Showing papers by "Sergey Y. Savrasov published in 2010"

Electronic Structure of Pyrochlore Iridates: From Topological Dirac Metal to Mott Insulator

Abstract: In 5d transition metal oxides such as the iridates, novel properties arise from the interplay of electron correlations and spin-orbit interactions. We investigate the electronic structure of the pyrochlore iridates, (such as Y$_{2}$Ir$_{2}$O$_{7}$) using density functional theory, LDA+U method, and effective low energy models. A remarkably rich phase diagram emerges on tuning the correlation strength U. The Ir magnetic moment are always found to be non-collinearly ordered. However, the ground state changes from a magnetic metal at weak U, to a Mott insulator at large U. Most interestingly, the intermediate U regime is found to be a Dirac semi-metal, with vanishing density of states at the Fermi energy. It also exhibits topological properties - manifested by special surface states in the form of Fermi arcs, that connect the bulk Dirac points. This Dirac phase, a three dimensional analog of graphene, is proposed as the ground state of Y$_{2}$Ir$_{2}$O$_{7}$ and related compounds. A narrow window of magnetic `axion' insulator, with axion parameter $\theta=\pi$, may also be present at intermediate U. An applied magnetic field induces ferromagnetic order and a metallic ground state.

Self consistent GW determination of the interaction strength: application to the iron arsenide superconductors

Abstract: Many metals, semiconductors, and insulators are well described by the "standard model" of solid state physics. In this picture the excitations are band electrons, and their dispersion can be computed quantitatively in perturbation theory starting from the density functional theory using the GW method [1]. When this standard model fails, we talk about strongly correlated electron systems. The presence of strong correlations is debated with each new material discovery, as for example in the context of the iron pnictide superconductors. On the experimental side, controversies arose because optical experiments revealed signicant mass renormalizations [2{4] while Xray absorption, core level spectroscopies and resonant inelastic Xray scattering indicated the absence of satellite peaks [5, 6], which are standard ngerprints of strong correlations. Photoemission studies indicate that the overall bandwidth is narrowed by a factor of two [7, 8] but substantially larger mass renormalizations are present near the Fermi level [9]. Similar controversies arose within the rst principles approaches to the treatment of correlations with some theoretical studies supporting the notion of weak correlations [10{14], while others advocate a more correlated picture [15{18]. To make progress on this issues one needs to develop fully ab initio tools for addressing the problem of determining the strength of correlations and test their predictions against experiments. In this letter we introduce a new rst principles methodology for evaluating the strength of the correlations based on the self-consistent GW method. This approach has been shown to predict accurate total energy [19, 20], and we expect to obtain reliable estimates for the interaction strength since this quantity can be thought as a second derivative of the total energy with respect to the occupation of the correlated orbitals. We test successfully the method on the well studied example of a correlated material NiO, and then we apply it to a prototypical iron pnictide BaFe2As2. We nd that the correlations in iron pnictides are strong, as pointed out in Refs. [15{18] but unlike earlier studies our ab-initio method accounts for the absence of well dened Hubbard bands in the spectral functions. Our results are thus in excellent agreement with experiment and reconcile the results of apparently conicting spectroscopies. We start with the one-particle electron Green’s function in the solid,G, which is measurable in photoemission experiments. We split it into G 1 = G 0 1 + , where G 0 describes the non-interacting system of electrons, and is the frequency dependent self-energy. Both G and are matrices in r;r 0 . The electrons interact among themselves via the Coulomb interactionVc(r;r 0 ) = 1 jr r0j , however, the mobile electrons screen it and is therefore useful to reformulate the problem in terms of a screened Coulomb interaction W dened

Magnetic quantum critical point and dimensionality trend in cerium-based heavy-fermion compounds

Abstract: We present realistic Kondo-lattice simulation results for the recently-discovered heavy-fermion antiferromagnet CePt2In7 comparing with its three-dimensional counterpart CeIn3 and the less two-dimensional ones, Ce-115’s. We find that the distance to the magnetic quantum critical point is the largest for CeIn3 and the smallest for Ce-115’s, and CePt2In7 falls in between. We argue that thetrend in quasi-two-dimensional materials stems from the frequency dependence of the hybridization between Cerium 4f-electrons and the conduction bands.

Relative importance of crystal field versus bandwidth to the high pressure spin transition in transition metal monoxides

Abstract: The crystal field splitting and d bandwidth of the 3d transition metal monoxides MnO, FeO, CoO and NiO are analyzed as a function of pressure within density functional theory. In all four cases the 3d bandwidth is significantly larger than the crystal field splitting over a wide range of compressions. The bandwidth actually increases more as pressure is increased than the crystal field splitting. Therefore the role of increasing bandwidth must be considered in any explanation of a possible spin collapse that these materials may exhibit under pressure.

Calculated Magnetic and Electronic Properties of Pyrochlore Iridates

Abstract: and rare–earth based pyrochlore iridates. Our study reveals that the groundstate is a non–collinear magnetic insulating state. Due to strong spin-orbit coupling in Ir 5d, there isan unusual correlation between the bands near Fermi level and the magnetization direction, resultingin a possibility of insulator–to–metal transition under applied magnetic field. This makes pyrochloreiridates a good candidate for possible magnetoressitance and magnetooptical applications.