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: 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: The optical phonons in semimetals, semiconductors, and superconductors were studied by the light reflection techniques with femtosecond time resolution and by the method of spontaneous Raman scattering. During measurements in the time domain, the phonon system is converted into a coherent state by the first ultrashort laser pulse and then probed at a variable delay by the second pulse. In this case, the phonons are shown to occur in a nonclassical state in which their fluctuational properties, different in various quadratures, are described by periodic functions of time. A comparison of the results obtained in the time and frequency domains gives evidence that the energies of thermal and coherent phonons coincide, while their dephasing and energy relaxation times are different.Journal of Experimental and Theoretical Physics 01/2001; 92(2):246-259. · 0.92 Impact Factor
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ABSTRACT: We have excited a coherent phonon field in the single crystal of YBa2Cu3O7-delta using femtosecond laser pulses and observed that the field exhibits phase-dependent fluctuation properties. The variance of phonon amplitude is a function of time delay and the oscillations of the coherent phonon and its variance occur at different frequencies. The dominant frequency of the coherent filed (/~ 4.52 THz) is that of the Ag phonon generated by the z-displacement of Cu ions belonging to the CuO2 planes. The squeezed field involves pairs of the phonons and it appears as a peak in the Fourier transformed variance at twice the phonon frequency.Physics Letters A 01/2000; 269:97-102. · 1.77 Impact Factor