Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma

Cheseaux, Switzerland

Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma

Cheseaux, Switzerland

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Lauchli A.M.,Max Planck Institute For Physik Komplexer Systeme | Lauchli A.M.,University of Innsbruck | Sudan J.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Sorensen E.S.,McMaster University
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We present a comprehensive list of the ground-state energies and the spin gaps of finite Kagomé clusters with up to 42 spins obtained using large-scale exact diagonalization techniques. This represents the current limit of this exact approach. For a fixed number of spins N, we study several cluster shapes under periodic boundary conditions in both directions resulting in a toroidal geometry. The clusters are characterized by their side length and diagonal as well as the shortest "Manhattan" diameter of the torii. A finite-size scaling analysis of the ground-state energy as well as the spin gap is then performed in terms of the shortest toroidal diameter as well as the shortest "Manhattan" diameter. The structure of the spin-spin correlations further supports the importance of short loops wrapping around the torii. © 2011 American Physical Society.


Ouyang C.Y.,Jiangxi Normal University | Ouyang C.Y.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Zeng X.M.,Jiangxi Normal University | Sljivancanin Z.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Baldereschi A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma
Journal of Physical Chemistry C | Year: 2010

The electronic structures of bulk and (001) surfaces of λ-MnO 2 and LiMn2O4 have been studied with density functional theory. In λ-MnO2, the Mn oxidation state is +4 both in the bulk and at the (001) surface. In LiMn2O4, however, Mn atoms exhibit mixed Mn3+/Mn4+ oxidation states in the bulk, while only Mn3+ at the (001) surface, due to lower coordination with O atoms. The technically undesired Mn3+ ions form at Li xMn2O4 (001) even for very small Li concentrations. Upon covering the LiMn2O4 (001) surface with Al2O3, the oxidation state of surface Mn atoms changes from +3 to +4, which explains the improved performance of coated LiMn2O4 cathodes in Li-ion batteries. © 2010 American Chemical Society.


Vukajlovic F.R.,Vinča Institute of Nuclear Sciences | Popovic Z.S.,Vinča Institute of Nuclear Sciences | Baldereschi A.,Ecole Polytechnique Federale de Lausanne | Baldereschi A.,University of Trieste | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

By means of ab initio calculations based on density-functional theory we demonstrate that magnetism in monoatomic Fe wires deposited on nanostructured Ir(100) surface can be tuned by their functionalization with hydrogen. The pristine monoatomic Fe wires deposited on nanostructured Ir(100) surface partially covered by H atoms are antiferromagnetic. However, the type of exchange interaction between Fe atoms can be changed by increasing H coverage. At fully hydrogenated Ir surface the Fe wires themselves are decorated with hydrogen, which gives rise to the ferromagnetic coupling between adjacent Fe atoms. © 2010 The American Physical Society.


Alkauskas A.,Ecole Polytechnique Federale de Lausanne | Alkauskas A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Broqvist P.,Ecole Polytechnique Federale de Lausanne | Broqvist P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | And 2 more authors.
Physica Status Solidi (B) Basic Research | Year: 2011

Hybrid density functional calculations applied to defect charge transition levels are explored in the attempt to overcome the band-gap problem of semilocal density functionals. Charge transition levels of a large set of point defects calculated with semilocal and hybrid density functionals are found to correspond closely when aligned with respect to the average electrostatic potential. This strongly suggests that the defect levels defined in this way are already accurately described at these levels of theory. In particular, this then also applies to the energy separation between different defect levels, which is directly accessible experimentally. At variance, within the same alignment scheme, the band edges obtained with hybrid functionals are found to undergo significant shifts with respect to those obtained with semilocal functionals. While these shifts systematically give larger band gaps, the agreement with experiment is not always satisfactory when a fixed fraction of exact exchange is admixed. This describes a current limitation of hybrid functional schemes. In the attempt of identifying a viable theoretical description within the class of one-parameter hybrid functionals based on bare exchange, we explore the validity of the empirical procedure which consists in tuning the fraction of nonlocal exchange to a value which gives a theoretical band gap reproducing the experimental one. Comparisons with experiment for band offsets and specific defect levels record very encouraging results. Despite its inherent limitations, such an empirical scheme based on hybrid functionals represents a definite improvement with respect to semilocal functionals. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Kibalchenko M.,University of Cambridge | Yates J.R.,University of Oxford | Massobrio C.,CNRS Institute of Genetics and of Molecular and Cellular Biology | Pasquarello A.,Ecole Polytechnique Federale de Lausanne | Pasquarello A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Structural assignments are determined for 77Se and 73Ge chemical shifts through density-functional NMR calculations for GeSe2, Ge4 Se9, and GeSe crystals. In particular, a very good agreement between calculated and measured 77Se isotropic chemical shifts and anisotropies is found for the GeSe2 crystal, for which experimental data are available. These assignments provide a consistent interpretation of experimental 77Se spectra of Gex Se1-x glasses, indicating that the contribution from Ge-Se-Se linkages overlaps with that from Ge-Se-Ge linkages in corner-sharing tetrahedral arrangements, thereby dismissing the occurrence of a bimodal phase. © 2010 The American Physical Society.


Kibalchenko M.,University of Cambridge | Yates J.R.,University of Oxford | Pasquarello A.,Ecole Polytechnique Federale de Lausanne | Pasquarello A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma
Journal of Physics Condensed Matter | Year: 2010

NMR parameters of 73Ge and 17O in vitreous GeO 2 and quartz GeO2, including the isotropic shifts, the quadrupole coupling constants CQ, and the electric-field-gradient asymmetry parameters η, are determined through density functional calculations. Clear correlations are established between 73Ge shifts and the mean of the four neighboring Ge-O-Ge bond angles, and between C Q and η parameters of 17O and the local Ge-O-Ge angle. Available experimental data for CQ and the corresponding established correlation are used to extract the value of 135° for the average Ge-O-Ge angle in vitreous GeO2. The features of the Ge-O-Ge bond angle distribution of vitreous GeO2 derived in this work are consistent with those inferred from other experimental probes. © 2010 IOP Publishing Ltd.


Dahinden P.,Ecole Polytechnique Federale de Lausanne | Dahinden P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Broqvist P.,Ecole Polytechnique Federale de Lausanne | Broqvist P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The defect levels of the nitrogen dangling bond at the Si/ SiO2 interface are determined through a density-functional approach. The composition grading at the interface is modeled through crystalline and amorphous models of stoichiometric SiO2, nitrided SiO2, and substoichiometric silicon oxides. The relevant charge transition levels are first determined with respect to the band edges of the parent oxides within a semilocal density-functional scheme. Through the use of band edge shifts and band offsets previously obtained with hybrid functionals, the calculated defect levels are then positioned with respect to a band diagram of the Si/ SiO2 interface which shows good agreement with the experimental one. We find that the 0/- charge transition level locates within the Si band gap. The level locates close to the Si valence band for nitrogen atoms in the stoichiometric oxide, but is found to rise across the silicon band gap as the environment of the nitrogen atoms becomes more silicon rich. The latter raise is accompanied by a stabilization of the incorporated nitrogen. © 2010 The American Physical Society.


Broqvist P.,Ecole Polytechnique Federale de Lausanne | Broqvist P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Alkauskas A.,Ecole Polytechnique Federale de Lausanne | Alkauskas A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | And 2 more authors.
Physica Status Solidi (A) Applications and Materials Science | Year: 2010

We introduce a theoretical scheme to study defect energy levels and band alignments at semiconductor-oxide interfaces. The scheme relies on hybrid functionals to overcome the band gap underestimation typically achieved with semilocal density functionals. For atomically localized defects, the more accurate description achieved with hybrid functionals does not lead to significant shifts of the charge transition levels, provided these levels are referred to a common reference potential. This result effectively decouples the shifts of the band edges with respect to the defect levels. We also show that relative shifts of conduction and valence band edges as determined by exact nonlocal exchange lead to band offsets in excellent agreement with experimental values for several semiconductor-oxide interfaces. The proposed scheme is illustrated through a series of applications, including the dangling bond defects in silicon and germanium, the charge state of the O2 molecule during silicon oxidation, and the oxygen vacancy in Si-SiO 2-HfO 2 stacks. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Yazyev O.V.,Ecole Polytechnique Federale de Lausanne | Yazyev O.V.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Yazyev O.V.,University of California at Berkeley | Pasquarello A.,Ecole Polytechnique Federale de Lausanne | Pasquarello A.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Periodically corrugated epitaxial graphene and hexagonal boron nitride (h-BN) on metallic substrates are considered as perspective templates for the self-assembly of nanoparticles arrays. By using first-principles calculations, we determine binding energies and diffusion activation barriers of metal adatoms on graphene and h-BN. The observed chemical trends can be understood in terms of the interplay between charge transfer and covalent bonding involving the adatom d electrons. We further investigate the electronic effects of the metallic substrate and find that periodically corrugated templates based on graphene in combination with strong interactions at the metal/graphene interface are the most suitable for the self-assembly of highly regular nanoparticle arrays. © 2010 The American Physical Society.


Komsa H.-P.,Ecole Polytechnique Federale de Lausanne | Komsa H.-P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | Broqvist P.,Ecole Polytechnique Federale de Lausanne | Broqvist P.,Institute Romand Of Recherche Numerique En Physique Des Materiaux Irrma | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We investigate how various treatments of exact exchange affect defect charge transition levels and band edges in hybrid functional schemes for a variety of systems. We distinguish the effects of long-range vs short-range exchange and of local vs nonlocal exchange. This is achieved by the consideration of a set of four functionals, which comprise the semilocal Perdew-Burke-Ernzerhof (PBE) functional, the PBE hybrid (PBE0), the Heyd-Scuseria-Ernzerhof (HSE) functional, and a hybrid derived from PBE0 in which the Coulomb kernel in the exact exchange term is screened as in the HSE functional but which, unlike HSE, does not include a local expression compensating for the loss of the long-range exchange. We find that defect levels in PBE0 and in HSE almost coincide when aligned with respect to a common reference potential, due to the close total-energy differences in the two schemes. At variance, the HSE band edges determined within the same alignment scheme are found to shift significantly with respect to the PBE0 ones: the occupied and the unoccupied states undergo shifts of about +0.4eV and -0.4eV, respectively. These shifts are found to vary little among the materials considered. Through a rationale based on the behavior of local and nonlocal long-range exchange, this conclusion is generalized beyond the class of materials used in this study. Finally, we explicitly address the practice of tuning the band gap by adapting the fraction of exact exchange incorporated in the functional. When PBE0-like and HSE-like functionals are tuned to yield identical band gaps, their respective results for the positions of defect levels within the band gap and for the band alignments at interfaces are found to be very close. © 2010 The American Physical Society.

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