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Polaronic superconductivity in the absence of electron-hole symmetry

Phys.Rev. B47, 5351 (1993)
A major drawback of theories of superconductivity based on small polarons has been that the effective mass of the
carriers becomes extremely large in the parameter regime where the effective interaction is attractive. An implicit
assumption in these theories has been the existence of electron-hole symmetry. We consider here the Holstein model for
small polarons and show that inclusion of electron-hole symmetry-breaking perturbations leads to a pairing mechanism
that circumvents the above-mentioned difficulty. Such perturbations arise from a dependence of coupling constant and
vibrational frequency on the density of carriers, as well as from anharmonicity in the vibrational potential. The possibility
of using such a polaronic model arising from purely electron-phonon interactions to describe superconductivity in
high-Tc oxides is considered. It is concluded that experimental evidence disfavors it, while it favors an electron-hole
asymmetric small-polaron model arising from excitations with energies of electronic scale together with a small admixture
of electron-phonon effects.

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