Hole localization, electronic structure and high- Tc superconductivity

Abstract

It is shown that the dielectric gap in the electronic spectrum of oxide high- T c systems appears because of ordering arrays of localized (heavy) holes or the charge density waves. Dielectric-metal (DM) phase transitions in these systems are the consequence of impurity bands appearing at doping, delocalization of the ordered heavy holes located on the metal-apical ion bonds. The free carriers introduced by doping can couple through excitation of these localized holes. This leads to high- T c superconductivity with small coherence length and anomalous temperature dependence of H c2( T). These results were obtained on the basis of experimental studies of specific heat, magnetic properties, thermal expansion and other characteristics. The physical nature of the observed DM transitions and negative thermal expansion in HTSC systems are discussed.