The Polaron Scenario for High-Tc Superconductivity

J. Ranninger Centre de Recherches sur les Très Basses Températures, Laboratoire associé à l'Université Joseph Fourier, CNRS, BP 166, 38042 Grenoble-Cédex 9, France

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On the basis of experimental evidence we conclude that high-Tc superconducting materials are composed of two distinct subsystems: one in which electrons are bound together in form of localized pairs of small polarons (bipolarons) and one in which electrons exist as strongly correlated Fermions. Upon doping one passes at a critical doping rate from the insulating parent compounds to metallic compounds. There, a charge transfer mechanism between the two subsystems sets in abruptly by which bipolarons can decay into pairs of electrons and vice versa. Such a picture can be modelled by a mixture of interacting Bosons and Fermions. A superconducting ground state for such a system develops at low temperature which is characterized by the opening of a gap in the single electron spectrum and the appearance of collective Boson excitations with a linear spectrum which is confined to the energy regime of the gap. Above Tc, a pseudo-gap remains upon closing the superconducting gap. There are strong indications that the normal state has non Fermi liquid behaviour.

DOI: 10.12693/APhysPolA.85.89 PACS numbers: 74.20.+z, 71.38.+i, 67.20.+k