Correlated pair-transfer mechanism for high temperature superconductivity. The case of YBa₂Cu₃O_6+x

Abstract

A theoretical study of high temperature superconductivity in the system YBa₂Cu₃O_6+x is made using the correlated pair transfer mechanism suggested recently. The mechanism involves the role of electron pairs in the spin singlet state of species X^↑↓ (= Cu¹⁺, O₂^2-1, or O^2-) and their virtual excited state X⁰(Cu³⁺, O⁰₂, or O⁰) in the pairing interaction of quasiholes. The two states of these species can be likened to a two-level Bose system. The strength of the interaction depends on the hole and species concentration. The expression for the transition temperature is a combination of phononic and the electronic mechanism and the calculated trend as a function of hole concentration (in effect excess O^-) is in agreement with the observed trend. The mechanism is applicable to other oxide systems where such variable species can exist (e. g. Bi³⁺, Bi⁵⁺ or Tl¹⁺, Tl³⁺, etc.). The theory has electron and hole symmetry, therefore it can also be applied to n-type high temperature superconductors.