Fermion local charged Boson model and cuprate superconductors

Abstract

One of the most exciting developments in Science in past few years is the discovery of high temperature superconductivity (HTSC) in cuprates. It has been observed that the superconducting state in these cuprates is rather "normal" compared to the "anomalous" normal state. This discovery has led to a deluge of experimental and theoretical researches all along the world. These cuprates are close to metal-insulator transition and the stability of the insulating and metallic phase depends on the degree of doping. Measurements of physical properties of these systems have revealed many anomalous results both in the superconducting and normal states, e.g. d-wave superconduting gap, the presence of pseudogap in the normal state, static or dynamic striped structure of CuO₂ planes etc. These have posed serious theoretical challenges towards formulating the mechanisms of pairing and explanation of anomalous behaviour. Several theoretical proposals have been advanced and only a few are likely to survive in the teeth of some reliable experimental data. A combined mechanism mediated by phonons and lochons (local charged bosons, local pairs or bipolarons) for the pairing of fermions (holes or electrons) belonging to a wide banci provides a microscopic explanation of anomalous normal state properties of HTSC cuprates and vindicates features of the phenomenological marginal Fermi liquid formulation. In the present review article detailed features of combined lochon and phonon mediated pairing mechanism are presented and a contact with the normal and superconducting state properties of HTSC cuprates have been made). With reasonable agreement with available experimental data, the combined pairing mechanism emerges as a strong contender for an acceptable model for HTSC cuprates.