In this work, results of ab-initio band structure calculations for A 2 BC Heusler compounds that have A and B sites occupied by transition metals and C by a main group element are presented.This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap and the peculiar magnetic behavior found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both A and B atoms to states near the Fermi energy and to the total magnetic moment. The magnitude of the total magnetic moment, which depends as well on the kind of C atoms, shows a trend consistent with the Slater-Pauling type behavior in several classes of these compounds. The localized moment in these magnetic compounds resides at the B site. Other than in the classical Cu 2 -based Heusler compounds, the A atoms in Co 2 , Fe 2 , and Mn 2 based compounds may contribute pronounced to the total magnetic moment.

In this work a simple concept was used for a systematic search for new materials with high spin polarization. It is based on two semi-empirical models. Firstly, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co2 based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment.Stimulated by these models, Co2FeSi was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of X-ray diffraction, X-ray absorption and Mößbauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment (6µB ) and Curietemperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations.

Na 2 IrO 3 , a honeycomb 5d 5 oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal-metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na 2 IrO 3 , the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic-structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant 6 New J. Phys. 16 (2014) 013056 V M Katukuri et al ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin-orbit interactions, this strong dependence on distortions of the Ir 2 O 2 plaquettes singles out the honeycomb 5d 5 oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems. IntroductionThe Heisenberg model of magnetic interactions, J S i · S j between spin moments at sites {i, j}, has been successfully used as an effective minimal model to describe the cooperative magnetic properties of both molecular and solid-state many-electron systems. A less conventional spin model-the Kitaev model [1]-has been recently proposed for honeycomb-lattice materials with 90 • metal-oxygen-metal bonds and strong spin-orbit interactions [2]. It has nontrivial topological phases with elementary excitations exhibiting Majorana statistics, which are relevant and much studied in the context of topological quantum computing [1,[3][4][5][6][7]. Candidate materials proposed to host such physics are the honeycomb oxides Na 2 IrO 3 and Li 2 IrO 3 [2]. The magnetically active sites, the Ir 4+ species, display in these compounds a 5d 5 valence electron configuration, octahedral ligand coordination and bonding of nearest-neighbor (NN) Ir ions through two ligands [8,9]. In the simplest approximation, i.e. for sufficiently large t 2g -e g octahedral crystal-field splittings within the Ir 5d shell and degenerate Ir t 2g levels, the ground-state (GS) electron configuration at each Ir site is a t 5 2g effective j = 1/2 spin-orbit doublet [2,[10][11][12]. The anisotropic, Kitaev type coupling then stems from the particular form the superexchange between the Ir j = 1/2 pseudospins takes for 90 • bond angles on the Ir-O 2 -Ir plaquette [2,13,14].Recent measurements on Na 2 IrO 3 [8,9] indicate significant lattice distortions away from the idealized case of cubic IrO 6 octahedra and 90 • Ir-O-Ir bond angles for which the Kitaev-Heis...

We use first-principles density functional theory to calculate the phonon frequencies, electron localization lengths, Born effective charges, dielectric response, and conventional electronic structures of the IV-VI chalcogenide series. The goals of our work are twofold: first, to determine the detailed chemical composition of lone pairs and, second, to identify the factors that cause lone pairs to favor high-or low-symmetry environments. Our results show that the traditional picture of cation s-p mixing causing localization of the lone pair lobe is incomplete, and instead the p states on the anion also play an important role. In addition these compounds reveal a delicate balance between two competing instabilities-structural distortion and tendency to metallicity-leading, at the same time, to anomalously large Born effective charges as well as large dielectric constants. The magnitude of the LO-TO splitting, which depends on the relative strength of both instabilities, shows a trend consistent with the structural distortions in these compounds.

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the κ-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.PACS numbers: 74.70. Kn,71.10.Fd,71.15.Mb,71.20.Rv A strong research trend of the new millennium has been the desire to understand complex manybody phenomena like superconductivity and magnetism by realistic modelling, i.e. to employ precise first principles calculations to feed the intricate details of real materials into the parameter sets of model Hamiltonians that are then solved with increasingly powerful manybody techniques. The κ-(BEDT-TTF) 2 X [1] organic charge transfer salts are a perfect example for a class of materials with such fascinating properties that they drive progress in experimental and manybody methods alike. Experimentally, the phase diagram shows Mott insulating, superconducting, magnetic and spin liquid phases [2,3,4,5]. Theoretically, the underlying anisotropic triangular lattice is a great challenge due to effects of frustration and the intense efforts to get a grip on the problem include studies with path integral renormalization group (PIRG) [6], exact diagonalization [7], variational Monte Carlo [8], cluster dynamical mean field theory [9, 10] and dual Fermions [11] to cite a few. In this rapidly expanding field of research, electronic structure calculations play the decisive role of mediating between the complex underlying structure and phenomenology of organic charge transfer salts and the models used for understanding the physics [12], and in this work, we will provide the perspective of precise, state of the art electronic structure calculations.Previously, κ-type CT salts have been investigated by semi-empiricial and first principles electronic structure calculations. The most commonly used t, t ′ , U parameter sets derive from extended Hückel molecular orbitals calculations [13,14] performed on different constellations of BEDT-TTF dimers.The main result reported here is that our first principles study shows all four considered κ-type CT salts to be less frustrated than previously assumed based on semiempirical theory. Most importantly, the often cited value of t ′ /t = 1.06 [13] for the spin liquid material κ-(ET) 2 Cu 2 (CN) 3 should be replaced by the significantly smaller value t ′ /t = 0.83 ± 0.08. This has fundamental implications on the systems' model description as we shall see below.In this Letter, we employ the Car-Parrinello [15] projector-augmented wave [16] molecular dynamics (CPMD)...