V. A. Gavrichkov

Evolution of the band structure of quasiparticles with doping in copper oxides on the basis of a generalized tight-binding method

Two methods for stabilizing the two-hole 3B1g state as the ground state instead of the Zhang-Rice singlet are determined on the basis of an orthogonal cellular basis for a realistic multiband pd model of a CuO2 layer and the dispersion relations for the valence band top in undoped and doped cases are calculated. In the undoped case, aside from the valence band, qualitatively corresponding to the experimental ARPES data for Sr2CuO2Cl2 and the results obtained on the basis of the t-t′-J model, the calculations give a zero-dispersion virtual level at the valence band top itself. Because of the zero amplitude of transitions forming the virtual level the response corresponding to it is absent in the spectral density function. In consequence, the experimental ARPES data do not reproduce its presence in this antiferromagnetic undoped dielectric. A calculation of the doped case showed that the virtual level transforms into an impurity-type band and acquires dispersion on account of the nonzero occupation number of the two-hole states and therefore should be detected in ARPES experiments as a high-energy peak in the spectral density. The computed dispersion dependence for the valence band top is identical to the dispersion obtained by the Monte Carlo method, and the ARPES data for optimally doped Bi2Sr2CaCu2O8+δ samples. The data obtained also make it possible to explain the presence of an energy pseudogap at the symmetric X point of the Brillouin band of HTSC compounds.

Influence of two-particle excited states on the interatomic exchange interaction in La2CuO4

The effective spin Hamiltonian for undoped cuprates is constructed in the framework of the realistic multiband p-d model with the parameters calculated from first principles. The exchange interaction parameter is defined as the sum of the antiferromagnetic and ferromagnetic contributions, which are determined by the two-hole triplet terms. The ferromagnetic and antiferromagnetic contributions of the excited terms compensate each other to a large extent. It is shown that the antiferromagnetic contribution of the two-hole ground singlet 1A1g to the exchange interaction is dominant.

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.

Temperature and concentration dependences of the electronic etructure of copper oxides in the generalized tight binding method

The electronic structure of p-type doped HTSC cuprates is calculated by explicitly taking into account strong electron correlations. The smooth evolution of the electronic structure from undoped antiferro-magnetic to optimally and heavily doped paramagnetic compositions is traced. For a low doping level, in-gap impurity-type states are obtained, at which the Fermi level is pinned in the low-doping region. These states are separated by a pseudogap from the valence band. The Fermi surfaces calculated for the paramagnetic phase for various concentrations of holes are in good agreement with the results of ARPES experiments and indicate a gradual change in the Fermi surface from the hole type to the electron type.

Comparison of the single-electron and many-electron mechanisms of the concentration dependence of the HTSC cuprate band structure

The band structure of the Nd2 − xCexCuO4HTSC is calculated using the LDA+GTB method, which combines the local density approximation (LDA) and the generalized tight-binding (GTB) method. Two mechanisms of the concentration dependence of the band structure (single-electron and many-electron mechanisms) are taken into account. It is demonstrated that the main contribution to the evolution of the band structure with doping comes from the many-electron mechanism.

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO3 and slightly doped La1−xSrxMnO3 manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO3 ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La1−xSrxMnO3. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).

The band structure of n-type cuprate superconductors with the T′(T) structure taking into account strong electron correlation

The spectral density, dispersion relations, and the position of the Fermi level for n-doped compositions based on NCO and LCO were calculated within the framework of the generalized tight binding method. As distinguished from LCO, the dielectric gap in NCO is nonlinear in character. We observe a virtual level both at the bottom of the conduction band and at the top of the valence band in both compounds. However, its position corresponds to the extreme bottom of the conduction band in LCO and is 0.1–0.2 eV above the bottom in NCO. This explains why we observe Fermi level pinning in n-LCO as the concentration of the doping component grows and reproduce its absence in NCCO at low doping values. We also found both compositions to be unstable in a narrow concentration range with respect to a nonuniform charge density distribution. The relation between the phase diagram for NCCO and the calculated electronic structure is discussed.

Electronic structure of p-type La1 − xMx2+MnO3 manganites in the ferromagnetic and paramagnetic phases in the LDA + GTB approach

The band structure, spectral intensity, and position of the Fermi level in doped p-type La1 − xMx/2+MnO3 manganites (M = Sr, Ca, Ba) is analyzed using the LDA + GBT method for calculating the electronic structure of systems with strong electron correlations, taking into account antiferro-orbital ordering and using the Kugel-Khomskii ideas and real spin S = 2. The results of the ferromagnetic phase reproduce the state of a spin half-metal with 100% spin polarization at T = 0, when the spectrum is of the metal type for a quasiparticle with one spin projection and of the dielectric type for the other. It is found that the valence band becomes approximately three times narrower upon a transition to the paramagnetic phase. For the paramagnetic phase, metal properties are observed because the Fermi level is located in the valence band for any nonzero x. The dielectrization effect at the Curie temperature is possible and must be accompanied by filling of dx orbitals upon doping. The effect itself is associated with strong electron correlations, and a complex structure of the top of the valence band is due to the Jahn-Teller effect in cubic materials.

A simple metal–insulator criterion for the doped Mott–Hubbard materials

Abstract A simple metal–insulator criterion for doped Mott–Hubbard materials has been derived. Its readings are closely related to the orbital and spin nature of the ground states of the unit cell. The available criterion readings (metal or insulator) in the paramagnetic phase reveal the possibility of the insulator state of doped materials with the forbidden first removal electron states. According to its physical meaning, the result is similar to the Wilson׳s criterion in itinerant electron systems. The application of the criterion to high- T c cuprates is discussed.