Publications
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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).
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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.
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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.
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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.
