Mechanism of magnetic moment collapse under pressure in ferropericlase.

We propose a new scenario for the magnetic collapse under pressure in ferropericlase (FP) (Fe(1/4)Mg(3/4))O without the presence of intermediate spin state, which contradicts the mechanism proposed in (2013 Phys. Rev. B 87 165113). This scenario is supported by results of combined local density approximation + dynamical mean-field theory method calculations for the paramagnetic phase at ambient and high pressures. At ambient pressure, calculation gave (Fe(1/4)Mg(3/4))O as an insulator with Fe 3d-shell in high-spin state. Experimentally observed high-spin to low-spin state transition of the Fe(2+) ion in the pressure range of 35-75 GPa is successfully reproduced in our calculations. The spin crossover is characterized by coexistence of Fe(2+) ions in high and low spin state but intermediate spin state is absent in the whole pressure range. Moreover, the probability of Fe ion d(7) onfiguration with S = 1 grows with pressure due to shortening of metal-oxygen distance. Also, no metal-insulator transition was obtained up to the pressure 140 GPa in agreement with experiment.

Investigation of electronic structure and magnetic properties of CaCo1.86As2 within the CPA method.

Recently in iron free arsenide compound CaCo(2)As(2) a 7(1)% of vacancies on the Co sites was detected (Quirinale D G et al 2013 Phys. Rev. B 88 174420). Here we report the investigation of electronic structure and magnetic properties of CaCo(1.86)As(2) within the coherent potential approximation (CPA). First, the CPA calculations are performed on the base of the local spin density approximation. Second, the possible role of Coulomb correlations is checked within the CPA scheme developed recently for strongly correlated systems. Then the spin-orbit coupling, which could be essential for Co, is also taken into account within the CPA calculation. The A type antiferromagnetic ground state and the value of magnetic moment obtained within the CPA approximation are in good agreement with experiment.

The coherent potential approximation for strongly correlated systems: electronic structure and magnetic properties of NiO-ZnO solid solutions.

A method of electronic structure calculations for strongly correlated disordered materials is developed employing the basic idea of the coherent potential approximation. The evolution of the electronic structure and spin magnetic moment value with the concentration x in strongly correlated Ni1-xZnxO solid solutions is investigated in the framework of this method. The values of the energy gap and magnetic moment obtained are in agreement with the available experimental data.

Effect of 3d doping on the electronic structure of BaFe2As2.

The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2.

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p(3/2) core level energies, 2p x-ray photoelectron spectra (XPS) and L(3,2) x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn(0.2)TiSe(2) were studied for the first time. The ionic-covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L(3,2) XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 +. In ResPES of Mn(0.2)TiSe(2) excited near the Ti 2p(3/2) and Mn 2p(3/2) absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K-Γ-M in the Brillouin zone of the crystal.

Structural models of FeSe(x).

Two different structural models for non-stoichiometric FeSe(x) are examined and compared with soft x-ray spectroscopy findings for FeSe(x) (x = 0.85, 0.50). A structural model of tetragonal FeSe with excess interstitial Fe gives better agreement with experiment than a structural model of tetragonal FeSe with Se vacancies. This interstitial Fe increases the number of 3d states at the Fermi level. We find evidence that large non-stoichiometric ratios of Fe:Se, such as that of FeSe(0.50), yield clusters of pure Fe in the crystal structure.

Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy.

Resonant x-ray emission spectroscopy (XES) measurements at Fe L(2,3) edges and electronic structure calculations for LiFeAs and NaFeAs are presented. Experiment and theory show that in the vicinity of the Fermi energy, the density of states is dominated by contributions from Fe 3d states. The comparison of Fe L(2,3) XES with spectra of related FeAs compounds reveals similar trends in energy and the ratio of intensities of the L(2) and L(3) peaks (I(L(2))/I(L(3)) ratio). The I(L(2))/I(L(3)) ratio for all FeAs-based superconductors is found to be closer to that of metallic Fe than that of the strongly correlated FeO. We conclude that iron-based superconductors are weakly or, at most, moderately correlated systems.

Band structure approach to resonant x-ray scattering.

We study the resonance behavior of the forbidden 600 and 222 x-ray Bragg peaks in Ge. These peaks remain forbidden in the resonant dipole scattering approximation, even taking into account the nonlocal nature of the band states. However, they become allowed at resonance if the eigenstates of the unoccupied conduction band involve a hybridization of p-like and d-like atomic states. We show that the energy dependence of the resonant behavior, including the phase of the scattering, is a direct measure of this p-d hybridization and obtain quantitative agreement with experiment. We discuss the implications of this to other materials like V2O3 in which the resonating atom is not at a center of inversion symmetry.