Electron-phonon interaction in a strongly correlated Hubbard system
ABSTRACT The electron-phonon (local) interactions have been considered in a single-band Hubbard model with strong on-site correlation. It has been shown that when no holes are present (i.e., one electron per site) the ground state of the system corresponds to the conventional coherent state of the phonon subsystem and the polaron has high effective mass, wheras for non-zero hole concentration the two-phonon coherent state of the phonon subsystem corresponds to the ground state of the system and the effective mass of the resulting squeezed polaron is reduced. If the superconductivity is due to Bose condensation of bipolarons the effective mass of the bisqueeps (squeezed bipolarons) at appropriate hole concentration may be reduced by 100 times or more in comparison to the conventional bipolarons and the corresponding Bose condensation temperature would be high.
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ABSTRACT: Cooperative properties of self-localized carriers on a lattice are discussed and reviewed at a fairly basic level with an emphasis on developments of the strong-coupling theory of superconductivity for the interpretation of high-Tc superconductors. Small polaron and bipolaron formation is shown to provide a number of new physical phenomena both in the normal and superconducting states. Two mechanisms of superconductivity are discussed in detail. The first one arises from the Cooper pairing of small polarons in momentum space (polaronic superconductivity) while the second one is due to polaron pairing in real space and analogous to the superfluidity of 4He (bipolaronic superconductivity). Highly non-adiabatic motion of bipolarons results in fundamental differences of bipolaronic superconductivity with respect to the BCS one including its well known strong-coupling generalization. The review covers the theoretical development and some experimental results in the past decade paying special attention to the physical properties of high-Tc oxides and their explanation with (bi)polarons. Basic properties of charged bosons are also considered.Reports on Progress in Physics 12/1998; 57(12):1197. · 13.23 Impact Factor
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ABSTRACT: The Holstein-Hubbard model serves as a useful framework to investigate this interplay between the phonon-induced electron-electron attractive interaction and the direct Coulomb repulsion and can afford interesting phase diagrams due to competition among charge-density wave (CDW), spin-density wave (SDW), and superconductivity. However the detailed nature of the CDW-SDW transition is still not very well known. It is generally believed that the system undergoes a direct insulator to insulator transition from CDW to SDW with the increase of the on-site Coulomb repulsion for a given strength of the electron-phonon coupling and this is the main bottleneck for the polaronic/bipolaronic mechanism of high-temperature superconductivity. We have recently made an investigation to study the nature of the transition from SDW phase to CDW phase within the framework of a one-dimensional Holstein-Hubbard model at half-filling using a variational method. We find that an intervening metallic phase may exist at the crossover region of the CDW-SDW transition. We have also observed that if the anharmonicity of the phonons is taken into account, this metallic phase widens and the polarons become more mobile, which is a more favorable situation from the point of view of superconductivity. We shall finally show that an improved variational calculation widens the metallic phase and makes the polarons more mobile, which reconfirms the existence of the intermediate metallic phase at the SDW-CDW crossover region.Advances in Condensed Matter Physics 01/2010; · 1.18 Impact Factor
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ABSTRACT: In order to get an insight into the nature of superconductivity in high-Tc copper oxide superconductors, we have studied the fermion pairing of the s- and d-wave-types with an extended Hubbard model for an anisotropic three-dimensional system for different values of the anisotropy parameter r, which is a measure of increased coupling between the planes. At low hole concentration the superconducting transition temperature Tc is higher for the s wave and decreases with increasing r. For higher hole concentration the d wave is stable and the corresponding Tc is weakly dependent on r. Comparison of Tc’s for the s-wave pairing and the boson-condensation temperature for varying r shows that there may be a crossover from fermion pairing to boson condensation as r increases.Physical review. B, Condensed matter 04/1991; 43(7):5606-5611. · 3.77 Impact Factor