High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

Abstract

The BCS theory is extended to a strong-coupling regime, λ≥1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.−z,74.65.+n,74.60.Mj