Effect of screening of the electron-phonon interaction on the temperature of Bose-Einstein condensation of intersite bipolarons

Here we consider an interacting electron-phonon system within the framework of extended Holstein-Hubbard model at strong enough electron-phonon interaction limit in which (bi)polarons are the essential quasiparticles of the system. It is assumed that the electron-phonon interaction is screened and its potential has Yukawa-type analytical form. An effect of screening of the electron-phonon interaction on the temperature of Bose-Einstein condensation of the intersite bipolarons is studied for the first time. It is revealed that the temperature of Bose-Einstein condensation of intersite bipolarons is higher in the system with the more screened electron-phonon interaction.Comment: 6 pages, 4 figure

Polaron features for long-range electron-phonon interaction

The polaron features for long-range electron-phonon interaction are investigated by extending a variational approach previously proposed for the study of systems with local coupling. The ground-state spectral weight, the average kinetic energy, the mean number of phonons, and the electron-lattice correlation function are discussed for a wide range of model parameters focusing on the adiabatic regime and comparing the results with the short-range case (Holstein model). A strong mixing of electronic and phononic degrees of freedom for small values of the electron-phonon coupling constant is found in the adiabatic case due to the long-range interaction. Finally a polaron "phase diagram" is proposed.Comment: 4 figs., to appear in J. Phys.:Condens. Matte

Electronic specific-heat anomalies in high-\u3ci\u3eT\u3csub\u3ec\u3c/sub\u3e\u3c/i\u3e cuprates

In this work, we study the electronic specific heat $C_e(T)$ of underdoped to overdoped high-$T_c$ cuprates, and identify the nature of anomalies in $C_e(T)$ at the superconducting transition temperature $T_c$ and at temperatures above $T_c$. The doped cuprate superconductor is considered as a multi-carrier model system which is composed of different types of charge carriers. The normal-state electronic specific heat $C_n(T)$ of high-$T_c$ cuprates below a characteristic pseudogap (PG) temperature $T^*$ is calculated taking into account three contributions coming from the excited components of Cooper pairs, the ideal Bose-gas of incoherent Cooper pairs and the unpaired carriers in the impurity band. Above $T^*$, two contributions to $C_n(T)$ coming from the unpaired intrinsic and extrinsic polarons are calculated within the two-component degenerate Fermi-gas model. The total electronic specific heat $C_e(T)=C_n(T)+C_s(T)$ below $T_c$ is calculated by considering the contribution $C_n(T)$ and the contribution $C_s(T)$ coming from the superfluid bosonic carriers. We have shown that our theoretical predictions of the behaviors of $C_e(T)$ near $T_c$ and above $T_c$ are strikingly similar to the behaviors of the electronic specific heat observed below and above $T_c$ in LSCO and YBCO. There is fair quantitative agreement between theoretical predictions about the anomalies in $C_e(T)$ (i.e. a $\lambda$-like anomaly near $T_c$ and a BCS-type anomaly above $T_c$ near $T^*$) and experimental data

Effect of screening of the electron-phonon interaction on mass renormalization and optical conductivity of the extended Holstein model polarons

An interacting electron-phonon system is considered within the Extended Holstein model at strong coupling regime and nonadiabatic approximation. It is assumed that screening of an electron-phonon interaction is due to the excess electrons in a lattice. An influence of the screening on the mass and optical conductivity of a lattice polarons is studied. A more general form Yukawa-type electron-phonon interaction potential potential is accepted and corresponding forces are derived in a lattice. It is emphasized that the screening effect is more pronounced at the values of screening radius comparable with a lattice constant. It is shown that the mass of a lattice polaron obtained using Yukawa-type electron-phonon interaction potential is less renormalized than those of the early studied works at the same screening regime. Optical conductivity of lattice polarons is calculated at different screening regimes. The screening lowers the value of energy that corresponds to the peak of the optical conductivity curve. The shift (lowering) is more pronounced at small values of screening radius too. The factors that give rise to this shift is briefly discussed.Comment: 8 pages, 10 figures, 1 table. arXiv admin note: substantial text overlap with arXiv:1308.219