Abstract

There is ample experimental evidence for localized polaronic charge carriers in high-Tc materials in the insulating phase as well as in the metallic phase at high temperatures. This would rule out a priori any condensation of bipolarons, since for that purpose they should be in free-particle-like states in the longwavelength limit. Yet, provided that the localized bipolarons hybridize with a band of itinerant electrons, such a mixture of Bosons (bipolarons) and Fermion pairs (pairs of conduction electrons) can undergo an instability towards a superconducting ground state in which at high temperatures the initially localized bipolarons become superfluid upon lowering of the temperature. The experimental situation leading up to such a picture and its physical consequences are discussed.

Introduction

The large values of the critical temperature Tc, the small number of charge carriers together with the short coherence length, the strong dependence of Tc on n/m (n being the carrier concentration and m their mass) and the large temperature regime near Tc (with a Ginzburg temperature of TG 0.1−0.01) controlled by X−Y universality strongly suggest that high-Tc superconductivity is more closely related to Bose–Einstein condensation of real-space pairs than to a BCS state of Cooper pairs. The polaronic nature of at least part of the charge carriers in these materials has been experimentally established in both the insulating and the metallic phase of these compounds. On theoretical grounds one expects small polarons to interact with each other over short distances and in a practically unretarded fashion.