124, 215301 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.215301] that disclosed that vortex arrays and capillary waves coexist in the equilibrium configuration of very large drops. In contrast to viscous droplets executing rigid-body rotation, the stability phase diagram of spinning He4 droplets cannot be universally described in terms of dimensionless angular momentum and angular velocity variables: Instead, the rotational properties of superfluid helium droplets display a clear dependence on the droplet size and the number of vortices they host. © 2021 American Physical Society.

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Conducting chiral nickel(ii) bis(dithiolene) complexes: Structural and electron transport modulation with the charge and the number of stereogenic centres
Abhervé A., Mroweh N., Cauchy T., Pop F., Cui H., Kato R., Vanthuyne N., Alemany P., Canadell E., Avarvari N. Journal of Materials Chemistry C; 9 (12): 4119 - 4140. 2021. 10.1039/d1tc00439e. IF: 7.393

Nickel(ii) bis(dithiolene) complexes can provide crystalline conducting materials either in their monoanionic or neutral forms. Here we show that the use of chiral dithiolene ligands with one or two stereogenic centres, together with variation of the counter-ion in the anionic complexes, represents a powerful strategy to modulate the conducting properties of such molecular materials. The chiral ligands 5-methyl-5,6-dihydro-1,4-dithiin-2,3-dithiolate (me-dddt) and 5,6-dimethyl-5,6-dihydro-1,4-dithiin-2,3-dithiolate (dm-dddt) have been generated from the thione precursors 1 and 2 which have been structurally and chiroptically characterized. Anionic Ni(ii) complexes of these two ligands with tetrabutyl-ammonium (TBA) and tetramethyl-ammonium (TMA) have been prepared and structurally characterized, suggesting that it is the nature of the counter-ion which mostly influences the solid state organization of the complexes. Both TBA and TMA radical anion salts are Mott insulators with antiferromagnetic ground state, as suggested by spin polarized DFT band structure calculations. However, the TMA salts are one order of magnitude more conducting than the TBA counterparts. The neutral materials [Ni(me-dddt)2] and [Ni(dm-dddt)2] are direct band gap semiconductors, as determined by DFT and extended Hückel band structure calculations, with their conductivity drastically increased up to 0.05-3.3 S cm-1 under the highest applied pressures of 10-11 GPa. At equivalent applied pressures the dm-dddt materials are more conducting than the me-dddt ones, in agreement with the lower calculated activation energy and larger bands dispersion for the former. This trend follows the structural change when going from one to two methyl substituents, since the packing and intermolecular interactions are completely different between [Ni(dm-dddt)2] and [Ni(me-dddt)2], the packing of the latter being related to the one of the achiral parent [Ni(dddt)2]. Subtle differences of conductivity are also observed within both series of neutral complexes between the enantiopure and racemic forms. This represents the first series of chiral nickel bis(dithiolene) complexes which shows modulation of the conducting properties with the number of stereogenic centres, the conductivity, measured on single crystals, strongly increasing upon applying hydrostatic pressure. This journal is © The Royal Society of Chemistry.

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DFT Electronic Properties and Synthesis Thermodynamics of LixLa1-xTiO3Electrolytes for Li-Ion Batteries
Cervantes J.M., Pilo J., Rosas-Huerta J.L., Antonio J.E., Munoz H., Oviedo-Roa R., Carvajal E. Journal of the Electrochemical Society; 168 (8, 080516) 2021. 10.1149/1945-7111/ac1a52. IF: 4.316

Lithium lanthanum titanate perovskites Li x La1-x TiO3 (LLTO) are promising solid-electrolytes for Li-ion batteries. We studied, in the Density-Functional-Theory framework, the thermodynamic stability and the electronic and magnetic properties of LLTO, as bulk materials and as thin slabs with (001) exposed surfaces. Results show that LaTiO3 (LTO) exhibits semiconductor behavior and G-type antiferromagnetic order (AFMG), whereas the TiO2-terminated LTO slab is a semiconductor with ferromagnetic (FM) order. Contrasting, the LTO slab exposing a LaO-terminated surface is a conductor with AFMG ordered Ti cations' magnetic moments (MMs), but at the surface there are some FM ordered MMs (La atoms). LLTO bulk electrolyte is a semiconductor (x = 0.25) or insulator (x = 0.50). The LLTO slab is a FM (non-magnetic) conductor (TiO2 (LiO)-terminated surface) or a FM semiconductor (LaO-terminated). Besides, the stability of the LLTO bulk and slabs structures was analyzed, as well as the slabs' preferences for LiO, LaO or TiO2 ends. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.

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Exploring the elastic and electronic properties of chromium molybdenum diboride alloys
Dovale-Farelo V., Tavadze P., Verstraete M.J., Bautista-Hernández A., Romero A.H. Journal of Alloys and Compounds; 866 (158885) 2021. 10.1016/j.jallcom.2021.158885. IF: 5.316

We perform first-principles calculations to study the structural, mechanical, thermal, electronic, and magnetic properties of Cr1−xMoxB2 for x = 0.25, 0.33, 0.50, 0.67 and 0.75. Based on structural search methods, we determine the ground-state structure for each concentration. The ternaries are either monoclinic (x = 0.25, 0.75) or trigonal (x = 0.33, 0.50, 0.67). The calculated mechanical properties reveal that the strength of Cr1−xMoxB2 is maximized for x = 0.50. Cr0.5Mo0.5B2 exhibits excellent mechanical properties (B = 298 GPa, Y = 558 GPa, G = 235 Gpa, ν = 0.19, Hv =27 GPa), surpassing those of β-MoB2 at a lower cost. All of these ternaries are hard alloys with Vickers hardness greater than 24 GPa. Chemical bonding analysis demonstrates that the strength of the new compounds is related to the alternating planar and buckled B-B layers, as well as the strong TM-B bonds. The enhanced strength of Cr0.5Mo0.5B2 is a consequence of the high density of strong interlayer Cr-Mo metallic bonds around the Fermi level. © 2021 Elsevier B.V.

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Giant thermoelectric figure of merit in multivalley high-complexity-factor LaSO
Farris R., Ricci F., Casu G., Dahliah D., Hautier G., Rignanese G.-M., Fiorentini V. Physical Review Materials; 5 (12, 125406) 2021. 10.1103/PhysRevMaterials.5.125406. IF: 3.989

We report a giant thermoelectric figure of merit ZT (up to six at 1100 K) in n-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed from ab initio bands within Bloch-Boltzmann theory in an energy-, chemical potential-, and temperature-dependent relaxation time approximation. The lattice thermal conductivity is estimated from a model employing the ab initio phonon and Grüneisen-parameter spectrum. The main source of the large ZT is the significant power factor which correlates with a large band complexity factor. We also suggest a possible n-type dopant for the material based on ab initio calculations. © 2021 American Physical Society.

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Localized electronic vacancy level and its effect on the properties of doped manganites
Juan D., Pruneda M., Ferrari V. Scientific Reports; 11 (1, 6706) 2021. 10.1038/s41598-021-85945-5. IF: 4.380

Oxygen vacancies are common to most metal oxides and usually play a crucial role in determining the properties of the host material. In this work, we perform ab initio calculations to study the influence of vacancies in doped manganites La (1 - x)Sr xMnO 3, varying both the vacancy concentration and the chemical composition within the ferromagnetic-metallic range (0.2<x<0.5). We find that oxygen vacancies give rise to a localized electronic level and analyse the effects that the possible occupation of this defect state can have on the physical properties of the host. In particular, we observe a substantial reduction of the exchange energy that favors spin-flipped configurations (local antiferromagnetism), which correlate with the weakening of the double-exchange interaction, the deterioration of the metallicity, and the degradation of ferromagnetism in reduced samples. In agreement with previous studies, vacancies give rise to a lattice expansion when the defect level is unoccupied. However, our calculations suggest that under low Sr concentrations the defect level can be populated, which conversely results in a local reduction of the lattice parameter. Although the exact energy position of this defect level is sensitive to the details of the electronic interactions, we argue that it is not far from the Fermi energy for optimally doped manganites (x∼1/3), and thus its occupation could be tuned by controlling the number of available electrons, either with chemical doping or gating. Our results could have important implications for engineering the electronic properties of thin films in oxide compounds. © 2021, The Author(s).

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Metallic Diluted Dimerization in VO2 Tweeds
Sandiumenge F., Rodríguez L., Pruneda M., Magén C., Santiso J., Catalan G. Advanced Materials; 33 (9, 2004374) 2021. 10.1002/adma.202004374. IF: 30.849

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale. © 2021 Wiley-VCH GmbH

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Multiscale modeling strategy to solve fullerene formation mystery
Popov A.M., Lebedeva I.V., Vyrko S.A., Poklonski N.A. Fullerenes Nanotubes and Carbon Nanostructures; 29 (10): 755 - 766. 2021. 10.1080/1536383X.2021.1900124. IF: 1.869

Since fullerene formation occurs under conditions where direct observation of atomic-scale reactions is not possible, modeling is the only way to reveal atomistic mechanisms which can lead to selection of abundant fullerene isomers (like C60-Ih). In the present paper we review the results obtained for different atomistic mechanisms by various modeling techniques. Although it seems that atomic-scale processes related to odd fullerenes (such as growth by consecutive insertions of single carbon atoms and rearrangements of the sp2 structure promoted by extra sp atoms) provide the main contribution to selection of abundant isomers, at the moment there is no conclusive evidence in favor of any particular atomistic mechanism. Thus, the following multiscale modeling strategy to solve the mystery of the high yield of abundant fullerene isomers is suggested. On the one hand, sets of reactions between fullerene isomers can be described using theoretical graph techniques. On the other hand, reaction schemes can be revealed by classical molecular dynamics simulations with subsequent refinement of the activation barriers by ab initio calculations. Based on the reaction sets with the reaction probabilities derived in this way, the different atomistic mechanisms of abundant fullerene isomer selection can be compared using kinetic models. © 2021 Taylor & Francis Group, LLC.

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Optical Signatures of Defect Centers in Transition Metal Dichalcogenide Monolayers
de Melo P.M.M.C., Zanolli Z., Verstraete M.J. Advanced Quantum Technologies; 4 (3, 2000118) 2021. 10.1002/qute.202000118. IF: 0.000

Even the best quality 2D materials have non-negligible concentrations of vacancies and impurities. It is critical to understand and quantify how defects change intrinsic properties, and use this knowledge to generate functionality. This challenge can be addressed by employing many-body perturbation theory to obtain the optical absorption spectra of defected transition metal dichalcogenides. Herein metal vacancies, which are largely unreported, show a larger set of polarized excitons than chalcogenide vacancies, introducing localized excitons in the sub-optical-gap region, whose wave functions and spectra make them good candidates as quantum emitters. Despite the strong interaction with substitutional defects, the spin texture and pristine exciton energies are preserved, enabling grafting and patterning in optical detectors, as the full optical-gap region remains available. A redistribution of excitonic weight between the A and B excitons is visible in both cases and may allow the quantification of the defect concentration. This work establishes excitonic signatures to characterize defects in 2D materials and highlights vacancies as qubit candidates for quantum computing. © 2021 The Authors. Advanced Quantum Technologies published by Wiley-VCH GmbH

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Relativistic first-principles theory of Yu-Shiba-Rusinov states applied to Mn adatoms and Mn dimers on Nb(110)
Nyári B., Lászlóffy A., Szunyogh L., Csire G., Park K., Ujfalussy B. Physical Review B; 104 (23, 235426) 2021. 10.1103/PhysRevB.104.235426. IF: 4.036

We present a fully relativistic first-principles-based theoretical approach for the calculation of the spectral properties of magnetic impurities on the surface of a superconducting substrate, providing a material specific framework for the investigation of the Yu-Shiba-Rusinov (YSR) states. By using a suitable orbital decomposition of the local densities of states we discuss in great detail the formation of the YSR states for an Mn adatom and for two kinds of Mn dimers placed on the Nb(110) surface and compare our results to recent experimental findings. In the case of the adatom we find that the spin-orbit coupling slightly shifts some of the YSR peaks and also the local spin polarization on the Nb atoms has marginal effects on their positions. Moreover, by scaling the exchange field on the Mn site we could explain the lack of the dx2−y2-like YSR state in the spectrum. While our results for a close packed ferromagnetic dimer are in satisfactory agreement with the experimentally observed splitting of the YSR states, in the case of an antiferromagnetic dimer we find that the spin-orbit coupling is not sufficiently large to explain the splitting of the YSR states seen in the experiment. Changing the relative orientation of the magnetic moments in this dimer induces splitting of the YSR states and also shifts their energy, leading even to the formation of a zero bias peak in the case of the deepest YSR state. ©2021 American Physical Society

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Spontaneous interlayer compression in commensurately stacked van der Waals heterostructures
Pike N.A., Dewandre A., Chaltin F., Garcia Gonzalez L., Pillitteri S., Ratz T., Verstraete M.J. Physical Review B; 103 (23, 235307) 2021. 10.1103/PhysRevB.103.235307. IF: 4.036

Interest in layered two-dimensional materials, particularly stacked heterostructures of transition-metal dichalcogenides, has led to the need for a better understanding of the structural and electronic changes induced by stacking. Here, we investigate the effects of idealized heterostructuring, with periodic commensurate stacking, on the structural, electronic, and vibrational properties, when compared to the counterpart bulk transition-metal dichalcogenide. We find that in heterostructures with dissimilar chalcogen species there is a strong compression of the interlayer spacing, compared to the bulk compounds. This compression of the heterostructure is caused by an increase in the strength of the induced polarization interaction between the layers, but not a full charge transfer. We argue that this effect is real, not due to the imposed commensurability, and should be observable in heterostructures combining different chalcogens. Interestingly, we find that incommensurate stacking of Ti-based dichalcogenides can lead to the stabilization of the charge-density wave phonon mode, which is unstable in the 1T phase at low temperature. Mixed Ti- and Zr-based heterostructures are still dynamically unstable, but TiS2/ZrS2 becomes ferroelectric. © 2021 American Physical Society.

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Spontaneous phase segregation of Sr2NiO3 and SrNi2O3 during SrNiO3 heteroepitaxy
Wang L., Yang Z., Yin X., Taylor S.D., He X., Tang C.S., Bowden M.E., Zhao J., Wang J., Liu J., Perea D.E., Wangoh L., Wee A.T.S., Zhou H., Chambers S.A., Du Y. Science Advances; 7 (10, eabe2866) 2021. 10.1126/sciadv.abe2866. IF: 14.143

Recent discovery of superconductivity in Nd0.8Sr0.2NiO2 motivates the synthesis of other nickelates for providing insights into the origin of high-temperature superconductivity. However, the synthesis of stoichiometric R1−xSrxNiO3 thin films over a range of x has proven challenging. Moreover, little is known about the structures and properties of the end member SrNiO3. Here, we show that spontaneous phase segregation occurs while depositing SrNiO3 thin films on perovskite oxide substrates by molecular beam epitaxy. Two coexisting oxygen-deficient Ruddlesden-Popper phases, Sr2NiO3 and SrNi2O3, are formed to balance the stoichiometry and stabilize the energetically preferred Ni2+ cation. Our study sheds light on an unusual oxide thin-film nucleation process driven by the instability in perovskite structured SrNiO3 and the tendency of transition metal cations to form their most stable valence (i.e., Ni2+ in this case). The resulting metastable reduced Ruddlesden-Popper structures offer a testbed for further studying emerging phenomena in nickel-based oxides. Copyright © 2021 The Authors, some rights reserved;

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Structural and magnetic phase diagram of epitaxial La0.7Sr0.3MnO3from first principles
Pilo J., Pruneda M., Bristowe N.C. Electronic Structure; 3 (2, 024001) 2021. 10.1088/2516-1075/abe6af. IF: 0.000

ABO3perovskites host a huge range of symmetry lowering structural distortions, each of which can tune, or even switch on or off, different functional properties due to the strong coupling between the lattice, spin and charge degrees of freedom in these materials. The sheer number of different meta-stable structures present in perovskites creates a challenge for materials design via theory and simulation. Here, we tackle this issue using a first principles structure searching method on a prototypical half-metallic perovskite, La0.7Sr0.3MnO3, to predict how epitaxial strain can engineer structural and magnetic properties.We reveal a rich structural phase diagram through strain engineering in which the octahedral tilt pattern, and hence the crystal symmetry, is altered from the bulk.We show how the low-symmetry of the various phases in turn induces new structural modes, an increase in the magnetic anisotropy energy, and weak antiferromagnetic spin-canting. © 2021 The Author(s).

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TB2J: A python package for computing magnetic interaction parameters
He X., Helbig N., Verstraete M.J., Bousquet E. Computer Physics Communications; 264 (107938) 2021. 10.1016/j.cpc.2021.107938. IF: 4.390

We present TB2J, a Python package for the automatic computation of magnetic interactions, including exchange and Dzyaloshinskii–Moriya, between atoms of magnetic crystals from the results of density functional calculations. The program is based on the Green's function method with the local rigid spin rotation treated as a perturbation. As input, the package uses the output of either Wannier90, which is interfaced with many density functional theory packages, or of codes based on localized orbitals. One of the main interests of the code is that it requires only one first-principles electronic structure calculation in the non-relativistic case (or three in the relativistic case) and from the primitive cell only to obtain the magnetic interactions up to long distances, instead of first-principles calculations of many different magnetic configurations and large supercells. The output of TB2J can be used directly for the adiabatic magnon band structure and spin dynamics calculations. A minimal user input is needed, which allows for easy integration into high-throughput workflows. Program summary: Program Title: TB2J CPC Library link to program files: https://doi.org/10.17632/dm45fcn69d.1 Developer's repository link: https://github.com/mailhexu/TB2J Code Ocean capsule: https://codeocean.com/capsule/6486145 Licensing provisions: BSD 2-clause Programming language: Python Nature of problem: TB2J is a package for the computing of parameters in the extended Heisenberg model of the magnetic interaction, including the isotropic exchange, anisotropic exchange and Dzyaloshinskii–Moriya interactions from first principles result. It can make use of the Wannier function Hamiltonian, which can be constructed from many first principles codes, or localized orbital based codes. Solution method: It uses the magnetic force theorem and takes the rigid spin rotation as a perturbation to the electronic structure. The energy variation is calculated from the Green's functions from tight-binding like Hamiltonian based on Wannier functions or localized orbitals. Additional comments including restrictions and unusual features: Isotropic exchange, anisotropic exchange, and Dzyaloshinskii–Moriya interactions can all be computed with the input of many DFT codes through the interface of Wannier90, or directly from localized orbital codes. The magnetic interaction parameters up to any distance can be computed from one DFT calculation. A minimum user-input is required which provides a black-box like experience. It generates output for several spin dynamics codes and thus bridges the first principles electronic structure simulation with the large scale spin dynamics simulation. © 2021 Elsevier B.V.

View abstract
Validity of the on-site spin-orbit coupling approximation
Cuadrado R., Robles R., García A., Pruneda M., Ordejón P., Ferrer J., Cerdá J.I. Physical Review B; 104 (19, 195104) 2021. 10.1103/PhysRevB.104.195104. IF: 4.036

Spin-orbit coupling (SOC) is generally understood as a highly localized interaction within each atom, whereby core electrons holding large J splittings transfer the SOC to the valence electrons of the same atom, while their direct impact on neighbor valence orbitals is usually small. Seivane and Ferrer [Phys. Rev. Lett. 99, 183401 (2007)PRLTAO0031-900710.1103/PhysRevLett.99.183401] proposed an approach within a tight-binding type ab initio framework assuming that the transfer of SOC from core to valence orbitals only takes place when both are on the same atom, leading to the so-called on-site approximation, which then has been successfully applied to a variety of systems. In this work we thoroughly test its general validity by confronting SOC related properties such as spin splittings, spin textures, or magnetic anisotropies calculated under the on-site approximation versus the more general approach where all the contributions to the SOC, including three-center integrals, are explicitly included. After considering a variety of systems with different dimensionalities, all presenting a strong SOC, we conclude that although the on-site approximation often provides accurate results, it breaks down in some systems where 5d electrons are close to the Fermi level due to their strong SOC and moderately large spatial extension. Furthermore, there are a few examples where subtle inaccuracies lead to qualitatively wrong conclusions, the most clear case being the doping of the topological surface state in Bi2Se3(0001). Finally, magnetic anisotropy energies calculated under this approximation tend to be underestimated. © 2021 American Physical Society.

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2020

ABINIT: Overview and focus on selected capabilities
Romero A.H., Allan D.C., Amadon B., Antonius G., Applencourt T., Baguet L., Bieder J., Bottin F., Bouchet J., Bousquet E., Bruneval F., Brunin G., Caliste D., Côté M., Denier J., Dreyer C., Ghosez P., Giantomassi M., Gillet Y., Gingras O., Hamann D.R., Hautier G., Jollet F., Jomard G., Martin A., Miranda H.P.C., Naccarato F., Petretto G., Pike N.A., Planes V., Prokhorenko S., Rangel T., Ricci F., Rignanese G.-M., Royo M., Stengel M., Torrent M., Van Setten M.J., Van Troeye B., Verstraete M.J., Wiktor J., Zwanziger J.W., Gonze X. Journal of Chemical Physics; 152 (12, 124102) 2020. 10.1063/1.5144261. IF: 2.991

abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned. © 2020 Author(s).

View abstract
Anion ordering transition and Fermi surface electron-hole instabilities in the (TMTSF)2ClO4and (TMTSF)2NO3Bechgaard salts analyzed through the first-principles Lindhard response function
Guster B., Pruneda M., Ordejón P., Canadell E., Pouget J.-P. Journal of Physics Condensed Matter; 33 (8, 085705) 2020. 10.1088/1361-648X/abc406. IF: 2.705

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2 X (X = ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron-hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2k F response of the quenched samples of both salts display a low temperature curved and tilted triangular continuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron-hole fluctuations. This broad maxima region is around 1% of the Brillouin zone area for the X = ClO4 salt (and X = PF6) but only 0.1% for the X = NO3 salt. It is suggested that the strong reduction of associated SDW fluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron-hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches. © 2020 IOP Publishing Ltd.

View abstract
Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films
Ronneberger I., Zanolli Z., Wuttig M., Mazzarello R. Advanced Materials; 32 (29, 2001033) 2020. 10.1002/adma.202001033. IF: 27.398

Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few-layer crystalline films of monochalcogenides display robust in-plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first-principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te-based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te-based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few-layers structures offers a powerful tool to tune materials properties for applications in information technology. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Cobalt atoms drive the anchoring of Co-TPP molecules to the oxygen-passivated Fe(0 0 1) surface
Calloni A., Jagadeesh M.S., Bussetti G., Fratesi G., Achilli S., Picone A., Lodesani A., Brambilla A., Goletti C., Ciccacci F., Duò L., Finazzi M., Goldoni A., Verdini A., Floreano L. Applied Surface Science; 505 (144213) 2020. 10.1016/j.apsusc.2019.144213. IF: 6.182

We present a multitechnique investigation of the structural and electronic properties of the prototypical system composed by ultra-thin films of magnetic molecules [Co-tetraphenyl-porphyrins (Co-TPP)] grown on a ferromagnetic substrate [oxygen passivated Fe(0 0 1), namely the Fe(0 0 1)-p(1 × 1)O surface]. Low Energy electron diffraction (LEED) and scanning tunneling microscopy (STM), coupled with first-principles calculations, reveal the formation of a commensurate superstructure at monolayer coverage, made by a square array of flat-lying TPP molecules. UV–photoemission and inverse photoemission spectroscopies (UPS and IPES) are used to investigate their electronic structure. Similar to our previous results on the Zn–TPP growth on Fe(0 0 1)–p(1 × 1)O, the passivation of the metallic surface is able to preserve the photoemission features characteristic of quasi-free molecules, opening the route towards an exploitation of single oxide layers as protective films in organic/inorganic junctions. X-ray photoemission (XPS) and near edge X-ray adsorption fine structure spectroscopies (NEXAFS), are used to reveal the details of the Co–TPP interaction with the substrate. © 2019 Elsevier B.V.

View abstract
Effect of Cr on the hydrogen storage and electronic properties of BCC alloys: Experimental and first-principles study
Balcerzak M., Wagstaffe M., Robles R., Pruneda M., Noei H. International Journal of Hydrogen Energy; 45 (53): 28996 - 29008. 2020. 10.1016/j.ijhydene.2020.07.186. IF: 4.939

Inventing an effective method to store large amounts of hydrogen at room temperature is one of the key challenges in developing a hydrogen-based economy. Metal hydrides have attracted attention owing to their promising hydrogen storage capabilities. We have systematically studied the structural and electronic properties of mechanically synthesized Ti0.5V1.5-xCrx (0 ≤ x ≤ 0.3) alloys and investigated the influence of the addition of Cr atoms on the hydrogen storage properties of vanadium-rich body-centered-cubic (V-BCC) alloys. X-ray diffraction (XRD) results indicate that all alloys are composed of BCC main phase, with the lattice parameters exhibiting no change following chemical modification. The kinetic measurements have revealed that Cr-containing alloys exhibit improved hydrogen uptake. X-ray photoelectron spectroscopy (XPS) measurements have shown that the addition of Cr has a significant effect on the anti-oxidation properties of V-BCC alloys, increasing their chemical activity and thus enhancing the hydrogen storage properties. Moreover, XPS results elucidate the role of activation of the studied materials. Additionally, the electrochemical properties of the negative electrodes (as part of Ni-MHx secondary batteries) made of Ti0.5V1.4-xNi0.1Crx (0 ≤ x ≤ 0.3) system have been studied by cyclic charge-discharge and demonstrate that doping of the V-BCC alloys with Cr can significantly improve the cycle-life stability of anode that exhibits similar discharge performance up to 50 cycles. First principles simulations are used to analyse the changes in the electronic density of states close to the Fermi level, as a function of Cr concentration, as well as binding energies and structural changes upon hydrogen absorption. Furthermore, ab initio studies confirmed that H absorption is favoured with increasing Cr-content. Our study highlights the importance of the addition of Cr to V-BCC alloys on both solid-gas and electrochemical hydrogenation reactions. © 2020 Hydrogen Energy Publications LLC

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Electron-Phonon beyond Fröhlich: Dynamical Quadrupoles in Polar and Covalent Solids
Brunin G., Miranda H.P.C., Giantomassi M., Royo M., Stengel M., Verstraete M.J., Gonze X., Rignanese G.-M., Hautier G. Physical Review Letters; 125 (13, 136601) 2020. 10.1103/PhysRevLett.125.136601. IF: 8.385

We include the treatment of quadrupolar fields beyond the Fröhlich interaction in the first-principles electron-phonon vertex in semiconductors. Such quadrupolar fields induce long-range interactions that have to be taken into account for accurate physical results. We apply our formalism to Si (nonpolar), GaAs, and GaP (polar) and demonstrate that electron mobilities show large errors if dynamical quadrupoles are not properly treated. © 2020 American Physical Society.

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Fermi surface electron-hole instability of the (TMTSF)2PF6 Bechgaard salt revealed by the first-principles Lindhard response function
Guster B., Pruneda M., Ordejón P., Canadell E., Pouget J.-P. Journal of Physics Condensed Matter; 32 (34, 345701) 2020. 10.1088/1361-648X/ab8522. IF: 2.707

We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2k F spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2k F broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron-hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2k F bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron-hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2k F SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors. © 2020 IOP Publishing Ltd.

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Gap anisotropy in multiband superconductors based on multiple scattering theory
Saunderson T.G., Annett J.F., Újfalussy B., Csire G., Gradhand M. Physical Review B; 101 (6, 064510) 2020. 10.1103/PhysRevB.101.064510. IF: 3.575

We implement the Bogoliubov-de Gennes equation in a screened Korringa-Kohn-Rostoker method for solving, self-consistently, the superconducting state for three-dimensional crystals. This method combines the full complexity of the underlying electronic structure and Fermi surface geometry with a simple phenomenological parametrization for the superconductivity. We apply this theoretical framework to the known s-wave superconductors Nb, Pb, and MgB2. In these materials multiple distinct peaks at the gap in the density of states were observed, showing significant gap anisotropy which is in good agreement with experiment. Qualitatively, the results can be explained in terms of the k-dependent Fermi velocities on the Fermi surface sheets exploiting concepts from BCS theory. © 2020 American Physical Society.

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LDA+U study of hydrostatic pressure effect on double perovskite Sr2FeNbO6: Crystal structure, mechanical and electronic properties
Rosas-Huerta J.L., Antonio J.E., Romero M., León-Flores J., Pilo J., Carvajal E., Escamilla R. Physica Scripta; 95 (11, 115704) 2020. 10.1088/1402-4896/abbf70. IF: 1.985

To study the effect of the applied hydrostatic pressure on the crystal structure and the electronic and mechanical properties of the Sr2FeNbO6 compound, computational calculations in the density functional theory framework, with the local density approximation and Hubbard correction as it is treated by the CA-PZ exchange-correlation functional were performed. The tetragonal structure with the I4/m space group is reported stable in the range from zero to 50 GPa according to Born's stability criterion. No crystal phase transition was found in agreement with experimental data; however, between 20 and 30 GPa, a brittle to ductile transition is confirmed by the Pugh's criterion and Poisson's ratio. Moreover, a change from ionic-covalent to metallic bonding is suggested by the Poisson's ratio. This behavior is reflected in the electronic properties, through the controlled modulation of the energy bandgap (Eg (eV)) as a function of pressure, according to a fitted linear equation, Eg = (-0.016)P + 2.040. At 50 GPa, Eg value is 1.236 eV, very close to the ideal 1.34 eV, which is required for hydrogen generation and photovoltaic applications. © 2020 IOP Publishing Ltd.

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Phonon-limited electron mobility in Si, GaAs, and GaP with exact treatment of dynamical quadrupoles
Brunin G., Miranda H.P.C., Giantomassi M., Royo M., Stengel M., Verstraete M.J., Gonze X., Rignanese G.-M., Hautier G. Physical Review B; 102 (9, 094308) 2020. 10.1103/PhysRevB.102.094308. IF: 3.575

We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense q meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology. © 2020 American Physical Society.

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Proximity effect in a superconductor-topological insulator heterostructure based on first principles
Park K., Csire G., Ujfalussy B. Physical Review B; 102 (13, 134504) 2020. 10.1103/PhysRevB.102.134504. IF: 3.575

Superconductor-topological insulator (SC-TI) heterostructures were proposed to be a possible platform to realize and control Majorana zero modes. Despite experimental signatures indicating their existence, univocal interpretation of the observed features demands theories including realistic electronic structures. To achieve this, we solve the Kohn-Sham-Dirac-Bogoliubov-de Gennes equations for ultrathin Bi2Se3 films on superconductor palladium telluride within the fully relativistic Korringa-Kohn-Rostoker method and investigate quasiparticle spectra as a function of chemical potential and film thickness. We find multiple proximity-induced gaps where the gap sizes highly depend on characteristics of the TI states. The TI Dirac interface state is relevant to the induced gap only when the chemical potential is close to the Dirac-point energy. Otherwise, at a given chemical potential, the largest induced gap arises from the highest-energy quantum-well states, whereas the smallest gap arises from the TI topological surface state with its gap size depending on the TI pairing potential. © 2020 American Physical Society.

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Quantitative theory of triplet pairing in the unconventional superconductor LaNiGa2
Ghosh S.K., Csire G., Whittlesea P., Annett J.F., Gradhand M., Újfalussy B., Quintanilla J. Physical Review B; 101 (10, 100506) 2020. 10.1103/PhysRevB.101.100506. IF: 3.575

The exceptionally low-symmetry crystal structures of the time-reversal symmetry-breaking superconductors LaNiC2 and LaNiGa2 lead to an internally antisymmetric nonunitary triplet state as the only possibility compatible with experiments. We argue that this state has a distinct signature: A double-peak structure in the density of states (DOS) which resolves in the spin channel in a particular way. We construct a detailed model of LaNiGa2 capturing its electronic band structure and magnetic properties ab initio. The pairing mechanism is described via a single adjustable parameter. The latter is fixed by the critical temperature Tc allowing parameter-free predictions. We compute the electronic specific heat and find excellent agreement with experiment. The size of the ordered moment in the superconducting state is compatible with zero-field muon spin relaxation experiments and the predicted spin-resolved DOS suggests the spin splitting is within the reach of present experimental technology. © 2020 American Physical Society.

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Real-space multiple scattering theory for superconductors with impurities
Saunderson T.G., Gyorgypál Z., Annett J.F., Csire G., Újfalussy B., Gradhand M. Physical Review B; 102 (24, 245106) 2020. 10.1103/PhysRevB.102.245106. IF: 3.575

We implement the Bogoliubov-de Gennes (BdG) equation in real-space using the screened Korringa-Kohn-Rostoker (KKR) method. This allows us to solve, self-consistently, the superconducting state for 3D crystals including substitutional impurities with a full normal-state DFT band structure. We apply the theoretical framework to bulk Nb with impurities. Without impurities, Nb has an anisotropic gap structure with two distinct peaks around the Fermi level. In the presence of nonmagnetic impurities, those peaks are broadened due to the scattering between the two bulk superconducting gaps, however the peaks remain separated. As a second example of self-consistent real-space solutions of the BdG equations, we examine superconducting clusters embedded within a nonsuperconducting bulk metallic host. This allows us to estimate the coherence length of the superconductor and we show that, within our framework, the coherence length of the superconductor is related to the inverse of the gap size, just as in bulk BCS theory. © 2020 American Physical Society.

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Siesta: Recent developments and applications
García A., Papior N., Akhtar A., Artacho E., Blum V., Bosoni E., Brandimarte P., Brandbyge M., Cerdá J.I., Corsetti F., Cuadrado R., Dikan V., Ferrer J., Gale J., García-Fernández P., García-Suárez V.M., García S., Huhs G., Illera S., Korytár R., Koval P., Lebedeva I., Lin L., López-Tarifa P., Mayo S.G., Mohr S., Ordejón P., Postnikov A., Pouillon Y., Pruneda M., Robles R., Sánchez-Portal D., Soler J.M., Ullah R., Yu V.W.-Z., Junquera J. The Journal of chemical physics; 152 (20): 204108. 2020. 10.1063/5.0005077. IF: 2.991

A review of the present status, recent enhancements, and applicability of the Siesta program is presented. Since its debut in the mid-1990s, Siesta's flexibility, efficiency, and free distribution have given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of Siesta combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here, we describe the more recent implementations on top of that core scheme, which include full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, density functional theory (DFT)+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as wannier90 and the second-principles modeling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering Siesta runs, and various post-processing utilities. Siesta has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low-level libraries, as well as data standards and support for them, particularly the PSeudopotential Markup Language definition and library for transferable pseudopotentials, and the interface to the ELectronic Structure Infrastructure library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.

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Supertetragonal Phases of Perovskite Oxides: Insights from Electronic Structure Calculations
Cohen N., Diéguez O. Israel Journal of Chemistry; 60 (8-9): 833 - 841. 2020. 10.1002/ijch.201900135. IF: 2.320

We review some of the insights that electronic-structure calculations has brought about the properties of the materials with the largest electric polarization known – supertetragonal perovskite oxides. These are materials whose structure corresponds to a perovskite that has been substantially strechted along one of its pseudocubic axes. They grow in different forms: bulk crystals (such as BiCoO3), epitaxial films (such as BiFeO3), nanowires whose inside is under negative pressure (such as PbTiO3), and others. Electronic structure calculations based on density-functional theory have revealed that supertetragonality potentially exist for many perovskite oxides under the right conditions, and they have helped explain why some of those conditions are easy to reach for some of the materials of the family, but not for others. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Unfolding method for periodic twisted systems with commensurate Moiré patterns
Sánchez-Ochoa F., Hidalgo F., Pruneda M., Noguez C. Journal of Physics Condensed Matter; 32 (2, 025501) 2020. 10.1088/1361-648X/ab44f0. IF: 2.707

We present a general unfolding method for the electronic bands of systems with double-periodicity. Within density functional theory with atomic orbitals as basis-set, our method takes into account two symmetry operations of the primitive cell: a standard expansion and a single rotation, letting to elucidate the physical effects associated to the mutual interactions between systems with more than one periodicity. As a result, our unfolding method allows studying the electronic properties of vertically stacked two-dimensional homo-or heterostructures. We apply our method to study 3 × 3single-layer graphene, √13×√ twisted single-layer graphene, and 2√3×2√3 graphene-√7×√7 tungsten disulfide heterostructure with an interlayer angle of 10.9°. Our unfolding method allows observing typical mini gaps reported in heterostructures, as well as other electronic deviations from pristine structures, impossible to distinguish without an unfolding method. We anticipate that this unfolding method can be useful to compare with experiments to elucidate the electronic properties of two-dimensional homo-or heterostructures, where the interlayer angle can be considered as an additional parameter. © 2019 IOP Publishing Ltd.

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2019

2 × 2 charge density wave in single-layer TiTe2
Guster B., Robles R., Pruneda M., Canadell E., Ordejón P. 2D Materials; 6 (1, 015027) 2019. 10.1088/2053-1583/aaf20b. IF: 7.343

A density functional theory study concerning the origin of the recently reported 2 × 2 charge density wave (CDW) instability in single-layer TiTe2 is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a 2 × 2 distortion. The study suggests that the magnitude of the semimetallic overlap between the valence band top at - and the conduction band bottom at M is a key factor controlling the tendency towards the distortion. It is also shown that tensile strain stabilizes a 2 × 2 CDW, and we suggest that this fact could be further used to induce the instability in doublelayers of TiTe2, which in the absence of strain remain undistorted in the experiment. The driving force for the CDW instability seems to be the same phonon mediated mechanism acting for singlelayer TiSe2, although in single-layer TiTe2 the driving force is smaller, and the semimetallic character is kept below the transition temperature. © 2018 IOP Publishing Ltd.

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Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe 2
Guster B., Rubio-Verdú C., Robles R., Zaldívar J., Dreher P., Pruneda M., Silva-Guillén J.Á., Choi D.-J., Pascual J.I., Ugeda M.M., Ordejón P., Canadell E. Nano Letters; 19 (5): 3027 - 3032. 2019. 10.1021/acs.nanolett.9b00268. IF: 12.279

Bulk and single-layer 2H-NbSe 2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2H-NbSe 2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2H-NbSe 2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2H-NbSe 2 may provide further understanding of its low temperature state and the competition between different instabilities. © 2019 American Chemical Society.

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Evidence for the weak coupling scenario of the Peierls transition in the blue bronze
Guster B., Pruneda M., Ordejón P., Canadell E., Pouget J.-P. Physical Review Materials; 3 (5, 055001) 2019. 10.1103/PhysRevMaterials.3.055001. IF: 2.926

On the basis of first-principles DFT calculations the wave-vector and temperature dependencies of the Lindhard response function of the blue bronze K0.3MoO3 have been calculated. The kFI+kFII interband component of the response, which is responsible for the Peierls instability, has been quantitatively analyzed. It is found that (i) the electron-hole coherence length of this response determines the length scale of the experimental intrachain CDW correlations, and (ii) the intrachain qâ¥ dependence of such a response also determines the shape of the Kohn anomaly experimentally measured. These findings provide compelling evidence that the Peierls transition of the blue bronze K0.3MoO3 follows the weak electron-phonon coupling scenario in the adiabatic approximation, something that had not yet been proved on the basis of first-principles calculations for a real material. It is proposed that the CDW interchain coupling occurs through a Coulomb coupling between dipolar CDWs. The nature of the phonon mode leading to the dipolar nature of the CDWs is also discussed, and the relevance of these results to rationalize the CDW instabilities in other oxides and bronzes is pointed out. These findings are also contrasted with recent results for other CDW materials like chalcogenides and tellurides. © 2019 American Physical Society.

View abstract
Guidelines for selecting interlayer spacers in synthetic 2D-based antiferromagnets from first-principles simulations
Cuadrado R., Pruneda M. Nanomaterials; 9 (12, 1764) 2019. 10.3390/nano9121764. IF: 4.034

Following the recent synthesis of graphene–based antiferromagnetic ultrathin heterostructures made of Co and Fe, we analyse the effect of the spacer between both ferromagnetic materials. Using density functional calculations, we carried out an exhaustive study of the geometric, electronic and magnetic properties for intercalated single Co MLs on top of Ir(111) coupled to monolayered Fe through n graphene layers (n = 1, 2, 3) or monolayered h-BN. Different local atomic arrangements have been considered to model the Moiré patterns expected in these heterostructures. The magnetic exchange interactions between both ferromagnets (JCo−Fe) are computed from explicit calculations of parallel and anti-parallel Fe/Co inter–layer alignments, and discussed in the context of recent experiments. Our analysis confirms that the robust antiferromagnetic superexchange–coupling between Fe and Co layers is mediated by the graphene spacer through the hybridization of C’s pz orbitals with Fe and Co’s 3d states. The hybridization is substantially suppressed for multilayered graphene spacers, for which the magnetic coupling between ferromagnets is critically reduced, suggesting the need for ultrathin (monolayer) spacers in the design of synthetic graphene-based antiferromagnets. In the case of h–BN, pz orbitals also mediate d(Fe/Co) coupling. However, there is a larger contribution of local ferromagnetic interactions. Magnetic anisotropy energies were also calculated using a fully relativistic description, and show out–of–plane easy axis for all the configurations, with remarkable net values in the range from 1 to 4 meV. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

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Low-Energy Phases of Bi Monolayer Predicted by Structure Search in Two Dimensions
Singh S., Zanolli Z., Amsler M., Belhadji B., Sofo J.O., Verstraete M.J., Romero A.H. Journal of Physical Chemistry Letters; 10 (23): 7324 - 7332. 2019. 10.1021/acs.jpclett.9b03043. IF: 7.329

We employ an ab-initio structure search algorithm to explore the configurational space of bismuth in quasi-two dimensions. A confinement potential is introduced to restrict the movement of atoms within a predefined thickness to find the stable and metastable forms of monolayer Bi. In addition to the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new phases: α, β, and γ. Each phase exhibits peculiar electronic properties, ranging from metallic (α and γ) to semiconducting (puckered monoclinic, buckled hexagonal, and β) monolayers. Topologically nontrivial features are predicted for buckled hexagonal and γphases. We also remark on the role of 5d electrons on the electronic properties of Bi monolayer. We conclude that Bi provides a rich playground to study distortion-mediated metal-insulator phase transitions in quasi-2D. Copyright © 2019 American Chemical Society.

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Real space manifestations of coherent screening in atomic scale Kondo lattices
Moro-Lagares M., Korytár R., Piantek M., Robles R., Lorente N., Pascual J.I., Ibarra M.R., Serrate D. Nature Communications; 10 (1, 2211) 2019. 10.1038/s41467-019-10103-5. IF: 11.878

The interaction among magnetic moments screened by conduction electrons drives quantum phase transitions between magnetically ordered and heavy-fermion ground states. Here, starting from isolated magnetic impurities in the Kondo regime, we investigate the formation of the finite size analogue of a heavy Fermi liquid. We build regularly-spaced chains of Co adatoms on a metallic surface by atomic manipulation. Scanning tunneling spectroscopy is used to obtain maps of the Kondo resonance intensity with sub-atomic resolution. For sufficiently small interatomic separation, the spatial distribution of Kondo screening does not coincide with the position of the adatoms. It also develops enhancements at both edges of the chains. Since we can rule out any other interaction between Kondo impurities, this is explained in terms of the indirect hybridization of the Kondo orbitals mediated by a coherent electron gas, the mechanism that causes the emergence of heavy quasiparticles in the thermodynamic limit. © 2019, The Author(s).

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Spectroscopic properties of few-layer tin chalcogenides
Dewandre A., Verstraete M. J., Grobert N., Zanolli Z. J. Phys. Mater.; 2 (44005) 2019. 10.1088/2515-7639/ab3513.

Stable structures of layered SnS and SnSe and their associated electronic and vibrational spectra arepredicted using first-principles DFT calculations. The calculations show that both materials undergo a phase transformation upon thinning whereby the in-plane lattice parameters ratioa/bconvergestowards 1, similar to the high-temperature behaviour observed for their bulk counterparts. The electronic properties of layered SnS and SnSe evolve to an almost symmetric dispersion whilst the gapchanges from indirect to direct. Characteristic signatures in the phonon dispersion curves and surface phonon states where only atoms belonging to surface layers vibrate should be observable experimentally.

View abstract
Spin States Protected from Intrinsic Electron-Phonon Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe2
Ersfeld M., Volmer F., De Melo P.M.M.C., De Winter R., Heithoff M., Zanolli Z., Stampfer C., Verstraete M.J., Beschoten B. Nano Letters; 19 (6): 4083 - 4090. 2019. 10.1021/acs.nanolett.9b01485. IF: 12.279

We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe2. These long lifetimes are accompanied by an intriguing temperature-dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations, we explain the latter behavior in terms of the intrinsic electron-phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photoexcited electron-hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated intervalley scattering rates. However, the electron-phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states, which are protected from the intrinsic electron-phonon coupling, and are most likely resident charge carriers, which are not part of the itinerant valence or conduction band states. © 2019 American Chemical Society.

View abstract

2018

A liquid-liquid transition in supercooled aqueous solution related to the HDA-LDA transition
Woutersen S., Ensing B., Hilbers M., Zhao Z., Austen Angell C. Science; 359 (6380): 1127 - 1131. 2018. 10.1126/science.aao7049. IF: 41.058

Simulations and theory suggest that the thermodynamic anomalies of water may be related to a phase transition between two supercooled liquid states, but so far this phase transition has not been observed experimentally because of preemptive ice crystallization. We used calorimetry, infrared spectroscopy, and molecular dynamics simulations to investigate a water-rich hydrazinium trifluoroacetate solution in which the local hydrogen bond structure surrounding a water molecule resembles that in neat water at elevated pressure, but which does not crystallize upon cooling. Instead, this solution underwent a sharp, reversible phase transition between two homogeneous liquid states. The hydrogen-bond structures of these two states are similar to those established for high- and low-density amorphous (HDA and LDA) water. Such structural similarity supports theories that predict a similar sharp transition in pure water under pressure if ice crystallization could be suppressed. © 2017 The Authors.

View abstract
A multiscale model of the effect of Ir thickness on the static and dynamic properties of Fe/Ir/Fe films
Cuadrado R., Oroszlány L., Szunyogh L., Hrkac G., Chantrell R.W., Ostler T.A. Scientific Reports; 8 (1, 3879) 2018. 10.1038/s41598-018-21934-5. IF: 4.122

The complex magnetic properties of Fe/Ir/Fe sandwiches are studied using a hierarchical multi-scale model. The approach uses first principles calculations and thermodynamic models to reveal the equilibrium spinwave, magnetization and dynamic demagnetisation properties. Finite temperature calculations show a complex spinwave dispersion and an initially counter-intuitive, increasing exchange stiffness with temperature (a key quantity for device applications) due to the effects of frustration at the interface, which then decreases due to magnon softening. Finally, the demagnetisation process in these structures is shown to be much slower at the interface as compared with the bulk, a key insight to interpret ultrafast laser-induced demagnetization processes in layered or interface materials. © The Author(s) 2018.

View abstract
Addressing the Environment Electrostatic Effect on Ballistic Electron Transport in Large Systems: A QM/MM-NEGF Approach
Feliciano G.T., Sanz-Navarro C., Coutinho-Neto M.D., Ordejón P., Scheicher R.H., Rocha A.R. Journal of Physical Chemistry B; 122 (2): 485 - 492. 2018. 10.1021/acs.jpcb.7b03475. IF: 3.146

The effects of the environment in nanoscopic materials can play a crucial role in device design. Particularly in biosensors, where the system is usually embedded in a solution, water and ions have to be taken into consideration in atomistic simulations of electronic transport for a realistic description of the system. In this work, we present a methodology that combines quantum mechanics/molecular mechanics methods (QM/MM) with the nonequilibrium Green's function framework to simulate the electronic transport properties of nanoscopic devices in the presence of solvents. As a case in point, we present further results for DNA translocation through a graphene nanopore. In particular, we take a closer look into general assumptions in a previous work. For this sake, we consider larger QM regions that include the first two solvation shells and investigate the effects of adding extra k-points to the NEGF calculations. The transverse conductance is then calculated in a prototype sequencing device in order to highlight the effects of the solvent. © 2017 American Chemical Society.

View abstract
First principles analysis of the CDW instability of single-layer 1T-TiSe2 and its evolution with charge carrier density
Guster B., Canadell E., Pruneda M., Ordejón P. 2D Materials; 5 (2, 025024) 2018. 10.1088/2053-1583/aab568. IF: 7.042

We present a density functional theory study of the electronic structure of single-layer TiSe2, and focus on the charge density wave (CDW) instability present on this 2D material. We explain the periodicity of the CDW from the phonon band structure of the undistorted crystal, which is unstable under one of the phonon modes at the M point. This can be understood in terms of a partial band gap opening at the Fermi level, which we describe on the basis of the symmetry of the involved crystal orbitals, leading to an energy gain upon the displacement of the atoms following the phonon mode in a 2 × 1 structure. Furthermore, the combination of the corresponding phonons for the three inequivalent M points of the Brillouin zone leads to the 2 × 2 distortion characteristic of the CDW state. This leads to a further opening of a full gap, which reduces the energy of the 2 × 2 structure compared to the 2 × 1 one of a single M point phonon, and makes the CDW structure the most stable one. We also analyze the effect of charge injection into the layer on the structural instability. We predict that the 2 × 2 structure only survives for a certain range of doping levels, both for electrons and for holes, as doping reduces the energy gain due to the gap opening. We predict the transition from the commensurate 2 × 2 distortion to an incommensurate one with increasing wavelength upon increasing the doping level, followed by the appearance of the undistorted 1 × 1 structure for larger carrier concentrations. © 2018 IOP Publishing Ltd.

View abstract
Hybrid quantum anomalous Hall effect at graphene-oxide interfaces
Zanolli Z., Niu C., Bihlmayer G., Mokrousov Y., Mavropoulos P., Verstraete M.J., Blügel S. Physical Review B; 98 (15, 155404) 2018. 10.1103/PhysRevB.98.155404. IF: 3.813

Interfaces are ubiquitous in materials science, and in devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications, here for spintronics. Using first-principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor BaMnO3. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from antiferromagnetic to ferromagnetic. The graphene-BaMnO3 system presents a Rashba gap 300 times larger than in pristine graphene, leading to a flavor of quantum anomalous Hall effect (QAHE), a hybrid QAHE, characterized by the coexistence of metallic and topological insulating states. These findings could be exploited to fabricate devices that use graphene to control the magnetic configuration of a substrate. © 2018 American Physical Society.

View abstract
Implementation of non-collinear spin-constrained DFT calculations in SIESTA with a fully relativistic Hamiltonian
R. Cuadrado, M. Pruneda, A. García, P. Ordejón Journal of Physics: Materials; 1 (1) 2018. 10.1088/2515-7639/aae7db.

An accurate and efficient general method to constrain the magnetization of individual atoms or groups of atoms within a fully relativistic non-collinear spin density functional theory formalism is presented and implemented within the SIESTA code. This approach can be applied to study a variety of complex magnetic configurations and to build effective magnetic Hamiltonians for multiscaling micromagnetic simulations. As an example, the method is applied to obtain constrained magnetic states for a Fe3 structure, and for a S = 1/2 kagome layer (vanadium oxyfluoride V7O6F18). Of paramount importance in spintronics is the control and manipulation of magnetic interactions between constituent species, characterized mainly by the pair-wise magnetic exchange tensor Jij. By constraining the atomic magnetizations of an infinite Fe linear chain, the total selfconsistent energy values are mapped to a generalized Heisenberg model, obtaining not only the diagonal terms of Jij but also the off-diagonal contributions due to the explicit presence of the spin–orbit coupling in the formalism. The diagonal values of Jij promote short ranged ferromagnetic alignment whilst the non-zero off-diagonal values can lead to the formation of the spiral states in the chain, as expected from theory.

View abstract
Mechanisms behind the enhancement of thermal properties of graphene nanofluids
Rodríguez-Laguna M.R., Castro-Alvarez A., Sledzinska M., Maire J., Costanzo F., Ensing B., Pruneda M., Ordejón P., Sotomayor Torres C.M., Gómez-Romero P., Chávez-Ángel E. Nanoscale; 10 (32): 15402 - 15409. 2018. 10.1039/c8nr02762e. IF: 7.233

While the dispersion of nanomaterials is known to be effective in enhancing the thermal conductivity and specific heat capacity of fluids, the mechanisms behind this enhancement remain to be elucidated. Herein, we report on highly stable, surfactant-free graphene nanofluids, based on N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), with enhanced thermal properties. An increase of up to 48% in thermal conductivity and 18% in specific heat capacity was measured. The blue shift of several Raman bands with increasing graphene concentration in DMF indicates that there is a modification in the vibrational energy of the bonds associated with these modes, affecting all the molecules in the liquid. This result indicates that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. Density functional theory and molecular dynamics simulations were used to gather data on the interaction between graphene and solvent, and to investigate a possible order induced by graphene on the solvent. The simulations showed a parallel orientation of DMF towards graphene, favoring π-π stacking. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the fluid's thermal properties. © The Royal Society of Chemistry.

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Optical and electronic properties of 2H−MoS2 under pressure: Revealing the spin-polarized nature of bulk electronic bands
Mauro Brotons-Gisbert, Alfredo Segura, Roberto Robles, Enric Canadell, Pablo Ordejón, and Juan F. Sánchez-Royo Physical Review Materials; 2 (5): 54602. 2018. 10.1103/PhysRevMaterials.2.054602. IF: 0.000

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk 2H−MoS2 hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of A and B excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of 2H−MoS2. Also, the pressure dependence of the indirect optical transitions of bulk 2H−MoS2 has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk MoS2 may occur at pressures higher than 26 Gpa.

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Site-Resolved Contributions to the Magnetic-Anisotropy Energy and Complex Spin Structure of Fe/MgO Sandwiches
Cuadrado R., Oroszlány L., Deák A., Ostler T.A., Meo A., Chepulskii R.V., Apalkov D., Evans R.F.L., Szunyogh L., Chantrell R.W. Physical Review Applied; 9 (5, 054048) 2018. 10.1103/PhysRevApplied.9.054048. IF: 4.782

Fe/MgO-based magnetic tunnel junctions are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite-size effects and temperature-dependent magnetic properties which must be carefully controlled for practical applications. In this article, we investigate the electronic, structural, and magnetic properties of MgO/Fe/MgO sandwiches using first-principles calculations and atomistic spin modeling based on a fully parametrized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin-spiral ground-state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layerwise temperature dependence of the magnetization. The different temperature dependences of the interface and bulklike layers results in an unexpected nonmonotonic temperature variation of the effective magnetic-anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing insight when interpreting experimental data of nanoscale magnetic tunnel junctions. © 2018 American Physical Society.

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Spin Proximity Effects in Graphene/Topological Insulator Heterostructures
Song K., Soriano D., Cummings A.W., Robles R., Ordejón P., Roche S. Nano Letters; 18 (3): 2033 - 2039. 2018. 10.1021/acs.nanolett.7b05482. IF: 12.080

Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. Here, we use ab initio simulations and tight-binding models to determine the precise spin texture of electronic states in graphene interfaced with a Bi2Se3 topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices. © 2018 American Chemical Society.

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Spin-Crossover in an Exfoliated 2D Coordination Polymer and Its Implementation in Thermochromic Films
Salvio Suárez-García, Nayarassery N. Adarsh, Gábor Molnár, Azzedine Bousseksou, Yann Garcia, Marinela M. Dîrtu, Javier Saiz-Poseu, Roberto Robles, Pablo Ordejón, and Daniel Ruiz-Molina ACS Applied Nano Materials; 1 (6): 2662 - 2668. 2018. 10.1021/acsanm.8b00341 . IF: 0.000

Development of novel 2D materials with singular and thrilling properties has aroused large interest due to the novel unexpected applications that can be derived from there. In this sense, coordination polymers (CPs) have appeared as matching candidates thanks to their rational chemical design and the added-value properties given by the presence of metal ions. This is the case of switchable spin-crossover systems that have been proposed as excellent candidates for data storage or sensing, among others. Here we report the delamination of crystals of the 2D spin-crossover (SCO) {[Fe(L1)2](ClO4)2}∝ (1) CP by liquid-phase exfoliation (LPE) in water. The application of this top-down technique results in the formation of flakes with controlled thicknesses, down to 1–2 nm thick (mostly mono- and bilayer), that retain the chemical composition and SCO interconversion of the bulk material. Moreover, these flakes can be handled as stable colloidal dispersions for many days. This allows for a controlled transfer to solid substrates and the formation of thermochromic polymeric films as a proof-of-concept of device. These first results will definitely open new venues and opportunities for the investigation and future integration of these original switchable 2D materials in devices.

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Structure evolution of mononuclear tungsten and molybdenum species in the protonation process: Insight from FPMD and DFT calculations
Zhang N., Yi H., Zeng D., Zhao Z., Wang W., Costanzo F. Chemical Physics; 502: 77 - 86. 2018. 10.1016/j.chemphys.2018.01.009. IF: 1.707

In this work, we apply static density functional theory (DFT) calculations, as well as classical and first-principles molecular dynamics (FPMD) simulations, using the free-energy perturbation method to study the protonation ability, active site and structures of W(VI) and Mo(VI) in acidic aqueous solution. Using FPMD simulations, utilizing the pKa's calculation technique, we concluded that the octahedral WO2(OH)2(H2O)2 is the true formula for tungstic acid (H2WO4), and the hydroxyl ligands are the acidic site. This aqueous structure of H2WO4 is analogous to the previously reported structure of molybdic acid (H2MoO4). The FPMD trajectories of the tungstic acid deprotonation show that the mono-protonated monotungstate ion (HWO4 −) may partially exist as a five-coordinated WO3(OH)(H2O)− species except for the four-coordinated WO3(OH)− species. This result is supported by DFT calculations, with an isoenergetic point (ΔE = 1.9 kcal·mol−1) for the WO3(OH)(H2O)− and WO3(OH)− species, when explicit solvent molecules are taken into account. In contrast, for the H2MoO4 acid, FPMD trajectories during the deprotonation process show that two H2O ligands immediately escape from the first coordinated sphere of Mo(VI) to form the four-coordinated MoO3(OH)− species. This difference indicates that structural expansion of W(VI) began in the first protonated step, while that of Mo(VI) only occurs in the second step. In addition, our calculated first and second acid constants for tungstic acid are higher than previously reported values for molybdic acid. This result suggests that WO4 2− is more easily protonated than the MoO4 2− anion in the same acidic solution, which is further confirmed by DFT calculations of hydrated oxoanions and its protonated species, based upon the hydration energy. © 2018 Elsevier B.V.

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2017

A tunable electronic beam splitter realized with crossed graphene nanoribbons
Brandimarte P., Engelund M., Papior N., Garcia-Lekue A., Frederiksen T., Sánchez-Portal D. Journal of Chemical Physics; 146 (9, 92318) 2017. 10.1063/1.4974895. IF: 2.965

Graphene nanoribbons (GNRs) are promising components in future nanoelectronics due to the large mobility of graphene electrons and their tunable electronic band gap in combination with recent experimental developments of on-surface chemistry strategies for their growth. Here, we explore a prototype 4-terminal semiconducting device formed by two crossed armchair GNRs (AGNRs) using state-of-the-art first-principles transport methods. We analyze in detail the roles of intersection angle, stacking order, inter-GNR separation, GNR width, and finite voltages on the transport characteristics. Interestingly, when the AGNRs intersect at θ=60°, electrons injected from one terminal can be split into two outgoing waves with a tunable ratio around 50% and with almost negligible back-reflection. The split electron wave is found to propagate partly straight across the intersection region in one ribbon and partly in one direction of the other ribbon, i.e., in analogy with an optical beam splitter. Our simulations further identify realistic conditions for which this semiconducting device can act as a mechanically controllable electronic beam splitter with possible applications in carbon-based quantum electronic circuits and electron optics. We rationalize our findings with a simple model suggesting that electronic beam splitters can generally be realized with crossed GNRs. © 2017 Author(s).

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Ab initio study of electron-phonon coupling in rubrene
Ordejón P., Boskovic D., Panhans M., Ortmann F. Physical Review B; 96 (3, 035202) 2017. 10.1103/PhysRevB.96.035202. IF: 3.836

The use of ab initio methods for accurate simulations of electronic, phononic, and electron-phonon properties of molecular materials such as organic crystals is a challenge that is often tackled stepwise based on molecular properties calculated in gas phase and perturbatively treated parameters relevant for solid phases. In contrast, in this work we report a full first-principles description of such properties for the prototypical rubrene crystals. More specifically, we determine a Holstein-Peierls-type Hamiltonian for rubrene, including local and nonlocal electron-phonon couplings. Thereby, a recipe for circumventing the issue of numerical inaccuracies with low-frequency phonons is presented. In addition, we study the phenyl group motion with a molecular dynamics approach. © 2017 American Physical Society.

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Anisotropic features in the electronic structure of the two-dimensional transition metal trichalcogenide TiS3: Electron doping and plasmons
Silva-Guillén J.A., Canadell E., Ordejón P., Guinea F., Roldán R. 2D Materials; 4 (2, 025085) 2017. 10.1088/2053-1583/aa6b92. IF: 6.937

Analysis of the band structure of TiS3 single-layers suggests the possibility of changing their physical behaviour by injecting electron carriers. The anisotropy of the valence and conduction bands is explained in terms of their complex orbital composition. The nature of the Fermi surface and Lindhard response function for different doping concentrations is studied by means of firstprinciples DFT calculations. It is suggested that for electron doping levels x (number of electrons per unit cell) ~0.18-0.30e- the system could exhibit incommensurate charge or spin modulations which, however, would keep the metallic state whereas systems doped with smaller x would be 2D metals without any electronic instability. The effect of spin-orbit coupling in the band dispersion is analysed. The DFT effective masses are used to study the plasmon spectrum from an effective low energy model. We find that this material supports highly anisotropic plasmons, with opposite anisotropy for the electron and hole bands.

View abstract
Building Complex Kondo Impurities by Manipulating Entangled Spin Chains
Choi D.-J., Robles R., Yan S., Burgess J.A.J., Rolf-Pissarczyk S., Gauyacq J.-P., Lorente N., Ternes M., Loth S. Nano Letters; 17 (10): 6203 - 6209. 2017. 10.1021/acs.nanolett.7b02882. IF: 12.712

The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a Cu2N surface with a scanning tunneling microscope. The atoms couple antiferromagnetically via superexchange interaction through the nitrogen atom network of the surface. The emergent Kondo resonance is spatially distributed along the chain. Its strength can be controlled by mixing atoms of different transition metal elements and manipulating their spatial distribution. We show that the Kondo screening of the full chain by the electrons of the nonmagnetic substrate depends on the interatomic entanglement of the spins in the chain, demonstrating the prerequisites to build and probe spatially extended strongly correlated nanostructures. © 2017 American Chemical Society.

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Electrical and Thermal Transport in Coplanar Polycrystalline Graphene-hBN Heterostructures
Barrios-Vargas J.E., Mortazavi B., Cummings A.W., Martinez-Gordillo R., Pruneda M., Colombo L., Rabczuk T., Roche S. Nano Letters; 17 (3): 1660 - 1664. 2017. 10.1021/acs.nanolett.6b04936. IF: 12.712

We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm-1 K-1. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials. © 2017 American Chemical Society.

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Electrochemical behavior of nanostructured La0.8Sr0.2MnO3 as cathodes for solid oxide fuel cells
Sacanell J., Sánchez J.H., Rubio Lopez A.E., Martinelli H., Siepe J., Leyva A.G., Ferrari V.P., Pruneda M., Juan D., Lamas D.G. ECS Transactions; 78 (1): 667 - 675. 2017. 10.1149/07801.0667ecst. IF: 0.000

La0.8Sr0.2MnO3 (LSM) is one of the most commonly used cathodes in Solid Oxide Fuel Cells (SOFC). In spite of the fact that nanostructured cathodes are expected to display improved performance, the high operating temperature (∼ 1000°C) of LSM-based SOFCs hinders their stability. In the present work, we have developed nanostructured cathodes prepared from LSM nanotubes of enhanced performance, allowing its use at lower temperatures (∼ 800°C). We observed that our cathodes have qualitative improvements compared with microstructured materials: firstly, the diffusion in the gas phase is optimized to a negligible level and secondly, evidence of ionic conduction is found, which is extremely rare in LSM cathodes. We propose that this important change in the electrochemical properties is due to the nanostructuration of the cathode and deserves further attention, including the exploration of other materials. © The Electrochemical Society.

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Enhanced Cooperativity in Supported Spin-Crossover Metal-Organic Frameworks
Groizard T., Papior N., Le Guennic B., Robert V., Kepenekian M. Journal of Physical Chemistry Letters; 8 (14): 3415 - 3420. 2017. 10.1021/acs.jpclett.7b01248. IF: 9.353

The impact of surface deposition on cooperativity is explored in Au(111)-supported self-assembled metal-organic frameworks (MOFs) based on Fe(II) ions. Using a thermodynamic model, we first demonstrate that dimensionality reduction combined with deposition on a metal surface is likely to deeply enhance the spin-crossover cooperativity, going from γ3D = 16 K for the bulk material to γ2Dsupp = 386 K for its 2D supported derivative. On the basis of density functional theory, we then elucidate the electronic structure of a promising Fe-based MOF. A chemical strategy is proposed to turn a weakly interacting magnetic system into a strongly cooperative spin-crossover monolayer with γMOFAu(111) = 83 K. These results open a promising route to the fabrication of cooperative materials based on SCO Fe(II) platforms. © 2017 American Chemical Society.

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Field Effect in Graphene-Based van der Waals Heterostructures: Stacking Sequence Matters
Stradi D., Papior N.R., Hansen O., Brandbyge M. Nano Letters; 17 (4): 2660 - 2666. 2017. 10.1021/acs.nanolett.7b00473. IF: 12.712

Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultrathin devices based on 2D atomic crystals. © 2017 American Chemical Society.

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Graphene-based synthetic antiferromagnets and ferrimagnets
Gargiani P., Cuadrado R., Vasili H.B., Pruneda M., Valvidares M. Nature Communications; 8 (1, 699) 2017. 10.1038/s41467-017-00825-9. IF: 12.124

Graphene-spaced magnetic systems with antiferromagnetic exchange-coupling offer exciting opportunities for emerging technologies. Unfortunately, the in-plane graphene-mediated exchange-coupling found so far is not appropriate for realistic exploitation, due to being weak, being of complex nature, or requiring low temperatures. Here we establish that ultra-thin Fe/graphene/Co films grown on Ir(111) exhibit robust perpendicular antiferromagnetic exchange-coupling, and gather a collection of magnetic properties well-suited for applications. Remarkably, the observed exchange coupling is thermally stable above room temperature, strong but field controllable, and occurs in perpendicular orientation with opposite remanent layer magnetizations. Atomistic first-principles simulations provide further ground for the feasibility of graphene-spaced antiferromagnetic coupled structures, confirming graphene's direct role in sustaining antiferromagnetic superexchange-coupling between the magnetic films. These results provide a path for the realization of graphene-based perpendicular synthetic antiferromagnetic systems, which seem exciting for fundamental nanoscience or potential use in spintronic devices. © 2017 The Author(s).

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Growth of Twin-Free and Low-Doped Topological Insulators on BaF2(111)
Bonell F., Cuxart M.G., Song K., Robles R., Ordejón P., Roche S., Mugarza A., Valenzuela S.O. Crystal Growth and Design; 17 (9): 4655 - 4660. 2017. 10.1021/acs.cgd.7b00525. IF: 4.055

We demonstrate the growth of twin-free Bi2Te3 and Sb2Te3 topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi2Se3 on lattice-matched BaF2(111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi2Se3 films are achieved by increasing the substrate roughness, the quality of our Bi2Te3 is superior on the flattest BaF2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab initio calculations. © 2017 American Chemical Society.

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Hydrogen Activation by Frustrated Lewis Pairs Revisited by Metadynamics Simulations
Liu L., Lukose B., Ensing B. Journal of Physical Chemistry C; 121 (4): 2046 - 2051. 2017. 10.1021/acs.jpcc.6b09991. IF: 4.536

Frustrated Lewis pairs have great potential as metal-free catalysts, for example, for the activation of molecular hydrogen. However, rational design of improved catalysts is hampered because the catalytic reaction mechanisms still remain largely unclear. In this study, we present a density-functional-theory-based metadynamics study of the hydrogen activation by a typical frustrated Lewis pair, tBu3P/B(C6F5)3. The computed free-energy landscape reveals a different reaction path compared with the ones in the literature. Importantly, we found different roles of the Lewis acid and base centers in the hydrogen activation. The rate-determining step is the hydride transfer to the Lewis acid, and the overall reaction is found to be exothermic once the proton transfer to the Lewis base step is accomplished. © 2017 American Chemical Society.

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Multiscale modeling of spin transport across a diffuse interface
Chureemart J., Cuadrado R., Chureemart P., Chantrell R.W. Journal of Magnetism and Magnetic Materials; 443: 287 - 292. 2017. 10.1016/j.jmmm.2017.07.085. IF: 2.630

We present multiscale calculations to describe the spin transport behavior of the Co/Cu bilayer structure including the effect of the interface. The multiscale approach introduces the connection between the ab initio calculation used to describe the electronic structure of the system and the generalized spin accumulation model employed to describe the spin transport behavior. We have applied our model to atomically smooth and diffuse interfaces. The results demonstrate the huge importance of the use of first principle calculations, not only due to the interfacial coordinates optimization but also the magnetic and electronic properties obtained through the electronic structure. The system including the effect of interface with and without the charge fluctuation are studied. The results indicate that changes of electronic structure at the Co/Cu interface give rise to an interfacial resistance distributed over several atomic planes, similar to the effect of interface diffusion. We argue that even atomically smooth Co/Cu interfaces have properties analogous to a diffuse interface due to the variation of electronic structure at the interface. © 2017 The Authors

View abstract
Oxygen Reduction Mechanisms in Nanostructured La0.8Sr0.2MnO3 Cathodes for Solid Oxide Fuel Cells
Sacanell J., Hernández Sánchez J., Rubio López A.E., Martinelli H., Siepe J., Leyva A.G., Ferrari V., Juan D., Pruneda M., Mejía Gómez A., Lamas D.G. Journal of Physical Chemistry C; 121 (12): 6533 - 6539. 2017. 10.1021/acs.jpcc.7b00627. IF: 4.536

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: (i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and (ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first-principles calculations point toward an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface. (Figure Presented). © 2017 American Chemical Society.

View abstract
Thermal and transport properties of pristine single-layer hexagonal boron nitride: A first principles investigation
Sergio Illera, Miguel Pruneda, Luciano Colombo, Pablo Ordejón Physical Review Materials; 1 (4): 44006. 2017. 10.1103/PhysRevMaterials.1.044006.

Molecular dynamics is used in combination with density functional theory to determine the thermal transport properties of the single-layer hexagonal boron nitride (SL h-BN) from ab initio calculations. Within this approach, the possible anisotropy in the thermal conductivity of SL h-BN was studied. For samples with finite length (of the order of 20 nm), we find a significant dependence of the conductivity on the transport direction. We make a direct comparison of the results obtained for two-dimensional (2D) layers and for nanoribbons with similar size, and show that, as a consequence of edge scattering, the ribbon geometry induces a significant decrease in the conductivity, and produces a strong change in the anisotropy. For the zigzag and armchair transport directions, the dependence of the thermal conductivity on the system length was also obtained, as well as its value in the 2D bulk limit case. A very small anisotropy was found for the limit of long samples, in contrast with the finite length ones. This is explained analyzing the dependence of the average square group velocities on the transport direction and the phonon frequency.

View abstract
Unconventional Current Scaling and Edge Effects for Charge Transport through Molecular Clusters
Obersteiner V., Huhs G., Papior N., Zojer E. Nano Letters; 17 (12): 7350 - 7357. 2017. 10.1021/acs.nanolett.7b03066. IF: 12.712

Metal-molecule-metal junctions are the key components of molecular electronics circuits. Gaining a microscopic understanding of their conducting properties is central to advancing the field. In the present contribution, we highlight the fundamental differences between single-molecule and ensemble junctions focusing on the fundamentals of transport through molecular clusters. In this way, we elucidate the collective behavior of parallel molecular wires, bridging the gap between single molecule and large-area monolayer electronics, where even in the latter case transport is usually dominated by finite-size islands. On the basis of first-principles charge-transport simulations, we explain why the scaling of the conductivity of a junction has to be distinctly nonlinear in the number of molecules it contains. Moreover, transport through molecular clusters is found to be highly inhomogeneous with pronounced edge effects determined by molecules in locally different electrostatic environments. These effects are most pronounced for comparably small clusters, but electrostatic considerations show that they prevail also for more extended systems. © 2017 American Chemical Society.

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Voltage-Induced Coercivity Reduction in Nanoporous Alloy Films: A Boost toward Energy-Efficient Magnetic Actuation
Quintana A., Zhang J., Isarain-Chávez E., Menéndez E., Cuadrado R., Robles R., Baró M.D., Guerrero M., Pané S., Nelson B.J., Müller C.M., Ordejón P., Nogués J., Pellicer E., Sort J. Advanced Functional Materials; 27 (32, 1701904) 2017. 10.1002/adfm.201701904. IF: 12.124

Magnetic data storage and magnetically actuated devices are conventionally controlled by magnetic fields generated using electric currents. This involves significant power dissipation by Joule heating effect. To optimize energy efficiency, manipulation of magnetic information with lower magnetic fields (i.e., lower electric currents) is desirable. This can be accomplished by reducing the coercivity of the actuated material. Here, a drastic reduction of coercivity is observed at room temperature in thick (≈600 nm), nanoporous, electrodeposited Cu–Ni films by simply subjecting them to the action of an electric field. The effect is due to voltage-induced changes in the magnetic anisotropy. The large surface-area-to-volume ratio and the ultranarrow pore walls of the system allow the whole film, and not only the topmost surface, to effectively contribute to the observed magnetoelectric effect. This waives the stringent “ultrathin-film requirement” from previous studies, where small voltage-driven coercivity variations were reported. This observation expands the already wide range of applications of nanoporous materials (hitherto in areas like energy storage or catalysis) and it opens new paradigms in the fields of spintronics, computation, and magnetic actuation in general. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
What Controls Photocatalytic Water Oxidation on Rutile TiO2(110) under Ultra-High-Vacuum Conditions?
Migani A., Blancafort L. Journal of the American Chemical Society; 139 (34): 11845 - 11856. 2017. 10.1021/jacs.7b05121. IF: 13.858

The photocatalytic O-H dissociation of water absorbed on a rutile TiO2(110) surface in ultrahigh vacuum (UHV) is studied with spin-polarized density functional theory and a hybrid exchange-correlation functional (HSE06), treating the excited-state species as excitons with triplet multiplicity. This system is a model for the photocatalytic oxidation of water by TiO2 in an aqueous medium, which is relevant for the oxygen evolution reaction and photodegradation of organic pollutants. We provide a comprehensive mechanistic picture where the most representative paths correspond to excitonic configurations with the hole located on three- and two-coordinate surface oxygen atoms (O3s and O2s). Our picture explains the formation of the species observed experimentally. At near band gap excitation, the O3s path leads to the generation of hydroxyl anions which diffuse on the surface, without net oxidation. In contrast, free hydroxyl radicals are formed at supra band gap excitation (e.g., 266 nm) from an interfacial exciton that undergoes O-H dissociation. The oxidation efficiency is low because the path associated with the O2s exciton, which is the most favored one thermodynamically, is unreactive because of a high propensity for charge recombination. Our results are also relevant to understand the reactivity in the liquid phase. We assign the photoluminescence measured for atomically flat TiO2(110) surfaces in an aqueous medium to the O3s exciton, in line with the proposal based on experiments, and we have identified a species derived from the O2s exciton with an activated O2s-Ti bond that may be relevant in photocatalytic applications in an aqueous medium. © 2017 American Chemical Society.

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2016

AFM Imaging of Mercaptobenzoic Acid on Au(110): Submolecular Contrast with Metal Tips
Hauptmann N., Robles R., Abufager P., Lorente N., Berndt R. Journal of Physical Chemistry Letters; 7 (11): 1984 - 1990. 2016. 10.1021/acs.jpclett.6b00684. IF: 8.539

A self-assembled monolayer of mercaptobenzoic acid (MBA) on Au(110) is investigated with scanning tunneling and atomic force microscopy (STM and AFM) and density functional calculations. High-resolution AFM images obtained with metallic tips show clear contrasts between oxygen atoms and phenyl moieties. The contrast above the oxygen atoms is due to attractive covalent interactions, which is different than previously reported high-resolution images, where Pauli repulsion dominated the image contrast. We show that the bonding of MBA to the substrate occurs mainly through dispersion interactions, whereas the thiol-Au bond contributes only a quarter of the adsorption energy. No indication of Au adatoms mediating the thiol-Au interaction was found in contrast to other thiol-bonded systems. However, MBA lifts the Au(110)-(2 × 1) reconstruction. © 2016 American Chemical Society.

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Effect of asymmetric concentration profile on thermal conductivity in Ge/SiGe superlattices
Hahn K.R., Cecchi S., Colombo L. Applied Physics Letters; 108 (20, 203102) 2016. 10.1063/1.4949491. IF: 3.142

The effect of the chemical composition in Si/Ge-based superlattices on their thermal conductivity has been investigated using molecular dynamics simulations. Simulation cells of Ge/SiGe superlattices have been generated with different concentration profiles such that the Si concentration follows a step-like, a tooth-saw, a Gaussian, and a gamma-type function in direction of the heat flux. The step-like and tooth-saw profiles mimic ideally sharp interfaces, whereas Gaussian and gamma-type profiles are smooth functions imitating atomic diffusion at the interface as obtained experimentally. Symmetry effects have been investigated comparing the symmetric profiles of the step-like and the Gaussian function to the asymmetric profiles of the tooth-saw and the gamma-type function. At longer sample length and similar degree of interdiffusion, the thermal conductivity is found to be lower in asymmetric profiles. Furthermore, it is found that with smooth concentration profiles where atomic diffusion at the interface takes place the thermal conductivity is higher compared to systems with atomically sharp concentration profiles. © 2016 Author(s).

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Electronic structure of 2H-NbSe2 single-layers in the CDW state
Silva-Guillén J.Á., Ordejón P., Guinea F., Canadell E. 2D Materials; 3 (3, 035028) 2016. 10.1088/2053-1583/3/3/035028. IF: 9.611

Adensity functional theory study of NbSe2"Qsingle-layers in the normal non-modulated and the 3"Q¡Ñ"Q3 CDWstates is reported.Weshow that, in the single layer, the CDW barely affects the Fermi surface of the system, thus ruling out a nesting mechanism as the driving force for the modulation. The CDW stabilizes levels lying around 1.35 eV below the Fermi level within the Se-based valence band but having a substantial Nb-VNb bonding character. The absence of interlayer interactions leads to the suppression of the pancake-like portion of the bulk Fermi surface in the single-layer.Weperform scanning tunneling microscopy simulations and find that the images noticeably change with the sign and magnitude of the voltage bias. The atomic corrugation of the Se sublayer induced by the modulation plays a primary role in leading to these images, but the electronic reorganization also has an important contribution. The analysis of the variation of these images with the bias voltage does not support a Fermi surface nesting mechanism for the CDW. It is also shown that underlying graphene layers (present in some of the recent experimental work) do not modify the conduction band, but do affect the shape of the valence band of NbSe2"Qsingle-layers. The relevance of these results in understanding recent physical measurements for NbSe2"Qsingle-layers is discussed. © 2016 IOP Publishing Ltd.

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Following the steps of a reaction by direct imaging of many individual molecules
Van Vörden D., Wortmann B., Schmidt N., Lange M., Robles R., Brendel L., Bobisch C.A., Möller R. Chemical Communications; 52 (49): 7711 - 7714. 2016. 10.1039/c6cc02959k. IF: 6.567

The dehydrogenation and dechlorination of FeOEP-Cl on Cu(111) has been studied in detail by scanning tunneling microscopy. Although, it is not possible to follow the reaction of an individual molecule, the complete pathway of the reaction with 22 inequivalent intermediate states and the rates of the involved processes are revealed. This is achieved by combining the analysis of a large data set showing thousands of molecules in the different stages of the reaction with numerical simulations. © 2016 The Royal Society of Chemistry.

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Heat transport through a solid-solid junction: The interface as an autonomous thermodynamic system
Rurali R., Colombo L., Cartoixà X., >Øivind W., Trinh T.T., Bedeaux D., Kjelstrup S. Physical Chemistry Chemical Physics; 18 (20): 13741 - 13745. 2016. 10.1039/c6cp01872f. IF: 4.449

We perform computational experiments using nonequilibrium molecular dynamics simulations, showing that the interface between two solid materials can be described as an autonomous thermodynamic system. We verify the local equilibrium and give support to the Gibbs description of the interface also away from the global equilibrium. In doing so, we reconcile the common formulation of the thermal boundary resistance as the ratio between the temperature discontinuity at the interface and the heat flux with a more rigorous derivation from nonequilibrium thermodynamics. We also show that thermal boundary resistance of a junction between two pure solid materials can be regarded as an interface property, depending solely on the interface temperature, as implicitly assumed in some widely used continuum models, such as the acoustic mismatch model. Thermal rectification can be understood on the basis of different interface temperatures for the two flow directions. © The Owner Societies 2016.

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How disorder affects topological surface states in the limit of ultrathin Bi2Se3 films
Song K., Soriano D., Robles R., Ordejon P., Roche S. 2D Materials; 3 (4, 045007) 2016. 10.1088/2053-1583/3/4/045007. IF: 9.611

We present a first-principles study of electronic properties of ultrathin films of topological insulators (TIs) and scrutinize the role of disorder on the robustness of topological surface states, which can be analysed through their spin textures. The presence of twin grain boundaries is found to increase the band gap of the film, while preserving the spin texture of states in first conduction and valence bands. Differently, partial hydrogenation of one surface not only results in some self-doping effect, but also provokes some alteration of the spin texture symmetry of the electronic states. The formation of a new Dirac cone at M-point of the Brillouin zone of the hydrogenated surface, together with a modified spin texture characteristics are consistent with a dominant Dresselhaus spin-orbit interaction type, more usually observed in 3D materials. Our findings indicate that defects can either be detrimental or beneficial for exploring spin transport of surface states in the limit of ultrathin films of TIs, which maximizes surface over bulk phenomena. © 2016 IOP Publishing Ltd.

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In-plane/out-of-plane disorder influence on the magnetic anisotropy of Fe1- yMnyPt-L10 bulk alloy
Cuadrado R., Liu K., Klemmer T.J., Chantrell R.W. Applied Physics Letters; 108 (12, 123102) 2016. 10.1063/1.4944534. IF: 3.142

The random substitution of a non-magnetic species instead of Fe atoms in FePt-L10 bulk alloy will permit to tune the magnetic anisotropy energy of this material. We have performed by means of first principles calculations a study of Fe1- yMnyPt-L10 (y = 0.0, 0.08, 0.12, 0.17, 0.22, and 0.25) bulk alloy for a fixed Pt concentration when the Mn species have ferro-/antiferromagnetic (FM,AFM) alignment at the same(different) atomic plane(s). This substitution will promote several in-plane lattice values for a fixed amount of Mn. Charge hybridization will change compared to the FePt-L10 bulk due to this lattice variation leading to a site resolved magnetic moment modification. We demonstrate that this translates into a total magnetic anisotropy reduction for the AFM phase and an enhancement for the FM alignment. Several geometric configurations were taken into account for a fixed Mn concentration because of different possible Mn positions in the simulation cell. © 2016 AIP Publishing LLC.

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Nanotexturing to Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap
Brotons-Gisbert M., Andres-Penares D., Suh J., Hidalgo F., Abargues R., Rodríguez-Cantó P.J., Segura A., Cros A., Tobias G., Canadell E., Ordejón P., Wu J., Martínez-Pastor J.P., Sánchez-Royo J.F. Nano Letters; 16 (5): 3221 - 3229. 2016. 10.1021/acs.nanolett.6b00689. IF: 13.779

Manipulating properties of matter at the nanoscale is the essence of nanotechnology, which has enabled the realization of quantum dots, nanotubes, metamaterials, and two-dimensional materials with tailored electronic and optical properties. Two-dimensional semiconductors have revealed promising perspectives in nanotechnology. However, the tunability of their physical properties is challenging for semiconductors studied until now. Here we show the ability of morphological manipulation strategies, such as nanotexturing or, at the limit, important surface roughness, to enhance light absorption and the luminescent response of atomically thin indium selenide nanosheets. Besides, quantum-size confinement effects make this two-dimensional semiconductor to exhibit one of the largest band gap tunability ranges observed in a two-dimensional semiconductor: from infrared, in bulk material, to visible wavelengths, at the single layer. These results are relevant for the design of new optoelectronic devices, including heterostructures of two-dimensional materials with optimized band gap functionalities and in-plane heterojunctions with minimal junction defect density. © 2016 American Chemical Society.

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Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface
Chaumeton F., Robles R., Pruneda M., Lorente N., Eydoux B., Bouju X., Gauthier S., Martrou D. Physical Review B; 94 (16, 165305) 2016. 10.1103/PhysRevB.94.165305.

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications. © 2016 American Physical Society.

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On-Surface Engineering of a Magnetic Organometallic Nanowire
Ormaza M., Robles R., Bachellier N., Abufager P., Lorente N., Limot L. Nano Letters; 16 (1): 588 - 593. 2016. 10.1021/acs.nanolett.5b04280. IF: 13.779

The manipulation of the molecular spin state by atom doping is an attractive strategy to confer desirable magnetic properties to molecules. Here, we present the formation of novel magnetic metallocenes by following this approach. In particular, two different on-surface procedures to build isolated and layer-integrated Co-ferrocene (CoFc) molecules on a metallic substrate via atomic manipulation and atom deposition are shown. The structure as well as the electronic properties of the so-formed molecule are investigated combining scanning tunneling microscopy and spectroscopy with density functional theory calculations. It is found that unlike single ferrocene a CoFc molecule possesses a magnetic moment as revealed by the Kondo effect. These results correspond to the first controlled procedure toward the development of tailored metallocene-based nanowires with a desired chemical composition, which are predicted to be promising materials for molecular spintronics. © 2015 American Chemical Society.

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Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO2(110)
Mowbray D.J., Migani A. Journal of Chemical Theory and Computation; 12 (6): 2843 - 2852. 2016. 10.1021/acs.jctc.6b00217. IF: 5.301

Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO2 nanoparticles. Our results suggest deprotonation of catechol's OH anchoring groups, while being nearly isoenergetic at high coverages, shifts the onset of the absorption spectra to lower energies, with a concomitant increase in photovoltaic efficiency. Further, the most relevant bright excitons in the visible region are rather intense charge transfer transitions with the electron and hole spatially separated in both the [110] and [001] directions. Such detailed information on the absorption spectra and excitons is only accessible via periodic models of the combined dye-substrate interface. © 2016 American Chemical Society.

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Reversible 2D Phase Transition Driven by an Electric Field: Visualization and Control on the Atomic Scale
Wortmann B., Vörden D.V., Graf P., Robles R., Abufager P., Lorente N., Bobisch C.A., Möller R. Nano Letters; 16 (1): 528 - 533. 2016. 10.1021/acs.nanolett.5b04174. IF: 13.779

We report on a reversible structural phase transition of a two-dimensional system that can be locally induced by an external electric field. Two different structural configurations may coexist within a CO monolayer on Cu(111). The balance between the two phases can be shifted by an external electric field, causing the domain boundaries to move, increasing the area of the favored phase controllable both in location and size. If the field is further enhanced new domains nucleate. The arrangement of the CO molecules on the Cu surface is observed in real time and real space with atomic resolution while the electric field driving the phase transition is easily varied over a broad range. Together with the well-known molecular manipulation of CO adlayers, our findings open exciting prospects for combining spontaneous long-range order with man-made CO structures such as "molecule cascades" or "molecular graphene". Our new manipulation mode permits us to bridge the gap between fundamental concepts and the fabrication of arbitrary atomic patterns in large scale, by providing unprecedented insight into the physics of structural phase transitions on the atomic scale. © 2015 American Chemical Society.

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Spin-polarised edge states in atomic Mn chains supported on Cu2N/Cu (100)
Choi D.-J., Robles R., Gauyacq J.-P., Rubio-Verdú C., Lorente N., Ignacio Pascual J. Journal of Physics Condensed Matter; 28 (23, 23LT01) 2016. 10.1088/0953-8984/28/23/23LT01. IF: 2.209

Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu2N/Cu (100) reveal an unsuspected electronic edge state at ∼ 1 eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at ∼ -1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences. © 2016 IOP Publishing Ltd.

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Structural and magnetic properties of FeMnx chains (x=1-6) supported on Cu2 N/Cu (100)
Choi D.-J., Robles R., Gauyacq J.-P., Ternes M., Loth S., Lorente N. Physical Review B; 94 (8, 085406) 2016. 10.1103/PhysRevB.94.085406.

Heterogeneous atomic magnetic chains are built by atom manipulation on a Cu2N/Cu (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the Cu2N surface. Here, we present results for FeMnx chains (x=1-6) emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements J for different chains. For the shorter chains, x≤2, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy D and transversal-anisotropy E constants. These parameters together with the values for J allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data. © 2016 American Physical Society.

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Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly
Knaak T., Gopakumar T.G., Schwager B., Tuczek F., Robles R., Lorente N., Berndt R. Journal of the American Chemical Society; 138 (24): 7544 - 7550. 2016. 10.1021/jacs.6b03710. IF: 13.038

Adding ligands to molecules can have drastic and unforeseen consequences in the final products of a reaction. Recently a surface trans effect due to the weakening of a molecule-surface bond was reported. Here, we show a surface cis effect where an axial ligand at adsorbed transition-metal complexes enables lateral bonding among the molecules. In the absence of this ligand, the intermolecular interaction is repulsive and supramolecular patterns are not observed. Fe-tetramethyl-tetraazaannulene on Au(111) was investigated using low-temperature scanning tunneling microscopy and spectroscopy along with density functional theory calculations. At low coverages, the molecules remain isolated. Exposure to CO leads to axial CO bonding and induces reordering into extended clusters of chiral molecular trimers. The changed self-assembly pattern is due to a CO-induced modification of the molecular structure and the corresponding charge transfer between the molecule and the substrate, which in turn changes the lateral intermolecular forces. © 2016 American Chemical Society.

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Theoretical studies of Rashba and Dresselhaus effects in hybrid organic-inorganic perovskites for optoelectronic applications
Pedesseau L., Kepenekian M., Robles R., Sapori D., Katan C., Even J. Proceedings of SPIE - The International Society for Optical Engineering; 9742 ( 97421B) 2016. 10.1117/12.2213618. IF: 0.000

In this paper, we propose a description of the Rashba-Dresselhaus effect in Hybrid Organic Perovskite (HOP). We show how the loss of the inversion symmetry leads to the loss of the spin degeneracy. An example of structure where both Rashba and Dresselhaus operate is illustrated with the formamidinium tin iodide CH(NH2)2SnI3. The control of this effect is as well addressed by two examples. A first example concerns the control with the temperature and is demonstrated for the 2D HOP Bz2PbCl4 (Bz = benzylammonium). Then the control with an external field is established for the 3D HOP CH3NH3PbBr3. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

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Thermal conductivity of MoS2 polycrystalline nanomembranes
Sledzinska M., Graczykowski B., Placidi M., Reig D.S., El Sachat A., Reparaz J.S., Alzina F., Mortazavi B., Quey R., Colombo L., Roche S., Torres C.M.S. 2D Materials; 3 (3, 035016) 2016. 10.1088/2053-1583/3/3/035016. IF: 9.611

Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS2 nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS2 by means of 2-laser Raman thermometry. The free-standing, drum-like MoS2 nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS2 and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m-1 K-1. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed. © 2016 IOP Publishing Ltd.

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Thermal rectification in silicon by a graded distribution of defects
Dettori R., Melis C., Rurali R., Colombo L. Journal of Applied Physics; 119 (21, 215102) 2016. 10.1063/1.4953142. IF: 2.101

We discuss about computer experiments based on nonequilibrium molecular dynamics simulations providing evidence that thermal rectification can be obtained in bulk Si by a non-uniform distribution of defects. We consider a graded population of both Ge substitutional defects and nanovoids, distributed along the direction of an applied thermal bias, and predict a rectification factor comparable to what is observed in other low-dimensional Si-based nanostructures. By considering several defect distribution profiles, thermal bias conditions, and sample sizes, the present results suggest that a possible way for tuning the thermal rectification is by defect engineering. © 2016 Author(s).

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Thermal transport in porous Si nanowires from approach-to-equilibrium molecular dynamics calculations
Cartoixà X., Dettori R., Melis C., Colombo L., Rurali R. Applied Physics Letters; 109 (1, 013107) 2016. 10.1063/1.4955038. IF: 3.142

We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology - such as the porosity and the pore diameter - and on the nanowire (NW) geometry - diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessen's rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term. © 2016 Author(s).

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2015

Assembly of ferrocene molecules on metal surfaces revisited
Ormaza M., Abufager P., Bachellier N., Robles R., Verot M., Le Bahers T., Bocquet M.-L., Lorente N., Limot L. Journal of Physical Chemistry Letters; 6 (3): 395 - 400. 2015. 10.1021/jz5026118. IF: 7.458

Metallocene (MCp2) wires have recently attracted considerable interest in relation to molecular spintronics due to predictions concerning their half-metallic nature. This exciting prospect is however hampered by the little and often-contradictory knowledge we have concerning the metallocene self-assembly and interaction with a metal. Here, we elucidate these aspects by focusing on the adsorption of ferrocene on Cu(111) and Cu(100). Combining low-temperature scanning tunneling microscopy and density functional theory calculations, we demonstrate that the two-dimensional molecular arrangement consists of vertical- and horizontal-lying molecules. The noncovalent T-shaped interactions between Cp rings of vertical and horizontal molecules are essential for the stability of the physisorbed molecular layer. These results provide a fresh insight into ferrocene adsorption on surfaces and may serve as an archetypal reference for future work with this important variety of organometallic molecules. (Figure Presented). © 2015 American Chemical Society.

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Capacitive DNA Detection Driven by Electronic Charge Fluctuations in a Graphene Nanopore
Feliciano G.T., Sanz-Navarro C., Coutinho-Neto M.D., Ordejón P., Scheicher R.H., Rocha A.R. Physical Review Applied; 3 (3, 034003) 2015. 10.1103/PhysRevApplied.3.034003. IF: 0.000

The advent of parallelized automated methods for rapid whole-genome analysis has led to an exponential drop in costs, thus greatly accelerating biomedical research and discovery. Third-generation sequencing techniques, which would utilize the characteristic electrical conductance of the four different nucleotides, could facilitate longer base read lengths and an even lower price per genome. In this work, we propose and apply a quantum-classical hybrid methodology to quantitatively determine the influence of the solvent on the dynamics of DNA and the resulting electron transport properties of a prototypic sequencing device utilizing a graphene nanopore through which the nucleic-acid chain is threaded. Our results show that charge fluctuations in the nucleotides are responsible for characteristic conductance modulations in this system, which can be regarded as a field-effect transistor tuned by the dynamic aqueous environment. © 2015 American Physical Society.

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Comparing Quasiparticle H2O Level Alignment on Anatase and Rutile TiO2
Sun H., Mowbray D.J., Migani A., Zhao J., Petek H., Rubio A. ACS Catalysis; 5 (7): 4242 - 4254. 2015. 10.1021/acscatal.5b00529. IF: 9.312

Knowledge of the alignment of molecular frontier levels in the ground state can be used to predict the photocatalytic activity of an interface. The position of the adsorbate's highest occupied molecular orbital (HOMO) levels relative to the substrate's valence band maximum (VBM) in the interface describes the favorability of photogenerated hole transfer from the VBM to the adsorbed molecule. This is a key quantity for assessing and comparing H2O photooxidation activities on two prototypical photocatalytic TiO2 surfaces: anatase (A)-TiO2(101) and rutile (R)-TiO2(110). Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) G0W0 calculations, we assess the relative photocatalytic activity of intact and dissociated H2O on coordinately unsaturated (Ticus) sites of idealized stoichiometric A-TiO2(101)/R-TiO2(110) and bridging O vacancies (Obrvac) of defective A-TiO2-x(101)/R-TiO2-x(110) surfaces (x = 1/4, 1/8) for various coverages. Such a many-body treatment is necessary to correctly describe the anisotropic screening of electron-electron interactions at a photocatalytic interface and, hence, obtain accurate interfacial level alignments. The more favorable ground state HOMO level alignment for A-TiO2(101) may explain why the anatase polymorph shows higher photocatalytic activities than the rutile polymorph. Our results indicate that (1) hole trapping is more favored on A-TiO2(101) than R-TiO2(110) and (2) HO@Ticus is more photocatalytically active than intact H2O@Ticus. © 2015 American Chemical Society.

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Electromechanical response at polar zigzag boundaries in hybrid monolayers
Martinez-Gordillo R., Pruneda M. Physical Review B - Condensed Matter and Materials Physics; 91 (4, 045411) 2015. 10.1103/PhysRevB.91.045411. IF: 3.736

First-principles calculations are used to demonstrate electromechanical control of charge and spin at zigzag-edged interfaces between graphene and boron-nitride domains in hybrid monolayers. We show how, through a direct piezoelectric effect, the interfacial bound charges and associated electric fields can be tuned by application of an external mechanical force (stress) on the system. This results in mechanical control of the edge magnetization (piezomagnetic effect), and the possibility to transform a semiconducting heterostructure into a half-metal. The inverse effect (application of an external electric field to induce a mechanical deformation) goes together with a magnetoelectric response, which under ideal conditions we estimate to be comparable to that of prototypical Cr2O3. These effects originate from the magnetic properties of graphene's zigzag edges and the dielectric properties of the boron-nitride domain, and can also be expected in any other coplanar heterostructures with polar discontinuities. © 2015 American Physical Society.

View abstract
FeCoCp3 Molecular Magnets as Spin Filters
Abufager P.N., Robles R., Lorente N. Journal of Physical Chemistry C; 119 (22): 12119 - 12129. 2015. 10.1021/acs.jpcc.5b01839. IF: 4.772

Metallorganic molecules have been proposed as excellent spin filters in molecular spintronics because of the large spin polarization of their electronic structure. However, most of the studies involving spin transport have disregarded fundamental aspects such as the magnetic anisotropy of the molecule and the excitation of spin-flip processes during electron transport. Here, we study a molecule containing a Co and an Fe atom stacked between three cyclopentadienyl rings, which presents a large magnetic anisotropy and a S = 1. These figures are superior to other molecules with the same transition metal and improves the spin-filtering capacities of the molecule. Nonequilibrium Green's functions calculations based on density functional theory predict excellent spin-filtering properties both in tunnel and contact transport regimes. However, exciting the first magnetic state drastically reduces the current's spin polarization. Furthermore, a difference of temperature between electrodes leads to strong thermoelectric effects that also suppress spin polarization. Our study shows that in principle good molecular candidates for spintronics need to be confronted with inelastic and thermoelectric effects. (Graph Presented). © 2015 American Chemical Society.

View abstract
Polar discontinuities and 1D interfaces in monolayered materials
Martinez-Gordillo R., Pruneda M. Progress in Surface Science; 90 (4): 444 - 463. 2015. 10.1016/j.progsurf.2015.08.001. IF: 5.696

Interfaces are the birthplace of a multitude of fascinating discoveries in fundamental science, and have enabled modern electronic devices, from transistors, to lasers, capacitors or solar cells. These interfaces between bulk materials are always bi-dimensional (2D) 'surfaces'. However the advent of graphene and other 2D crystals opened up a world of possibilities, as in this case the interfaces become one-dimensional (1D) lines. Although the properties of 1D nanoribbons have been extensively discussed in the last few years, 1D interfaces within infinite 2D systems had remained mostly unexplored until very recently. These include grain boundaries in polycrystalline samples, or interfaces in hybrid 2D sheets composed by segregated domains of different materials (as for example graphene/BN hybrids, or chemically different transition metal dichalcogenides). As for their 2D counterparts, some of these 1D interfaces exhibit polar characteristics, and can give rise to fascinating new physical properties. Here, recent experimental discoveries and theoretical predictions on the polar discontinuities that arise at these 1D interfaces will be reviewed, and the perspectives of this new research topic, discussed. © 2015 Elsevier Ltd. All rights reserved.

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Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2O on TiO2(110)
Migani A., Mowbray D.J., Zhao J., Petek H. Journal of Chemical Theory and Computation; 11 (1): 239 - 251. 2015. 10.1021/ct500779s. IF: 5.498

Knowledge of the frontier levels' alignment prior to photoirradiation is necessary to achieve a complete quantitative description of H2O photocatalysis on TiO2(110). Although H2O on rutile TiO2(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H2O occupied levels is still lacking. For experiment, this is due to the H2O levels being obscured by hybridization with TiO2(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H2O-TiO2(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) G0W0, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H2O on TiO2(110). We perform this separation as a function of H2O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO2(110) surface, the H2O 3a1 and 1b1 levels are broadened into several peaks between 5 and 1 eV below the TiO2(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H2O adsorbed intact and dissociated on stoichiometric TiO2(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H2O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QPGW (scQPGW1) to obtain the ionization potential of the H2O-TiO2(110) interface. (Figure Presented). © 2014 American Chemical Society.

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Quasiparticle spectra of 2H-NbSe2: Two-band superconductivity and the role of tunneling selectivity
Noat Y., Silva-Guillén J.A., Cren T., Cherkez V., Brun C., Pons S., Debontridder F., Roditchev D., Sacks W., Cario L., Ordejón P., García A., Canadell E. Physical Review B - Condensed Matter and Materials Physics; 92 (13, 134510) 2015. 10.1103/PhysRevB.92.134510. IF: 3.736

We have studied the superconducting state of 2H-NbSe2 by scanning tunneling spectroscopy along two different crystal orientations, the c and the a/b axes. Along the c axis a large gap is dominant in the spectra, while a smaller gap is measured along the a/b axis. We show that these spectra are accurately described by the McMillan model where the small gap is induced through the coupling to the band associated with the large gap. In order to assign the small and large gaps to specific parts of the 2H-NbSe2 Fermi surface, the electronic structure was studied using first-principles calculations. While we cannot exclude the possibility of intrinsic anisotropy of the gaps, we propose that the large gap opens in the Fermi surface cylinders located around the corner K points while the sheets located around Γ are associated with the small gap. An additional component of the Fermi surface, a selenium based pocket, plays an essential role in the tunneling process. The role of the charge density wave occurring in this material is also discussed. Finally, we are able to give a coherent description of the observed characteristics of the tunneling spectra of 2H-NbSe2 as well as the differences with 2H-NbS2 where no charge density wave state is present. Further experimental work, such as high-resolution ARPES, would be very useful to confirm our interpretation. The approach and modeling developed here could also be relevant for other compounds of the dichalcogenide family. © 2015 American Physical Society.

View abstract
Rashba and Dresselhaus Effects in Hybrid Organic-Inorganic Perovskites: From Basics to Devices
Kepenekian M., Robles R., Katan C., Sapori D., Pedesseau L., Even J. ACS Nano; 9 (12): 11557 - 11567. 2015. 10.1021/acsnano.5b04409. IF: 12.881

We use symmetry analysis, density functional theory calculations, and k·p modeling to scrutinize Rashba and Dresselhaus effects in hybrid organic-inorganic halide perovskites. These perovskites are at the center of a recent revolution in the field of photovoltaics but have also demonstrated potential for optoelectronic applications such as transistors and light emitters. Due to a large spin-orbit coupling of the most frequently used metals, they are also predicted to offer a promising avenue for spin-based applications. With an in-depth inspection of the electronic structures and bulk lattice symmetries of a variety of systems, we analyze the origin of the spin splitting in two- and three-dimensional hybrid perovskites. It is shown that low-dimensional nanostructures made of CH3NH3PbX3 (X = I, Br) lead to spin splittings that can be controlled by an applied electric field. These findings further open the door for a perovskite-based spintronics. © 2015 American Chemical Society.

View abstract
Tunneling and electronic structure of the two-gap superconductor MgB2
Silva-Guillén J.A., Noat Y., Cren T., Sacks W., Canadell E., Ordejón P. Physical Review B - Condensed Matter and Materials Physics; 92 (6, 064514) 2015. 10.1103/PhysRevB.92.064514. IF: 3.736

A combined experimental (superconductor-insulator-superconductor tunneling spectra) and theoretical (density functional theory) study of the two-gap superconductor MgB2 is reported. The calculations confirm that the small gap is associated with a π band mostly based on the boron pz orbitals leading to the three-dimensional band component of the Fermi surface. This channel almost completely dominates the tunneling images and spectra for c-axis-oriented samples and not the two-dimensional σ band. The origin of this effect is due to the faster decay of the electronic states associated with the boron px and py orbitals compared to those associated with the boron pz orbitals, together with the symmetry properties of the wave functions. The calculated tunneling channels and partial density of states for each band agree with the values deduced from precise fits of experimental tunneling spectra. The present approach provides a framework for the understanding of tunneling spectra and the nature of superconducting gaps of other multigap superconductors. © 2015 American Physical Society.

View abstract
Using G0W0 Level Alignment to Identify Catechol's Structure on TiO2(110)
Mowbray D.J., Migani A. Journal of Physical Chemistry C; 119 (34): 19634 - 19641. 2015. 10.1021/acs.jpcc.5b05392. IF: 4.772

We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol on rutile TiO2(110). Catechol is often used as an anchoring group for larger more complex organic and inorganic dyes on TiO2 and forms type II heterojunctions on TiO2(110). In particular, we compare quasiparticle (QP) G0W0 with hybrid exchange correlation functional (HSE) density functional theory (DFT) calculations for the catechol-rutile TiO2(110) interface. In so doing, we provide a theoretical interpretation of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) experiments for this prototypical system. Specifically, we demonstrate that the position, presence, and intensity of peaks associated with catechol's HOMO, intermolecular OH-O bonds, and interfacial hydrogen bonds to the surface bridging O atoms (ObrH-C and ObrH-O) may be used to fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our results suggest deprotonation of these groups, while being nearly isoenergetic at high coverages, may significantly increase the photovoltaic efficiency of catechol-TiO2(110) interfaces. (Figure Presented). © 2015 American Chemical Society.

View abstract

2014

Classical limit of a quantal nano-magnet in an anisotropic environment
Gauyacq, J.P.; Lorente, N. Surface Science; 630: 325 - 330. 2014. 10.1016/j.susc.2014.07.013. IF: 1.870
Coverage dependence of the level alignment for methanol on TiO2(110)
Migani, A.; Mowbray, D.J. Computational and Theoretical Chemistry; 1040-1041: 259 - 265. 2014. 10.1016/j.comptc.2014.03.007. IF: 1.368
Difficulties in the ab initio description of electron transport through spin filters
Kepenekian, M.; Gauyacq, J.-P.; Lorente, N. Journal of Physics Condensed Matter; 2014. 10.1088/0953-8984/26/10/104203. IF: 2.223
Electron transport signature of H2 dissociation on atomic gold wires
Zanchet, A.; Roncero, O.; Dorta-Urra, A.; Aguado, A.; Martínez, J.I.; Flores, F.; Lorente, N. Physical Review B; 90 (4) 2014. 10.1103/PhysRevB.90.041404.
Electronic properties of single-layer and multilayer transition metal dichalcogenides MX2 (M = Mo, W and X = S, Se)
Roldán R., Silva-Guillén J.A., López-Sancho M.P., Guinea F., Cappelluti E., Ordejón P. Annalen der Physik; 526 (9-10): 347 - 357. 2014. 10.1002/andp.201400128.

Single- and few-layer transition metal dichalcogenides have recently emerged as a new family of layered crystals with great interest, not only from the fundamental point of view, but also because of their potential application in ultrathin devices. Here the electronic properties of semiconducting MX2, where M = Mo or W and X = S or Se, are reviewed. Based on of density functional theory calculations, which include the effect of spin-orbit interaction, the band structure of single-layer, bilayer and bulk compounds is discussed. The band structure of these compounds is highly sensitive to elastic deformations, and it is reviewed how strain engineering can be used to manipulate and tune the electronic and optical properties of those materials. Further, the effect of disorder and imperfections in the lattice structure and their effect on the optical and transport properties of MX2 is discussed. The superconducting transition in these compounds, which has been observed experimentally, is analyzed, as well as the different mechanisms proposed so far to explain the pairing. Finally, a discussion on the excitonic effects which are present in these systems is included. © 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

View abstract
Excitation of bond-alternating spin-1/2 Heisenberg chains by tunnelling electrons
Gauyacq, J.P.; Lorente, N. Journal of Physics Condensed Matter; 2014. 10.1088/0953-8984/26/39/394005. IF: 2.223
Momentum dependence of spin-orbit interaction effects in single-layer and multi-layer transition metal dichalcogenides
Roldán R., López-Sancho M.P., Guinea F., Cappelluti E., Silva-Guillén J.A., Ordejón P. 2D Materials; 1 (3, 034003) 2014. 10.1088/2053-1583/1/3/034003.

One of the main characteristics of the new family of two-dimensional crystals of semiconducting transition metal dichalcogenides (TMDs) is the strong spin-orbit interaction, which makes them very promising for future applications in spintronics and valleytronics devices. Here we present a detailed study of the effect of spin-orbit coupling (SOC) on the band structure of single-layer and bulk TMDs, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms. To this aim, we combine density functional theory (DFT) calculations with a Slater-Koster tight-binding (TB) model. Whereas most of the previous TB models have been restricted to the K and K' points of the Brillouin zone (BZ), here we consider the effect of SOC in the whole BZ, and the results are compared to the band structure obtained by DFT methods. The TB model is used to analyze the effect of SOC in the band structure, considering separately the contributions from the transition metal and the chalcogen atoms. Finally, we present a scenario where, in the case of strong SOC, the spin/orbital/valley entanglement at the minimum of the conduction band at Q can be probed and be of experimental interest in the most common cases of electron-doping reported for this family of compounds. © 2014 IOP Publishing Ltd.

View abstract
Orbital redistribution in molecular nanostructures mediated by metal-organic bonds
Yang, Z.; Corso, M.; Robles, R.; Lotze, C.; Fitzner, R.; Mena-Osteritz, E.; Bäuerle, P.; Franke, K.J.; Pascual, J.I. ACS Nano; 8 (10): 10715 - 10722. 2014. 10.1021/nn504431e. IF: 12.033
Oxygen vacancies in self-assemblies of ceria nanoparticles
Sk, M.A.; Kozlov, S.M.; Lim, K.H.; Migani, A.; Neyman, K.M. Journal of Materials Chemistry A; 2 (43): 18329 - 18338. 2014. 10.1039/c4ta02200a. IF: 0.000
Piezoelectric monolayers as nonlinear energy harvesters
López-Suárez, M.; Pruneda, M.; Abadal, G.; Rurali, R. Nanotechnology; 2014. 10.1088/0957-4484/25/17/175401. IF: 3.672
Quasiparticle level alignment for photocatalytic interfaces
Migani, A.; Mowbray, D.J.; Zhao, J.; Petek, H.; Rubio, A. Journal of Chemical Theory and Computation; 10 (5): 2103 - 2113. 2014. 10.1021/ct500087v. IF: 5.310
Spin transport in dangling-bond wires on doped H-passivated Si(100)
Kepenekian, M.; Robles, R.; Rurali, R.; Lorente, N. Nanotechnology; 2014. 10.1088/0957-4484/25/46/465703. IF: 3.672
Theoretical study on the influence of the Mg2+ and Al3+ octahedral cations on the vibrational spectra of the hydroxy groups of dioctahedral 2:1 phyllosilicate models
Hernández-Haro, N.; Ortega-Castro, J.,; Pruneda, M.,; Sainz-Díaz, C.I.,; Hernández-Laguna, A. Journal of Molecular Modeling; 20 (9): 2402. 2014. 10.1007/s00894-014-2402-6. IF: 1.867
Transport fingerprints at graphene superlattice Dirac points induced by a boron nitride substrate
Martinez-Gordillo, R.; Roche, S.; Ortmann, F.; Pruneda, M. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.89.161401. IF: 3.664

2013

Adsorption site determination of a molecular monolayer via inelastic tunneling
Wegner, D.; Yamachika, R.; Zhang, X.; Wang, Y.; Crommie, M.F.; Lorente, N. Nano Letters; 13: 2346 - 2350. 2013. 10.1021/nl304081q. IF: 13.025
Controlled manipulation of single atoms and small molecules using the scanning tunnelling microscope
Morgenstern, K.; Lorente, N.; Rieder, K.-H. Physica Status Solidi (B): Basic Research; 250: 1671 - 1751. 2013. 10.1002/pssb.201248392. IF: 1.489
Correlation-mediated processes for electron-induced switching between Néel states of Fe antiferromagnetic chains
Gauyacq, J.-P.; Yaro, S.M.; Cartoixà, X.; Lorente, N. Physical Review Letters; 110 2013. 10.1103/PhysRevLett.110.087201. IF: 7.943
Electron transport through dangling-bond silicon wires on H-passivated Si(100)
Kepenekian, M.; Novaes, F.D.; Robles, R.; Monturet, S.; Kawai, H.; Joachim, C.; Lorente, N. Journal of Physics Condensed Matter; 25 2013. 10.1088/0953-8984/25/2/025503. IF: 2.355
Gold and methane: A noble combination for delicate oxidation
Mowbray, D.J.; Migani, A.; Walther, G.; Cardamone, D.M.; Rubio, A. Journal of Physical Chemistry Letters; 4: 3006 - 3012. 2013. 10.1021/jz401553p. IF: 6.585
Imaging the dynamics of individually adsorbed molecules
Schaffert, J.; Cottin, M.C.; Sonntag, A.; Karacuban, H.; Bobisch, C.A.; Lorente, N.; Gauyacq, J.-P.; Möller, R. Nature Materials; 12: 223 - 227. 2013. 10.1038/nmat3527. IF: 35.749
Impact of vacancies on diffusive and pseudodiffusive electronic transport in graphene
Cresti, A.; Louvet, T.; Ortmann, F.; Van Tuan, D.; Lenarczyk, P.; Huhs, G.; Roche, S. Crystals; 3 (2): 289 - 305. 2013. 10.3390/cryst3020289. IF: 0.000
Leakage current in atomic-size surface interconnects
Kepenekian, M.; Robles, R.; Joachim, C.; Lorente, N. Applied Physics Letters; 103 2013. 10.1063/1.4825375. IF: 3.794
Level alignment of a prototypical photocatalytic system: Methanol on TiO2(110)
Migani, A.; Mowbray, D.J.; Iacomino, A.; Zhao, J.; Petek, H.; Rubio, A. Journal of the American Chemical Society; 135: 11429 - 11432. 2013. 10.1021/ja4036994. IF: 10.677
Magnetic reversal of a quantum nanoferromagnet
Gauyacq, J.P.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 87 2013. 10.1103/PhysRevB.87.195402. IF: 3.767
Manganese 3×3 and √3×√3-R30 â̂̃ structures and structural phase transition on w-GaN(0001̄) studied by scanning tunneling microscopy and first-principles theory
Chinchore A.V., Wang K., Shi M., Mandru A., Liu Y., Haider M., Smith A.R., Ferrari V., Barral M.A., Ordejón P. Physical Review B - Condensed Matter and Materials Physics; 87 (16, 165426) 2013. 10.1103/PhysRevB.87.165426.

Manganese deposited on the N-polar face of wurtzite gallium nitride [GaN (0001̄)] results in two unique surface reconstructions, depending on the deposition temperature. At lower temperature (less than 105 â̂̃C), it is found that a metastable 3×3 structure forms. Mild annealing of this Mn 3×3 structure leads to an irreversible phase transition to a different, much more stable √3×√3-R30â̂̃ structure which can withstand high-temperature annealing. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction data are compared with results from first-principles theoretical calculations. Theory finds a lowest-energy model for the 3×3 structure consisting of Mn trimers bonded to the Ga adlayer atoms but not with N atoms. The lowest-energy model for the more stable √3×√3-R30â̂̃ structure involves Mn atoms substituting for Ga within the Ga adlayer and thus bonding with N atoms. Tersoff-Hamman simulations of the resulting lowest-energy structural models are found to be in very good agreement with the experimental STM images. © 2013 American Physical Society.

View abstract
Modelling the growth of ZnO thin films by PVD methods and the effects of post-annealing
Blackwell, S.; Smith, R.; Kenny, S.D.; Walls, J.M.; Sanz-Navarro, C.F. Journal of Physics Condensed Matter; 25 2013. 10.1088/0953-8984/25/13/135002. IF: 2.355
Nitrogen-doped graphitic nanoribbons: Synthesis, characterization, and transport
Ortiz-Medina, J.; García-Betancourt, M.L.; Jia, X.; Martínez-Gordillo, R.; Pelagio-Flores, M.A.; Swanson, D.; Elías, A.L.; Gutiérrez, H.R.; Gracia-Espino, E.; Meunier, V.; Owens, J.; Sumpter, B.G.; Cruz-Silva, E.; Rodríguez-Macías, F.J.; Lõpez-Urías, F.; Muñoz-Sandoval, E.; Dresselhaus, M.S.; Terrones, H.; Terrones, M. Advanced Functional Materials; 23: 3755 - 3762. 2013. 10.1002/adfm.201202947. IF: 9.765
Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines
Krull, C.; Robles, R.; Mugarza, A.; Gambardella, P. Nature Materials; 12: 337 - 343. 2013. 10.1038/nmat3547. IF: 35.749
Surface-induced dechlorination of FeOEP-Cl** on Cu(111)
Van Vörden, D.; Lange, M.; Schaffert, J.; Cottin, M.C.; Schmuck, M.; Robles, R.; Wende, H.; Bobisch, C.A.; Möller, R. Chemphyschem : a European journal of chemical physics and physical chemistry; 14: 3472 - 3475. 2013. 10.1002/cphc.201300497. IF: 3.349
Surface-state engineering for interconnects on H-passivated Si(100)
Kepenekian, M.; Robles, R.; Joachim, C.; Lorente, N. Nano Letters; 13: 1192 - 1195. 2013. 10.1021/nl304611m. IF: 13.025
Surface-supported supramolecular pentamers
Karan, S.; Wang, Y.; Robles, R.; Lorente, N.; Berndt, R. Journal of the American Chemical Society; 135: 14004 - 14007. 2013. 10.1021/ja405456k. IF: 10.677
Tight-binding model and direct-gap/indirect-gap transition in single-layer and multilayer MoS2
Cappelluti E., Roldán R., Silva-Guillén J.A., Ordejón P., Guinea F. Physical Review B - Condensed Matter and Materials Physics; 88 (7, 075409) 2013. 10.1103/PhysRevB.88.075409.

In this paper we present a paradigmatic tight-binding model for single-layer as well as multilayered semiconducting MoS2 and similar transition metal dichalcogenides. We show that the electronic properties of multilayer systems can be reproduced in terms of a tight-binding modeling of the single-layer hopping terms by simply adding the proper interlayer hoppings ruled by the chalcogenide atoms. We show that such a tight-binding model makes it possible to understand and control in a natural way the transition between a direct-gap band structure, in single-layer systems, and an indirect gap in multilayer compounds in terms of a momentum/orbital selective interlayer splitting of the relevant valence and conduction bands. The model represents also a suitable playground to investigate in an analytical way strain and finite-size effects. © 2013 American Physical Society.

View abstract
Tunneling electron induced rotation of a copper phthalocyanine molecule on Cu(111)
Schaffert, J.; Cottin, M.C.; Sonntag, A.; Bobisch, C.A.; Möller, R.; Gauyacq, J.-P.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 88 2013. 10.1103/PhysRevB.88.075410. IF: 3.767
Vibrational transition rule during a through-bond electron transfer process
Monturet, S.; Kepenekian, M.; Robles, R.; Lorente, N.; Joachim, C. Chemical Physics Letters; 567: 1 - 5. 2013. 10.1016/j.cplett.2013.02.057. IF: 2.145

2012

A theoretical rationalization of a total inelastic electron tunneling spectrum: The comparative cases of formate and benzoate on Cu(111)
Burema, S.R.; Lorente, N.; Bocquet, M.-L. Journal of Chemical Physics; 136 2012. 10.1063/1.4730168.
Calculation of core level shifts within DFT using pseudopotentials and localized basis sets
García-Gil S., García A., Ordejón P. European Physical Journal B; 85 (7, 239) 2012. 10.1140/epjb/e2012-30334-5.

The calculation of core level shifts can be done in the context of density functional theory (DFT) using different approaches and physical approximations to the photoemission process. The initial state and the SCF approximations are the most commonly used ones. Here, we describe the details of their implementation in the context of DFT using pseudopotentials and localized atomic orbitals as a basis set, and in particular as applied to the Siesta code. We give a full account of the technicalities involved in these calculations, including the details of the ionic pseudopotential generation, basis sets, charge states and reference potential. We test the method by computing the core level shifts of the Si 2p level for a series of molecules and the p(2×2) asymmetric-dimer reconstruction of the Si(001) surface. © EDP Sciences, Societá Italiana di Fisica, Springer-Verlag 2012.

View abstract
Carbon nanotubes as substrates for molecular spiropyran-based switches
Malic, E.; Setaro, A.; Bluemmel, P.; Sanz-Navarro, C.F.; Ordejón, P.; Reich, S.; Knorr, A. Journal of Physics Condensed Matter; 24: 394006. 2012. .
Dielectric screening in extended systems using the self-consistent Sternheimer equation and localized basis sets
Hübener H., Pérez-Osorio M.A., Ordejón P., Giustino F. Physical Review B - Condensed Matter and Materials Physics; 85 (24, 245125) 2012. 10.1103/PhysRevB.85.245125.

We develop a first-principles computational method for investigating the dielectric screening in extended systems using the self-consistent Sternheimer equation and localized nonorthogonal basis sets. Our approach does not require the explicit calculation of unoccupied electronic states, uses only two-center integrals, and has a theoretical scaling of order O(N3). We demonstrate this method by comparing our calculations for silicon, germanium, diamond, and LiCl with reference plane-wave calculations. We show that accuracy comparable to that of plane-wave calculations can be achieved via a systematic optimization of the basis set. © 2012 American Physical Society.

View abstract
Electrochemical ferroelectric switching: Origin of polarization reversal in ultrathin films
Bristowe, N.C.; Stengel, M.; Littlewood, P.B.; Pruneda, J.M.; Artacho, E. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.024106.
Electronic and magnetic properties of molecule-metal interfaces: Transition-metal phthalocyanines adsorbed on Ag(100)
Mugarza, A.; Robles, R.; Krull, C.; Korytár, R.; Lorente, N.; Gambardella, P. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.155437.
Electroresistance effect in ferroelectric tunnel junctions with symmetric electrodes
Bilc D.I., Novaes F.D., Íñiguez J., Ordejón P., Ghosez P. ACS Nano; 6 (2): 1473 - 1478. 2012. 10.1021/nn2043324.

Understanding the effects that govern electronic transport in ferroelectric tunnel junctions (FTJs) is of vital importance to improve the efficiency of devices such as ferroelectric memories with nondestructive readout. However, our current knowledge (typically based on simple semiempirical models or first-principles calculations restricted to the limit of zero bias) remains partial, which may hinder the development of more efficient systems. For example, nowadays it is commonly believed that the tunnel electroresistance (TER) effect exploited in such devices mandatorily requires, to be sizable, the use of two different electrodes, with related potential drawbacks concerning retention time, switching, and polarization imprint. In contrast, here we demonstrate at the first-principles level that large TER values of about 200% can be achieved under finite bias in a prototypical FTJ with symmetric electrodes. Our atomistic approach allows us to quantify the contribution of different microscopic mechanisms to the electroresistance, revealing the dominant role of the inverse piezoelectric response of the ferroelectric. On the basis of our analysis, we provide a critical discussion of the semiempirical models traditionally used to describe FTJs. © 2012 American Chemical Society.

View abstract
Energetics and stability of dangling-bond silicon wires on H passivated Si(100)
Robles, R.; Kepenekian, M.; Monturet, S.; Joachim, C.; Lorente, N. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/44/445004.
Excitation of local magnetic moments by tunneling electrons
Gauyacq, J.-P.; Lorente, N.; Novaes, F.D. Progress in Surface Science; 87: 63 - 107. 2012. 10.1016/j.progsurf.2012.05.003.
Insulating behavior of an amorphous graphene membrane
Van Tuan D., Kumar A., Roche S., Ortmann F., Thorpe M.F., Ordejon P. Physical Review B - Condensed Matter and Materials Physics; 86 (12, 121408) 2012. 10.1103/PhysRevB.86.121408.

We investigate the charge transport properties of planar amorphous graphene that is fully topologically disordered, in the form of sp2 threefold coordinated networks consisting of hexagonal rings but also including many pentagons and heptagons distributed in a random fashion. Using the Kubo transport methodology and the Lanczos method, the density of states, mean free paths, and semiclassical conductivities of such amorphous graphene membranes are computed. Despite a large increase in the density of states close to the charge neutrality point, all electronic properties are dramatically degraded, evidencing an Anderson insulating state caused by topological disorder alone. These results are supported by Landauer-Büttiker conductance calculations, which show a localization length as short as 5 nm. © 2012 American Physical Society.

View abstract
Lifetime of magnetic excitations in supported ferromagnetic and antiferromagnetic spin-12 Heisenberg chains
Gauyacq, J.P.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.115420.
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Korytár, R.; Lorente, N.; Gauyacq, J.-P. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.125434.
Native defects in hybrid C/BN nanostructures by density functional theory calculations
Pruneda, J.M. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.045422.
Performance of local orbital basis sets in the self-consistent Sternheimer method for dielectric matrices of extended systems
Hubene, H.; Perez-Osorio, M.A.; Ordejon, P.; Giustino, F. European Physical Journal B; 85: 321. 2012. .
Spin doping of individual molecules by using single-atom manipulation
Robles, R.; Lorente, N.; Isshiki, H.; Liu, J.; Katoh, K.; Breedlove, B.K.; Yamashita, M.; Komeda, T. Nano Letters; 12: 3609 - 3612. 2012. 10.1021/nl301301e.
Y:BaZrO 3 perovskite compounds I: DFT study on the unprotonated and protonated local structures
Cammarata A., Ordejón P., Emanuele A., Duca D. Chemistry - An Asian Journal; 7 (8): 1827 - 1837. 2012. 10.1002/asia.201100974.

Y-doped BaZrO 3 derivatives were studied by density functional theory (DFT) to investigate the local arrangements of the octahedral sites in Pm${\bar 3}$m cubic frameworks. Single- and double substitution of zirconium by yttrium were considered, including in the presence of a nearby oxygen vacancy. Although the structural symmetry of undoped barium zirconate was not modified after yttrium doping, the presence of yttrium induced several differences in the oxygen sites around it, according to the local geometrical arrangement of yttrium in the host matrix. As an example, the differences between such oxygen sites were shown in the presence of a proton. In this case, different stabilization energies characterized the protonated fragments. Only in those structures, in which two yttrium atoms were neighbors (i.e., formed Y-O-Y moieties), were the relative energy differences between the corresponding proton stable sites in agreement with the order of magnitude of the experimental proton-hopping activation energies. The distribution of such energy differences suggested a grouping of the oxygen atoms into three sets, which had peculiar structural features that weren't easily deducible from their topologies. The existence of proton traps was also discussed on the basis of the energy-difference distributions. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

View abstract

2011

An efficient implementation of a QM-MM method in SIESTA
Sanz-Navarro C.F., Grima R., García A., Bea E.A., Soba A., Cela J.M., Ordejón P. Theoretical Chemistry Accounts; 128 (4): 825 - 833. 2011. 10.1007/s00214-010-0816-5.

We present the major features of a new implementation of a QM-MM method that uses the DFT code Siesta to treat the quantum mechanical subsystem and the AMBER force field to deal with the classical part. The computation of the electrostatic interaction has been completely revamped to treat periodic boundary conditions exactly, using a real-space grid that encompasses the whole system. Additionally, we present a new parallelization of the Siesta grid operations that provides near-perfect load balancing for all the relevant operations and achieves a much better scalability, which is important for efficient massive QM-MM calculations in which the grid can potentially be very large. © 2010 Springer-Verlag.

View abstract
Cis-dicarbonyl binding at cobalt and iron porphyrins with saddle-shape conformation
Seufert, K.; Bocquet, M.-L.; Auwärter, W.; Weber-Bargioni, A.; Reichert, J.; Lorente, N.; Barth, J.V. Nature Chemistry; 3: 114 - 119. 2011. 10.1038/nchem.956.
Excitation of spin waves by tunneling electrons in ferromagnetic and antiferromagnetic spin-12 Heisenberg chains
Gauyacq, J.P.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.035418.
Magnetic excitation by tunneling electrons of frustrated ferromagnetic spin-12 chains and rings
Gauyacq, J.P.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 84 2011. 10.1103/PhysRevB.84.085415.
Magnetism-dependent transport phenomena in hydrogenated graphene: From spin-splitting to localization effects
Leconte N., Soriano D., Roche S., Ordejon P., Charlier J.-C., Palacios J.J. ACS Nano; 5 (5): 3987 - 3992. 2011. 10.1021/nn200558d.

Spin-dependent transport in hydrogenated two-dimensional graphene is explored theoretically. Adsorbed atomic hydrogen impurities can either induce a local antiferromagnetic, ferromagnetic, or nonmagnetic state depending on their density and relative distribution. To describe the various magnetic possibilities of hydrogenated graphene, a self-consistent Hubbard Hamiltonian, optimized by ab initio calculations, is first solved in the mean field approximation for small graphene cells. Then, an efficient order N Kubo transport methodology is implemented, enabling large scale simulations of functionalized graphene. Depending on the underlying intrinsic magnetic ordering of hydrogen-induced spins, remarkably different transport features are predicted for the same impurity concentration. Indeed, while the disordered nonmagnetic graphene system exhibits a transition from diffusive to localization regimes, the intrinsic ferromagnetic state exhibits unprecedented robustness toward quantum interference, maintaining, for certain resonant energies, a quasiballistic regime up to the micrometer scale. Consequently, low temperature transport measurements could unveil the presence of a magnetic state in weakly hydrogenated graphene. © 2011 American Chemical Society.

View abstract
Magnetoresistance and magnetic ordering fingerprints in hydrogenated graphene
Soriano D., Leconte N., Ordejón P., Charlier J.-C., Palacios J.-J., Roche S. Physical Review Letters; 107 (1, 016602) 2011. 10.1103/PhysRevLett.107.016602.

Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to ∼7% are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene. © 2011 American Physical Society.

View abstract
Modulation of surface charge transfer through competing long-range repulsive versus short-range attractive interactions
Fraxedas J., García-Gil S., Monturet S., Lorente N., Fernández-Torrente I., Franke K.J., Pascual J.I., Vollmer A., Blum R.-P., Koch N., Ordejón P. Journal of Physical Chemistry C; 115 (38): 18640 - 18648. 2011. 10.1021/jp2050838.

We report a combined experimental and theoretical study of the modulation of surface charge transfer on the tetrathiafulvalene (TTF)/Au(111) interface as a function of coverage in the submonolayer regime by combining low-temperature scanning tunneling microscopy, high-resolution photoemission spectroscopy using synchrotron radiation, and density functional theory (DFT) calculations. The modulation is induced by the competition between long-range repulsive Coulombic interactions and short-range attractive hydrogen-bonding interactions. The system shows the characteristic pattern evolution, from monomeric stripes at low coverages to two-dimensional islands, with the formation of labyrinths in the crossover. © 2011 American Chemical Society.

View abstract
Multi-orbital non-crossing approximation from maximally localized Wannier functions: The Kondo signature of copper phthalocyanine on Ag(100)
Korytr, R.; Lorente, N. Journal of Physics Condensed Matter; 23 2011. 10.1088/0953-8984/23/35/355009.
Observation and electric current control of a local spin in a single-molecule magnet
Komeda, T.; Isshiki, H.; Liu, J.; Zhang, Y.F.; Lorente, N.; Katoh, K.; Breedlove, B.K.; Yamashita, M. Nature Communications; 2 2011. 10.1038/ncomms1210.
Oxygen surface functionalization of graphene nanoribbons for transport gap engineering
Cresti A., Lopez-Bezanilla A., Ordejón P., Roche S. ACS Nano; 5 (11): 9271 - 9277. 2011. 10.1021/nn203573y.

We numerically investigate the impact of epoxide adsorbates on the transport properties of graphene nanoribbons with width varying from a few nanometers to 15 nm. For the wider ribbons, a scaling analysis of conductance properties is performed for adsorbate density ranging from 0.1% to 0.5%. Oxygen atoms introduce a large electron-hole transport asymmetry with mean free paths changing by up to 1 order of magnitude, depending on the hole or electron nature of charge carriers. The opening of a transport gap on the electron side for GNRs as wide as 15 nm could be further exploited to control current flow and achieve larger ON/OFF ratios, despite the initially small intrinsic energy gap. The effect of the adsorbates in narrow ribbons is also investigated by full ab initio calculations to explore the limit of ultimate downsized systems. In this case, the inhomogeneous distribution of adsorbates and their interplay with the ribbon edge are found to play an important role. © 2011 American Chemical Society.

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Quantum transport in chemically modified two-dimensional graphene: From minimal conductivity to Anderson localization
Leconte N., Lherbier A., Varchon F., Ordejon P., Roche S., Charlier J.-C. Physical Review B - Condensed Matter and Materials Physics; 84 (23, 235420) 2011. 10.1103/PhysRevB.84.235420.

An efficient computational methodology is used to explore charge transport properties in chemically modified (and randomly disordered) graphene-based materials. The Hamiltonians of various complex forms of graphene are constructed using tight-binding models enriched by first-principles calculations. These atomistic models are further implemented into a real-space order-N Kubo-Greenwood approach, giving access to the main transport length scales (mean free paths, localization lengths) as a function of defect density and charge carrier energy. An extensive investigation is performed for epoxide impurities with specific discussions on both the existence of a minimum semiclassical conductivity and a crossover between weak to strong localization regime. The 2D generalization of the Thouless relationship linking transport length scales is here illustrated based on a realistic disorder model. © 2011 American Physical Society.

View abstract
Spin coupling and relaxation inside molecule-metal contacts.
Mugarza, A.; Krull, C.; Robles, R.; Stepanow, S.; Ceballos, G.; Gambardella, P. Nature Communications; 2 2011. 10.1038/ncomms1497.
Structural, dynamical, and electronic transport properties of modified DNA duplexes containing size-expanded nucleobases
Blas J.R., Huertas O., Tabares C., Sumpter B.G., Fuentes-Cabrera M., Orozco M., Ordejón P., Luque F.J. Journal of Physical Chemistry A; 115 (41): 11344 - 11354. 2011. 10.1021/jp205122c.

Among the distinct strategies proposed to expand the genetic alphabet, size-expanded nucleobases are promising for the development of modified DNA duplexes with improved biotechnological properties. In particular, duplexes built up by replacing canonical bases with the corresponding benzo-fused counterparts could be valuable as molecular nanowires. In this context, this study reports the results of classical molecular dynamics simulations carried out to examine the structural and dynamical features of size-expanded DNAs, including both hybrid duplexes containing mixed pairs of natural and benzo-fused bases (xDNA) and pure size-expanded (xxDNA) duplexes. Furthermore, the electronic structure of both natural and size-expanded duplexes is examined by means of density functional computations. The results confirm that the structural and flexibility properties of the canonical DNA are globally little affected by the presence of benzo-fused bases. The most relevant differences are found in the enhanced size of the grooves, and the reduction in the twist. However, the analysis also reveals subtle structural effects related to the nature and sequence of benzo-fused bases in the duplex. On the other hand, electronic structure calculations performed for xxDNAs confirm the reduction in the HOMO-LUMO gap predicted from the analysis of the natural bases and their size-expanded counterparts, which suggests that pure size-expanded DNAs can be good conductors. A more complex situation is found for xDNAs, where fluctuations in the electrostatic interaction between base pairs exerts a decisive influence on the modulation of the energy gap. © 2011 American Chemical Society.

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Theoretical study of magnetic moments induced by defects at the SiC(110) surface
Poissier, A.; Lorente, N.; Yndurain, F. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.035322.

2010

Band selection and disentanglement using maximally localized Wannier functions: The cases of Co impurities in bulk copper and the Cu(111) surface
Korytár R., Pruneda M., Junquera J., Ordejón P., Lorente N. Journal of Physics Condensed Matter; 22 (38, 385601) 2010. 10.1088/0953-8984/22/38/385601.

We have adapted the maximally localized Wannier function approach of Souza et al (2002 Phys. Rev. B 65 035109) to the density functional theory based Siesta code (Soler et al 2002 J. Phys.: Condens. Mater. 14 2745) and applied it to the study of Co substitutional impurities in bulk copper as well as to the Cu(111) surface. In the Co impurity case, we have reduced the problem to the Co d-electrons and the Cu sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined density functional parameters in a fully orthonormal basis set. In order to test the quality of the Wannier approach to surfaces, we have studied the electronic structure of the Cu(111) surface by again transforming the density functional problem into the Wannier representation. An excellent description of the Shockley surface state is attained, permitting us to be confident in the application of this method to future studies of magnetic adsorbates in the presence of an extended surface state. © 2010 IOP Publishing Ltd.

View abstract
Damaging graphene with ozone treatment: A chemically tunable metal - Insulator transition
Leconte N., Moser J., Ordejón P., Tao H., Lherbier A., Bachtold A., Alsina F., Sotomayor Torres C.M., Charlier J.-C., Roche S. ACS Nano; 4 (7): 4033 - 4038. 2010. 10.1021/nn100537z.

We present a multiscale ab initio study of electronic and transport properties of two-dimensional graphene after epoxide functionalization via ozone treatment. The orbital rehybridization induced by the epoxide groups triggers a strong intervalley scattering and changes dramatically the conduction properties of graphene. By varying the coverage density of epoxide defects from 0.1 to 4%, charge conduction can be tuned from a diffusive to a strongly localized regime, with localization lengths down to a few nanometers long. Experimental results supporting the interpretation as a metal - insulator transition are also provided. © 2010 American Chemical Society.

View abstract
Density-wave instability in α- ( BEDT-TTF)2KHg(SCN) 4 studied by x-ray diffuse scattering and by first-principles calculations
Foury-Leylekian P., Pouget J.-P., Lee Y.-J., Nieminen R.M., Ordejón P., Canadell E. Physical Review B - Condensed Matter and Materials Physics; 82 (13, 134116) 2010. 10.1103/PhysRevB.82.134116.

α- (BEDT-TTF)2KHg(SCN)4 develops a density wave ground state below 8 K whose origin is still debated. Here we report a combined x-ray diffuse scattering and first-principles density functional theory study supporting the charge density wave (CDW) scenario. In particular, we observe a triply incommensurate anharmonic lattice modulation with intralayer wave vector components which coincide within experimental errors to the maximum of the calculated Lindhard response function. A detailed study of the structural aspects of the modulation shows that the CDW instability in α- ( BEDT-TTF) 2 KHg ( SCN) 4 is considerably more involved than those following a standard Peierls mechanism. We thus propose a microscopic mechanism where the CDW instability of the BEDT-TTF layer is triggered by the anion sublattice. Our mechanism also emphasizes the key role of the coupling of the BEDT-TTF and anion layers via the hydrogen bond network to set the global modulation. © 2010 The American Physical Society.

View abstract
Electronic transport between graphene layers covalently connected by carbon nanotubes
Novaes F.D., Rurali R., Ordejón P. ACS Nano; 4 (12): 7596 - 7602. 2010. 10.1021/nn102206n.

We present a first-principles study of the electronic transport properties of metallic and semiconducting carbon nanotube (CNT) junctions connecting two graphene layers, for different CNT lengths and link structures. Transport is analyzed in terms of the scattering states originated from the π and π* states of the finite-length CNTs, which couple to the graphene states producing resonances in the transmission curves. We find that, for metallic CNTs, the conductance is nearly independent of the tube length, but changes strongly with the link structure, while the opposite occurs for semiconducting CNTs, where the conductance in the tunneling regime is mainly controlled by the tube length and independent of the link structure. The sizable band offset between graphene and the CNTs yields to considerable effects on the transport properties, which cannot be captured using simple empirical models and highlights the need for a first-principles description. © 2010 American Chemical Society.

View abstract
Graphene on Ru(0001): Contact formation and chemical reactivity on the atomic scale
Altenburg, S.J.; Kröger, J.; Wang, B.; Bocquet, M.L.; Lorente, N.; Berndt, R. Physical Review Letters; 105 2010. 10.1103/PhysRevLett.105.236101.
Growth and Structure of Self-assembled Monolayers of a TTF Derivative on Au(111)
Urban, C.; Écija, D.; Wang, Y.; Trelka, M.; Preda, I.; Vollmer, A.; Lorente, N.; Arnau, A.; Alcamí, M.; Soriano, L.; Martín, N.; Martín, F.; Otero, R.; Gallego, J.M.; Miranda, R. Journal of Physical Chemistry C; 114: 6503 - 6510. 2010. 10.1021/jp911839b.
Magnetic transitions induced by tunneling electrons in individual adsorbed M -phthalocyanine molecules (M=Fe and Co)
Gauyacq, J.-P.; Novaes, F.D.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.165423.
Mixed-valency signature in vibrational inelastic electron tunneling spectroscopy
Alducin, M.; Sánchez-Portal, D.; Arnau, A.; Lorente, N. Physical Review Letters; 104 2010. 10.1103/PhysRevLett.104.136101.
Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking
Mugarza A., Lorente N., Ordejón P., Krull C., Stepanow S., Bocquet M.-L., Fraxedas J., Ceballos G., Gambardella P. Physical Review Letters; 105 (11, 115702) 2010. 10.1103/PhysRevLett.105.115702.

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers. © 2010 The American Physical Society.

View abstract
Origin of half-semimetallicity induced at interfaces of C-BN heterostructures
Pruneda, J.M. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.161409.
Preface: Phys. stat. sol. (c) 7/11-12
Correia, A.; Sáenz, J.J.; Ordejon, P.; Roche, S. Physica Status Solidi (C) Current Topics in Solid State Physics; 7: 2593 - 2595. 2010. 10.1002/pssc.201060100.
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
Novaes, F.D.; Lorente, N.; Gauyacq, J.-P. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.155401.
Real-time TD-DFT simulations in dye sensitized solar cells: The electronic absorption spectrum of alizarin supported on TiO2 nanoclusters
Sánchez-De-Armas R., Oviedo López J., San-Miguel M.A., Sanz J.F., Ordejón P., Pruneda M. Journal of Chemical Theory and Computation; 6 (9): 2856 - 2865. 2010. 10.1021/ct100289t.

The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO 2)n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient. © 2010 American Chemical Society.

View abstract
Role of molecular electronic structure in inelastic electron tunneling spectroscopy: O2 on Ag(110)
Monturet, S.; Alducin, M.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.085447.

2009

Efficient spin transitions in inelastic electron tunneling spectroscopy
Lorente, N.; Gauyacq, J.-P. Physical Review Letters; 103 2009. 10.1103/PhysRevLett.103.176601.
He-LiF surface interaction potential from fast atom diffraction
Schüller, A.; Winter, H.; Gravielle, M.S.; Pruneda, J.M.; Miraglia, J.E. PHYSICAL REVIEW A; 80 2009. 10.1103/PhysRevA.80.062903.
Nonadiabatic wavepacket dynamics: K-space formulation
Pruneda, J.M.; Souza, I. Physical Review B - Condensed Matter and Materials Physics; 79 2009. 10.1103/PhysRevB.79.045127.
Optimal strictly localized basis sets for noble metal surfaces
García-Gil S., García A., Lorente N., Ordejón P. Physical Review B - Condensed Matter and Materials Physics; 79 (7, 075441) 2009. 10.1103/PhysRevB.79.075441.

The properties of the (111) surfaces of Cu, Ag, and Au are evaluated using a first-principles approach with strictly localized basis sets. These surfaces present metallic and extended properties that are a priori difficult to describe with a local-basis approach. We explore methodologies to enhance the basis sets of the surface atoms in order to accurately describe surface properties such as surface energies, surface states, and work functions. In this way, the advantages of local-basis computations (namely, efficiency, optimum size scaling, and a natural description of bonding in real space) can be retained, while keeping the accuracy in the description of the properties of the surface at a very good level. © 2009 The American Physical Society.

View abstract
Physica Status Solidi C: Preface
Correia, A.; Sáenz, J.J.; Ordejón, P. Physica Status Solidi (C) Current Topics in Solid State Physics; 6: 2093 - 2095. 2009. 10.1002/pssc.200960073.
Probing the proton location in a water bilayer on Pd (111) by inelastic spectroscopy simulations
Bocquet, M.-L.; Lorente, N. Journal of Chemical Physics; 130 2009. 10.1063/1.3089824.
Selective scanning tunnelling microscope electron-induced reactions of single biphenyl molecules on a Si(100) surface
Riedel, D.; Bocquet, M.-L.; Lesnard, H.; Lastapis, M.; Lorente, N.; Sonnet, P.; Dujardin, G. Journal of the American Chemical Society; 131: 7344 - 7352. 2009. 10.1021/ja8101133.
Solid-state physics: Lost magnetic moments
Korytár, R.; Lorente, N. Nature; 458: 1123 - 1124. 2009. 10.1038/4581123a.
Tunneling spectroscopy in core/shell structured Fe/MgO nanospheres
Martínez-Boubeta C., Balcells Li., Monty C., Ordejon P., Martínez B. Applied Physics Letters; 94 (6, 062507) 2009. 10.1063/1.3080657.

We report on tunneling conductance properties of a single Fe island enclosed by an epitaxial MgO shell. The results obtained show clear oscillations in the current-voltage curves that, along with the theoretical modeling, allow determining the electronic density of states of the iron core. Moreover, the correlation of these results with the temperature dependence of the electrical conductivity of assemblies of Fe/MgO nanocrystals provides evidence of the Δ1 symmetry-sensitive tunneling across a crystalline magnesia barrier. © 2009 American Institute of Physics.

View abstract

2008

Adsorption of N2O on Cu(100): A combined scanning tunneling microscopy and density functional theory study
Franke, K.J.; Fernández-Torrente, I.; Pascual, J.I.; Lorente, N. Physical Chemistry Chemical Physics; 10: 1640 - 1647. 2008. 10.1039/b716952c.
Band bending and quasi-2deg in the metallized β-SiC(001) surface
Rurali R., Wachowicz E., Hyldgaard P., Ordejón P. Physica Status Solidi - Rapid Research Letters; 2 (5): 218 - 220. 2008. 10.1002/pssr.200802166.

We study the mechanism leading to the metallization of the β-SiC(001) Si-rich surface induced by hydrogen adsorption. We analyze the effects of band bending and demonstrate the existence of a quasi-2D electron gas, which originates from the donation of electrons from adsorbed hydrogen to bulk conduction states. We also provide a simple model that captures the main features of the results of first-principles calculations, and uncovers the basic physics of the process. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

View abstract
Creating pseudo-Kondo resonances by field-induced diffusion of atomic hydrogen
Hofer, W.A.; Teobaldi, G.; Lorente, N. Nanotechnology; 19 2008. 10.1088/0957-4484/19/30/305701.
Dynamic Jahn-Teller effect in electronic transport through single C 60 molecules
Frederiksen, T.; Franke, K.J.; Arnau, A.; Schulze, G.; Pascual, J.I.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 78 2008. 10.1103/PhysRevB.78.233401.
Formation of dispersive hybrid bands at an organic-metal interface
Gonzalez-Lakunza, N.; Fernández-Torrente, I.; Franke, K.J.; Lorente, N.; Arnau, A.; Pascual, J.I. Physical Review Letters; 100 2008. 10.1103/PhysRevLett.100.156805.
Inelastic effects in electron transport studied with wave packet propagation
Monturet, S.; Lorente, N. Physical Review B - Condensed Matter and Materials Physics; 78 2008. 10.1103/PhysRevB.78.035445.
Reducing the molecule-substrate coupling in C60-based nanostructures by molecular interactions
Franke, K.J.; Schulze, G.; Henningsen, N.; Fernández-Torrente, I.; Pascual, J.I.; Zarwell, S.; Rück-Braun, K.; Cobian, M.; Lorente, N. Physical Review Letters; 100 2008. 10.1103/PhysRevLett.100.036807.
The SIESTA method; Developments and applicability
Artacho E., Anglada E., Diéguez O., Gale J.D., García A., Junquera J., Martin R.M., Ordejón P., Pruneda J.M., Sánchez-Portal D., Soler J.M. Journal of Physics Condensed Matter; 20 (6, 064208) 2008. 10.1088/0953-8984/20/6/064208.

Recent developments in and around the SIESTA method of first-principles simulation of condensed matter are described and reviewed, with emphasis on (i) the applicability of the method for large and varied systems, (ii) efficient basis sets for the standards of accuracy of density-functional methods, (iii) new implementations, and (iv) extensions beyond ground-state calculations. © 2008 IOP Publishing Ltd.

View abstract
Theoretical study of benzene and pyridine STM-induced reactions on copper surfaces
Lesnard, H.; Lorente, N.; Bocquet, M.-L. Journal of Physics Condensed Matter; 20 2008. 10.1088/0953-8984/20/22/224012.
Unified description of inelastic propensity rules for electron transport through nanoscale junctions
Paulsson, M.; Frederiksen, T.; Ueba, H.; Lorente, N.; Brandbyge, M. Physical Review Letters; 100 2008. 10.1103/PhysRevLett.100.226604.

2007

Chemisorption of sulfur and sulfur-based simple molecules on Au(111)
Gonzalez-Lakunza, N.; Lorente, N.; Arnau, A. Journal of Physical Chemistry C; 111: 12383 - 12390. 2007. 10.1021/jp0726586.
GaS and InSe equations of state from single crystal diffraction
J. Pellicer-Porres; E. Machado-Charry; A. Segura; S. Gilliland; E. Canadell; P. Ordejón; A. Polian; P. Munsch; A. Chevy y N. Guignot Physica Status Solidi (B): Basic Research; 2007. .
Including the probe tip in theoretical models of inelastic scanning tunneling spectroscopy: CO on Cu(100)
Teobaldi, G.; Peñalba, M.; Arnau, A.; Lorente, N.; Hofer, W.A. Physical Review B - Condensed Matter and Materials Physics; 76 2007. 10.1103/PhysRevB.76.235407.
Interaction of copper organometallic precursors with barrier layers of Ti, Ta and W and their nitrides: a first-principles molecular dynamics study
E. Machado; M. Kaczmarski; B. Braida; P. Ordejón; D. Garg; J. Norman; H. Cheng Journal of Molecular Modeling; 2007. .
Long-range repulsive interaction between molecules on a metal surface induced by charge transfer
Fernandez-Torrente, I.; Monturet, S.; Franke, K.J.; Fraxedas, J.; Lorente, N.; Pascual, J.I. Physical Review Letters; 99 2007. 10.1103/PhysRevLett.99.176103.
Resistive and rectifying effects of pulling gold atoms at thiol-gold nanocontacts
R. J. C. Batista; P. Ordejón; H. Chacham; E. Artacho Physical Review B; 2007. .
The Calculation of Free-Energiesin Semiconductors: Defects, Transitionsand Phase Diagrams
E. R. Hernández; A. Antonelli; L. Colombo; P. Ordejón Topics in Applied Physics; 104: 115 - 140. 2007. .
Transport measurements under pressure in III¿IV layered semiconductors
A. Segura; D. Errandonea; D. Martínez García; F. J. Manjón; A. Chevy; G. Tobías; P. Ordejón; E. Canadell Physica Status Solidi (B): Basic Research; 244: 162 - 168. 2007. .

2006

Electron transport via local polarons at interface atoms
Berthe, M.; Urbieta, A.; Perdigão, L.; Grandidier, B.; Deresmes, D.; Delerue, C.; Stiévenard, D.; Rurali, R.; Lorente, N.; Magaud, L.; Ordejón, P. Physical Review Letters; 97 2006. 10.1103/PhysRevLett.97.206801.
Methylthiolate adsorption on Au(1 1 1): Energetics, vibrational modes and STM imaging
Gonzalez, N.; Lorente, N.; Arnau, A. Surface Science; 600: 4039 - 4043. 2006. 10.1016/j.susc.2006.01.119.

2005

First-principles study of structural, elastic, and bonding properties of pyrochlores
J.M. Pruneda; Emilio Artacho Physical Review B - Condensed Matter and Materials Physics; 2005. .
Self-doped titanium oxide thin films for efficient visible light photocatalysis: An example: Nonylphenol photodegradation
Justicia, I.; Garcia, G.; Vázquez, L.; Santiso, J.; Ordejón, P.; Battiston, G.; Gerbasi, R.; Figueras, A. Sensors and Actuators, B: Chemical; 109: 52 - 56. 2005. 10.1016/j.snb.2005.03.021.

2004

Intrinsic point defects and volume swelling in ZrSiO4 under irradiation
J. M. Pruneda; T. D. Archer; Emilio Artacho Physical Review B - Condensed Matter and Materials Physics; 2004. .
Short-range repulsive interatomic interactions in energetic processes in solids
J. M. Pruneda; Emilio Artacho Physical Review B - Condensed Matter and Materials Physics; 2004. .