The pseudogap in the boson-fermion model for high-T-c superconductivity

High-T-c superconductors are considered as composite systems made up of iti nerant electrons and localized bipolarons trapped in unstable ligand config urations, responsible for triggering a charge exchange between the two spec ies. As a consequence, the intrinsically localized bipolarons become itiner ant upon lowering the temperature and eventually condense into a superfluid state. Concomitantly a pseudogap in the density of states of the electrons opens up, deepens upon decreasing the temperature and finally develops int o a true gap as superconductivity sets in. The photo-emission spectral prop erties exhibit a large, broad, incoherent contribution coming from the phon on shake-off of the bipolarons. (C) 1998 Elsevier Science Ltd. All rights r eserved.