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Large polaron and bipolaron formation is reviewed with particular emphasis on the screening effects which become important at relatively high carrier density (plasma frequency, ω, larger than longitudinal optical frequency, ω). At high density polarons are mobile particles even at very high electron-phonon coupling constant (a»l) and bipolaron binding energy tends to vanish giving evidence of the existence of metastable bipolaron states. The coexistence of metastable bipolarons (bosons) and polarons (fermions) supports the so-called boson-fermion model of superconductivity. This model can be generalized in order to take into account the dependence on the carrier density given in the polaron-bipolaron framework. The results are in good agreement with the density dependence of the critical temperature exhibited by many high- T superconductors. [ABSTRACT FROM AUTHOR]

The existence of pair excitations in a Fermi gas interacting via a short-range attractive potential is investigated. Within the ladder approximation to the Bethe-Salpeter equation for the effective two-particle interaction, evidence of pair excitation is found at energies slightly larger than the chemical potential. The link between those excitations and a boson-fermion model of superconductivity ( Phys. Lett. A, 196 (1995) 359) is discussed. In particular it is shown that the charge carrier density dependence of the pair excitation (boson) energy, assumed phenomenologically in the boson-fermion model, is consistent with the properties of the interacting Fermi gas studied. These results give support to the microscopic origin of the phenomenological boson-fermion model of superconductivity. [ABSTRACT FROM AUTHOR]

The screening effects of the formation of large bipolarons have been studied. It is shown that the dependence of the bipolaron binding energy on the electron density does not change qualitatively by using different dielectric functions for describing electron screening. The same result is found if the electron system is described as a bipolaron gas (charged bosons). [ABSTRACT FROM AUTHOR]

The mobility of the Fröhlich bipolarons is a challenging matter for the bipolaronic theory of high- T superconductivity. Here we focus our attention on the problem of the effective mass of a bipolaron in a strongly ionic system (like the new H(igh) T S(uper)C(onductors) are) and in the presence of an electron gas. We show that there is a sizeable renormalization of the bipolaron mass due to the polarization effects of both the optical phonons and plasmons. Moreover, we show that the effective mass decreases increasing the density of carriers, because the screening effects become more important. In all cases, the values of the bipolaron effective mass are not greater than some tens of electronic masses ( m ). [ABSTRACT FROM AUTHOR]

We calculate the binding energy of a two-dimensional exciton for a set of states labelled by the quantum number m associated to the angular momentum in the direction perpendicular to the surface. The Fröhlich electron-phonon and hole-phonon interactions are taken into account. The state m=0 is more bound with respect to that obtained by the Wannier exciton theory with the screening given by the static dielectric constant and the reduced mass calculated by the electron and hole polaronic masses. The opposite effect is found for the state m=1,2,... When m becomes large, the hydrogenic series is recovered with the screening and reduced mass defined above. Our results are compared with the experimental data concerning exciton luminescence GaAs/AlGaAs, CuCl/CaF and CdTe/CdZnTe quantum wells. [ABSTRACT FROM AUTHOR]

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: D is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: D is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: D is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento della Societa Italiana di Fisica: D is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)