A. O. Shorikov

Magnetic state and electronic structure of the δ and α phases of metallic Pu and its compounds

By LDA+U method with spin-orbit coupling (LDA+U+SO) the magnetic state and electronic structure have been investigated for plutonium in \delta and \alpha phases and for Pu compounds: PuN, PuCoGa5, PuRh2, PuSi2, PuTe, and PuSb. For metallic plutonium in both phases in agreement with experiment a nonmagnetic ground state was found with Pu ions in f^6 configuration with zero values of spin, orbital, and total moments. This result is determined by a strong spin-orbit coupling in 5f shell that gives in LDA calculation a pronounced splitting of 5f states on f^{5/2} and f^{7/2} subbands. A Fermi level is in a pseudogap between them, so that f^{5/2} subshell is already nearly completely filled with six electrons before Coulomb correlation effects were taken into account. The competition between spin-orbit coupling and exchange (Hund) interaction (favoring magnetic ground state) in 5f shell is so delicately balanced, that a small increase (less than 15%) of exchange interaction parameter value from J_H=0.48eV obtained in constrain LDA calculation would result in a magnetic ground state with nonzero spin and orbital moment values. For Pu compounds investigated in the present work, predominantly f^6 configuration with nonzero magnetic moments was found in PuCoGa5, PuSi2, and PuTe, while PuN, PuRh2, and PuSb have f^5 configuration with sizeable magnetic moment values. Whereas pure jj coupling scheme was found to be valid for metallic plutonium, intermediate coupling scheme is needed to describe 5f shell in Pu compounds. The results of our calculations show that both spin-orbit coupling and exchange interaction terms in the Hamiltonian must be treated in a general matrix form for Pu and its compounds.

Orbital-selective pressure-driven metal to insulator transition in FeO from dynamical mean-field theory

In this Letter we report the first LDA+DMFT (method combining Local Density Approximation with Dynamical Mean-Field Theory) results of magnetic and spectral properties calculation for paramagnetic phases of FeO at ambient and high pressures (HP). At ambient pressure (AP) calculation gave FeO as a Mott insulator with Fe 3d-shell in high-spin state. Calculated spectral functions are in a good agreement with experimental PES and IPES data. Experimentally observed metal-insulator transition at high pressure is successfully reproduced in calculations. In contrast to MnO and Fe2O3 (d 5 configuration) where metal-insulator transition is accompanied by high-spin to low-spin transition, in FeO (d configuration) average value of magnetic moment √

Coulomb correlation effects in LaFeAsO: An LDA + DMFT(QMC) study

Effects of Coulomb correlation on the LaFeAsO electronic structure are investigated by the LDA + DMFT(QMC) method (combination of the local density approximation with the dynamic mean-field theory; impurity solver is a quantum Monte Carlo algorithm). The calculation results show that LaFeAsO is in the regime of intermediate correlation strength with a significant part of the spectral density moved from the Fermi energy to the Hubbard bands and far from the edge of the metal-insulator transition. Correlations affect iron d-orbitals differently. The t2g states (xz, yz and x2 − y2 orbitals) have a higher energy due to crystal field splitting and are nearly half-filled. Their spectral functions have a pseudogap with the Fermi level position on the higher subband slope. The lower energy eg set (xy and 3z2 − r2 orbitals) have occupancies significantly larger than 1/2 with typically metallic spectral functions.

Orbital-selective formation of local moments in α -iron: First-principles route to an effective model

We revisit a problem of theoretical description of alpha-iron. By performing LDA+DMFT calculations in the paramagnetic phase we find that Coulomb interaction and, in particular Hund exchange, yields the formation of local moments in e_g electron band, which can be traced from imaginary time dependence of the spin-spin correlation function. This behavior is accompanied by non-Fermi-liquid behavior of e_g electrons and suggests using local moment variables in the effective model of iron. By investigating orbital-selective contributions to the Curie-Weiss law for Hund exchange I=0.9 eV we obtain an effective value of local moment of e_g electrons 2p=1.04 mu_B. The effective bosonic model, which allows to describe magnetic properties of iron near the magnetic phase transition, is proposed.

Crystal-field splitting for low symmetry systems in ab initio calculations

In the framework of the LDA+U approximation we propose the direct way of calculation of crystal-field excitation energy and apply it to La and Y titanates. The method developed can be useful for comparison with the results of spectroscopic measurements because it takes into account fast relaxations of electronic system. For titanates these relaxation processes reduce the value of crystal-field splitting by similar to 30% as compared with the difference of LDA one electron energies. However, the crystal-field excitation energy in these systems is still large enough to make an orbital liquid formation rather unlikely and experimentally observed isotropic magnetism remains unexplained.

Density-functional calculation of the Coulomb repulsion and correlation strength in superconducting LaFeAsO

V. I. Anisimov, Dm. M. Korotin, S. V. Streltsov, A. V. Kozhevnikov, 2 J. Kuneš, A. O. Shorikov, and M. A. Korotin Institute of Metal Physics, Russian Academy of Sciences, 620041 Yekaterinburg GSP-170, Russia Joint Institute for Computational Sciences, Oak Ridge National Laboratory P.O. Box 2008 Oak Ridge, TN 37831-6173, USA Theoretical Physics III, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Augsburg 86135, GermanyConstrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund’s exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3–4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F0 = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure.

First-order transition between a small gap semiconductor and a ferromagnetic metal in the isoelectronic alloy FeSi1-xGex.

The contrasting ground states of isoelectronic, isostructural FeSi and FeGe are explained within an extended local density approximation scheme (LDA+U) by an appropriate choice of the Coulomb repulsion U on the Fe sites. A minimal two-band model with interband interactions leads to a phase diagram for the alloys FeSi1-xGex. A mean field approximation gives a first-order transition between a small gap semiconductor and a ferromagnetic metal as a function of magnetic field, temperature, and concentration x. Unusually the transition from metal to insulator is driven by broadening, not narrowing, the bands and it is the metallic state that shows magnetic order.

Metal-insulator transitions and magnetism in correlated band insulators: FeSi and Fe 1 − x Co x Si

The

LDA+DMFT spectral functions and effective electron mass enhancement in the superconductor LaFePO

\text{LDA}+\text{DMFT}

Electronic structure and magnetic state of transuranium metals under pressure

(local density approximation combined with dynamical mean-field theory) computation scheme has been used to study spectral and magnetic properties of FeSi and