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Polaronic superconductivity in the absence of electron-hole
symmetry

Phys. Rev. B
**47**, 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 $T_c$ 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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