Hybridization-induced wide-band bipolaron model for high-T_c superconductors

Abstract

A microscopic theory based on the idea of hybridization-induced wide-band bipolarons is proposed to explain the occurrence of high-temperature superconductors such as La_2−x-Ba_xCuO₄, and La_2−xSr_xCuO₄. In our view, these are such that bipolarons are formed whose singlet ground state possesses a large binding energy (Δ) with bandwidth W (>Δ) as a result of two distinct electron-phonon mechanisms and two types of hybridization between Cu²⁺ d electrons and other electrons. This wide-band bipolaron gas has features similar to Cooper pairs and leads to a superconducting state with high T_c in which all occupied electron states participate. The large electron-phonon coupling and a large W both cause the transition temperature T_c, to rise spectacularly.