Pressure-Driven Metal-Insulator Transition in Hematite from Dynamical Mean-Field Theory

The Local Density Approximation combined with Dynamical Mean-Field Theory (LDA+DMFT method) is applied to the study of the paramagnetic and magnetically ordered phases of hematite Fe$_2$O$_3$ as a function of volume. As the volume is decreased, a simultaneous 1st order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the charge gap with increasing pressure, upon closing of which the high spin phase becomes unstable. We conclude that the transition in Fe$_2$O$_3$ at $\approx$50 GPa can be described as an electronically driven volume collapse.Comment: 5 pages, 4 figure

Electronic correlations and crystal structure distortions in BaBiO3

BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states form the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by the breathing distortion of the BiO6 octahedra (extension and contraction of the Bi-O bond lengths). Standard Density Functional Theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively experimentally observed insulator with monoclinic crystal structure but gives strongly underestimate breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA+U method using a Wannier functions basis set for the Bi 6s-band. Due to high oxidation state of bismuth in this material the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part a of d-band Wannier function is concentrated on metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA+U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA+U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting

First principle computation of stripes in cuprates

We present a first principle computation of vertical stripes in $La_{15/8}Sr_{1/8}CuO_4$ within the LDA+U method. We find that Cu centered stripes are unstable toward O centered stripes. The metallic core of the stripe is quite wide and shows reduced magnetic moments with suppressed antiferromagnetic (AF) interactions. The system can be pictured as alternating metallic and AF two-leg ladders the latter with strong AF interaction and a large spin gap. The Fermi surface shows warping due to interstripe hybridization. The periodicity and amplitude of the warping is in good agreement with angle resolved photoemission experiment. We discuss the connection with low-energy theories of the cuprates.Comment: 5 pages,4 figure

Coulomb Parameter U and Correlation Strength in LaFeAsO

First principles constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund's exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and corresponding bands are included, computation results in U=3-4 eV, however, with the basis set restricted to Fe-3d orbitals and bands only, computation gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local Density Approximation combined with the Dynamical Mean-Field Theory) calculation with this parameters results in weakly correlated electronic structure that is in agreement with X-ray experimental spectra

Structural relaxation due to electronic correlations in the paramagnetic insulator KCuF3

A computational scheme for the investigation of complex materials with strongly interacting electrons is formulated which is able to treat atomic displacements, and hence structural relaxation, caused by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The equilibrium Jahn-Teller distortion and antiferro-orbital order found for paramagnetic KCuF3 agree well with experiment.Comment: 4 pages, 3 figure

Calculation of the exchange constants of the Heisenberg model in the plane-wave based methods using the Green's function approach

An approach to compute exchange parameters of the Heisenberg model in plane-wave-based methods is presented. This calculation scheme is based on the Green's function method and Wannier function projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, Li2MnO3, and KCuF3. The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and KCuF3. In the case of FeO our calculations explain the pressure dependence of the N\'eel temperature. Li2MnO3 turns out to be a Slater insulator with antiferromagnetic nearest-neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling