Abstract

The electrical features in the normal phase of high-Tc superconducting materials can be explained by the coexistence model of small polarons and Anderson-localized carriers. According to this model, with increasing carrier concentration, the degree of Anderson localization decreases, and then the concentration of coexisting small polarons increases, attains a maximum and decreases with this variation. If Tc is determined by the concentration of bosons (bipolarons) as in Shafroth's formula for Bose condensation, then the shape of the superconducting phase can be explained by this behavior of small polarons. The degree of localization in the oxides without superconductivity is too large for coexistence to be attained.

Introduction

In the high-Tc superconducting oxides, the superconducting phase appears just in the composition region in which a metal–insulator transition takes place. This fact leads to the idea that the electronic states proper to this transition are responsible for the origin of superconductivity. This transition accompanying a gradual change in electronic nature in accordance with stoichiometric variation or with carrier doping has the following characteristic features [1].

1. The electrical conductivity can be described by the variable-range hopping (VRH) mechanism at low temperatures in the insulator (semiconductor) region, which means that the carriers in this region are Anderson-localized. The degree of localization decreases with increasing carrier concentration resulting in the occurrence of the metallic phase. So this transition should be classified as an Anderson transition.

2. Plural types of carriers, itinerant and localized ones, coexist in both the metallic and the semiconducting phase.