Correlated Pair‐Transfer Mechanism for High Temperature Superconductivity. The Case of YBa₂Cu₃O6+x

A theoretical study of high temperature superconductivity in the system YBa 2 Cu 3 O 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 {article}{empty}{document}X↑ ↓ (= Cu^ 1 + , O₂^ 2 - , or O^ 2 - ){document} and their virtual excited state X 0 (Cu 3+ , O , or O 0 ) 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. Bi 3+ , Bi 5+ or Tl 1+ , Tl 3+ , etc.). The theory has electron and hole symmetry, therefore it can also be applied to n‐type high temperature superconductors.