Coulomb repulsion and correlation strength in LaFeAsO from density functional and dynamical mean-field theories.

The LDA+DMFT (local density approximation combined with dynamical mean-field theory) computation scheme has been used to calculate spectral properties of LaFeAsO-the parent compound of the new high-T(c) iron oxypnictides. The average Coulomb repulsion [Formula: see text] and Hund's exchange J parameters for iron 3d electrons were calculated using the first-principles constrained density functional theory scheme in the Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in the calculations: when the full set of O-2p, As-4p and Fe-3d orbitals and the corresponding bands are included, the interaction parameters [Formula: see text] eV and J = 0.8 eV are obtained. In contrast, when the basis set is restricted to the Fe-3d orbitals and bands only, the calculation gives much smaller values of [Formula: see text] eV, J = 0.5 eV. Nevertheless, DMFT calculations with both parameter sets and the corresponding basis sets result in a weakly correlated electronic structure that is in agreement with the experimental x-ray and photoemission spectra.

Hellmann-Feynman forces within the DFT + U in Wannier functions basis.

The most general way to describe localized atomic-like electronic states in strongly correlated compounds is to use Wannier functions. In the present paper we continue development of widely-used DFT + U method with the Wannier function basis set and propose a technique to calculate Hubbard contribution to atomic forces. The technique was implemented as a part of plane-waves pseudopotential code Quantum-ESPRESSO and tested on two compounds: charge transfer insulator NiO with cubic crystal structure and correlated metal SrVO3 with perovskite structure.

Correlation effects and phonon modes softening with doping in Ba1-xKxBiO3.

The monoclinic crystal structure of the undoped BaBiO3 can be described as a cubic perovskite which is distorted by the frozen breathing and tilting phonon modes of the BiO6 octahedra. The phonon mode softening is experimentally observed (Braden et al 1996 Europhys. Lett. 34 531) in Ba1-xKxBiO3 through potassium doping followed by a transition into an ideal cubic perovskite structure at x = 0.37, close to the emergence of superconductivity. In our previous paper (Korotin et al 2012 J. Phys.: Condens. Matter 24 415603) we demonstrated that it is necessary to take into account correlation effects using the DFT+U method in Wannier functions as a basis to obtain a good agreement between the calculated and experimental values of crystal structure distortion and the energy gap in BaBiO3. In the present work, using the same method, we calculated the breathing mode phonon frequencies as a function of the potassium doping level in Ba1-xKxBiO3. The obtained frequencies are in good agreement with experimental values and the breathing mode softening with doping is reproduced, contrary to calculations made without consideration of correlation effects. We show that the cubic crystal structure becomes stable at x = 0.30 in agreement with the experimental transition to cubic perovskite at x = 0.37. The possible connections between the correlation effects, phonon mode softening and superconductivity in Ba1-xKxBiO3 are discussed.

Pressure-driven metal-insulator transition in hematite from dynamical mean-field theory.

The local density approximation combined with dynamical mean-field theory is applied to study the paramagnetic and magnetically ordered phases of hematite Fe2O3 as a function of volume. As the volume is decreased, a simultaneous first-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 spectral gap with increasing pressure, upon closing of which the high-spin phase becomes unstable. We conclude that the transition in Fe2O3 at approximately 50 GPa can be described as an electronically driven volume collapse.