Modelling the electronic structure and magnetic properties of LiFeAs and FeSe using hybrid-exchange density functional theory

The electronic structure and magnetic properties of LiFeAs and FeSe have been studied using hybrid exchange density functional theory. The total energies for a unit cell in LiFeAs and FeSe with different spin states including non-magnetic and spin-2 are calculated. The spin-2 configuration has the lower energy for both LiFeAs and FeSe. The computed anti-ferromagnetic exchange interactions between spins on the nearest (next nearest) neighbouring Fe atoms in LiFeAs and FeSe are approximately 14 (17) meV and 6 (13) meV respectively. The total energies of the checkerboard and stripe-type anti-ferromagnetic ordering for LiFeAs and FeSe are compared, yielding that for LiFeAs the checkerboard is lower whereas for FeSe the stripe-type is lower. However, owing to the fact that the exchange interaction of the next nearest neighbour is larger than that of the nearest one, which means that the collinear ordering might be the ground state. These results are in agreement with previous theoretical calculations and experiments. Especially the calculations for LiFeAs indicate a co-existence of conducting d-bands at the Fermi surface and d-orbital magnetism far below the Fermi surface. The theoretical results presented here might be useful for the experimentalists working on the electronic structure and magnetism of iron-based superconductors.Comment: 7 pages, 4 figures, 1 table, accepted by Solid State Communication

Etude des proprietes electriques des molecules a partir d'une methode invariante de jauge: application a la determination des polarisabilites et des hyperpolarisabilites dynamiques de systemes simples

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 77873 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Modélisation du champ électrique interne d'un solide (Application à la détermination des propriétés dérivées de la polarisation.)

L'objectif de cette thèse est la modélisation du champ électrique interne dans les solides cristallins et l'étude des propriétés qui lui sont dérivées (polarisation spontanée, piézoélectricité, charges dynamiques de Born,...). L'effet de la contrainte imposée aux différents systèmes pour le calcul des constantes piézoélectrique, a été utilisé pour la détermination du jeu complet des constantes élastiques de la plupart des systèmes considérés dans ce mémoire. L'analyse du comportement dynamique par l'étude du spectre de phonons au centre de la zone de Brillouin d'une part, et le calcul spécifique des fréquences anharmoniques de vibration du groupement hydroxyle d'hydroxydes alcalins ou alcalino-terreux, d'autre part, complète ce travail.This thesis deals with modeling internal electric field in solids and the study of related properties (spontaneous polarisation, piezoelectricity, dynamical Born charges,...). For most of the studied compounds, the strain applied for the determination of piezoelectric constants is also used for the calculation of the full set of elastic constants. Analysis of the dynamical behaviour via the phonon spectrum at the center of the Brillouin zone simulation and calculation of the OH stretching mode anharmonicity of some alkali and alkali-earth metals hydroxides complete this work.PAU-BU Sciences (644452103) / SudocSudocFranceF

Density-functional Lcao Calculations of Periodic-systems - Effect of An A-posteriori Correction of the Hartree-fock Energy On the Physical-properties of Ionic Sulfur-compounds

Density functional theory is taken into account to correct 'a posteriori' the total energy calculated with ab initio Hartree-Fock theory. Two models are adopted: the first one is the local density approximation parametrized by Ceperley and Alder while the second one is non-local and corresponds to the formulation proposed by Perdew. The changes in physical properties of ionic sulfide compounds (equilibrium geometry, formation energy, elastic constants, phase transition B-1 --> B-3 in MgS and surface relaxation) associated with the electronic correlation are evaluated