Sergei G. Ovchinnikov

The mechanism of the electronic transition in ferroborates under high pressure

A novel mechanism for the insulator–semiconductor transition and magnetic collapse in FeBO3 is proposed in the framework of the multielectron model with account taken of strong electron correlations. The electronic transition results from the crossover of the high spin and low spin Fe3+ states induced by the crystal field increasing with pressure. In the high pressure phase a semiconductor–metal transition is expected.

Prediction of the in-gap states above the top of the valence band in undoped insulating cuprates due to the spin-polaron effect

In the framework of the generalized tight binding method we have calculated the quasiparticle band structure and the spectral functions of the undoped cuprates such as La2CuO4, Sr2CuO2Cl2 etc. Due to spin fluctuations the in-gap state appears above the top of the valence band in the undoped antiferromagnetic insulator similar to in-gap states induced by hole doping. In the ARPES experiments the in-gap states can be detected as weak low energy satellites.

Theory of X-ray absorption spectra of strongly correlated copper oxides

Abstract It has been shown that theoretical X-ray absorption spectra of highly correlated systems can be presented as a product of a single-electron part obtained by the self-consistent field Xα-scattered wave (SCFXα-SW) method, and of a multi-electron part obtained by exact diagonalization of the Hamiltonian of the multi-band multi-electron p-d model. Using that model, the influence of strong correlation effects on the Cu K- and CuL2,3-spectra of La2−xSrxCuO4 (x = 0, 0.2, 1) has been studied. In terms of that model, the main peak of the Cu K-spectrum for x = 0 was assigned to the Cu d 10 L - configuration and only one satellite was assigned to the Cud9-configuration. Comparison of the theoretical with the experimental data shows that the ground state of the two-holes in the CuO4 cell is triplet. In that case additional satellites of Cu d 9 L - and Cud8-configurations are observed. The same conclusions have been made concerning polarized CuL2,3-spectra for the fully doped LaSrCuO4 excluding the peak with the energy 2.8 eV above the threshold, which was assigned to the transitions into quasi-stationary states due to existence of a high barrier in the Cud-state Hartree-Fock potential.

Phonon density of states in γ-, β-, and α-AgCuS

The ternary superionic conductor AgCuS was studied by means of inelastic neutron scattering at temperatures of 150 to 398 K. The experimental time-of-flight spectra were recalculated into the generalized phonon density of states G(ɛ) in an incoherent approximation. It is shown that G(ɛ) of AgCuS has a nontrivial temperature dependence over the low-energy range. The relation between the temperature dependence of G(ɛ) and the existence of a low-energy mode in AgCuS is discussed.

Electron spectral density of the half-filled Hubbard model in the atomic limit at finite temperature

We have calculated the spectral function and density of states of halffilled two-dimensional Hubbard model in the Hubbard-I approximation assuming an antiferromagnetic long range order at low temperature and compared results to the QMC data. It occurs that calculated functions are in a qualitative agreement with the QMC one. We have also shown that Neel ordered state dispersion has the similar form to the spin density wave one.

Influence of the type of dopant on the Néel temperature of copper oxides

Abstract The similarity of the magnetic properties of La 2 Cu 1− x Zn x O 4 and Pr 2− x Ce x CuO 4 is explained by the peculi arities of the electronic energy structure of strongly correlated electrons of the CuO 2 layers. The covalent admixture of oxygen states to the upper Hubbard band results in a small difference in the slope of T N ( x ) dependence in both systems.

Quasiparticle band structure, spectral weights and degree ofd-electron localization in iron silicides

Quasiparticle hand structure, spectral weights and degree of d-electron localization in Iron Silicides

Spin fluctuations influence on quasiparticle spectrum of realistic p–d model

Abstract In the present work the multiband p–d model for CuO 2 -layer is treated. It was shown that for the realistic set of parameters besides Zhang–Rice two-particle singlet state there is non-negligible contribution of two-particle triplet state to the top of the valence band. Also shown, that to gain quantitative agreement with experimental data the minimal approximation should include the spin fluctuations beyond the Hubbard-I scheme. Quasiparticle spectrum, obtained in this approximation, is in fairly good agreement with ARPES data on Bi2212 High— T c compound.