Are Metal-Oxide Superconductors Charged Bosonic Superfluids?

Abstract

Prior to a discussion of the superconductivity mechanisms in the high-T _c metal oxides, the nature of the conductivity mechanism itself, and in particular the type of charge carriers should be considered. As advocated by the present author [1–3], the materials should be viewed as doped, large-gap semiconductors, where the excess charges introduced in the lattice by the doping form mixed-valence small-polarons (highly local, charged defects). At sufficiently high concentration, the mobility of the defects is guaranteed by quantum-mechanical tunneling in a band motion for temperatures smaller than the polaron bandwidth, and by thermally activated hopping in the higher temperature range [4]. Since the moving polarons will see a random electric potential from a.o. the impurity atoms (Sr²⁺, oxygen deficiency), localization will occur at too low carrier concentrations, leading to a metal-insulator transition of the Anderson type. This behaviour is clearly observable in the experimental resistivity curves as a function of doping level [2]. In the semiconducting samples, thermally activated hopping has indeed been observed.