Correlated pair-transfer mechanism for high temperature superconductivity

A theoretical study of high temperature superconductivity in the system YBa2Cu3O6+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+, O22-, or O2-) and their virtual excited state X0(Cu3+, O20 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. (author)