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

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 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.Comment: Submitted to ICM 2003 Conference 3 pages, 2 figure

Triplet p-wave superconductivity in low-density extended Hubbard model with Coulomb repulsion

We analyze superconducting instabilities in 3D and 2D extended Hubbard model with Coulomb repulsion between electrons on neighboring sites in the limit of low electron density ($n_{el} \rightarrow 0$) on simple cubic (square) lattice. We show that in a realistic strong-coupling case $U\gg V\gg W$ ($U$ and $V$ are the onsite and the intersite Coulomb repulsions, $W$ the bandwidth) the main SC instability corresponds to the p-wave pairing and in the leading order is correctly described by the equations obtained earlier in the absence of the intersite Coulomb interaction V=0.Comment: 6 pages, 4 figures, accepted for JETP Letter

Lifshitz quantum phase transitions and Fermi surface transformation with hole doping in high-TcT_c superconductors

We study the doping evolution of the electronic structure in the normal phase of high-$T_c$ cuprates. Electronic structure and Fermi surface of cuprates with single CuO$_2$ layer in the unit cell like La$_{2-x}$Sr$_x$CuO$_4$ have been calculated by the LDA+GTB method in the regime of strong electron correlations (SEC) and compared to ARPES and quantum oscillations data. We have found two critical concentrations, $x_{c1}$ and $x_{c2}$, where the Fermi surface topology changes. Following I.M. Lifshitz ideas of the quantum phase transitions (QPT) of the 2.5-order we discuss the concentration dependence of the low temperature thermodynamics. The behavior of the electronic specific heat $\delta(C/T) \sim (x - x_c)^{1/2}$ is similar to the Loram and Cooper experimental data in the vicinity of $x_{c1} \approx 0.15$.Comment: 8 pages, 4 figure

Anomalous resistivity and the origin of a heavy mass in the two-band Hubbard model with one narrow band

We search for marginal Fermi-liquid behavior in the two-band Hubbard model with one narrow band. We consider the limit of low electron densities in the bands and strong intraband and interband Hubbard interactions. We analyze the influence of electron polaron effect and other mechanisms of mass-enhancement (related to momentum dependence of the self-energies) on effective mass and scattering times of light and heavy components in the clean case (electron - electron scattering and no impurities). We find the tendency towards phase-separation (towards negative partial compressibility of heavy particles) in a 3D case for large mismatch between the densities of heavy and light bands in a strong coupling limit. We also observe that for low temperatures and equal densities the resistivity in a homogeneous state R(T) ~ T^2 - behaves in a Fermi-liquid fashion both in 3D and 2D cases. For temperatures higher then effective bandwidth for heavy electrons T > Wh* the coherent behavior of heavy component is totally destroyed. The heavy particles move diffusively in the surrounding of light particles. At the same time the light particles scatter on the heavy ones as if on immobile (static) impurities. In this regime the heavy component is marginal, while the light one is not. The resistivity goes on saturation for T > Wh* in the 3D case. In 2D the resistivity has a maximum and localization tail due to weak - localization corrections of Altshuler - Aronov type. Such behavior of resistivity in 3D could be relevant for some uranium-based heavy-fermion compounds like UNi2Al3 and in 2D for some other mixed-valence compounds possibly including the layered manganites. We also consider briefly the superconductive (SC) instability in the model. The leading instability is towards p-wave pairing and is governed by enhanced Kohn - Luttinger mechanism of SC at low electron density.Comment: 37 pages, 9 figures, accepted for JET

Energy structure of high-temperature superconductors with the intersite Coulomb interaction

71.27.+a Strongly correlated electron systems; heavy fermions, 74.72.-h Cuprate superconductors, 71.28.+d Narrow-band systems; intermediate-valence solids,