Exchange-Like Integrals Enhancing the Superconducting Transition Temperature in Copper Oxide Compounds

Abstract

We developed a two-band model of superconductivity including the interband BCS-form matrix elements originating in the exchange-like integral, K , at each atomic site as well as the usual intraband BCS interactions and the on-site Coulomb energy, U . The mean field treatment in a few typical cases discloses that K is directly subtracted from U in the expression of the transition temperature, T c , and that T c is substantially enhanced by the interaction with K in the Cu oxide superconductors having multiple bands, if U is 1∼2 eV. Further, when the two bands are quasi-1D, the ladder and bubble diagrams divergently enhance the effect of K in elevating T c . This effect is claimed to be the origin of T c elevated up to 94 K and explains the precipitous decrease of T c with a decrease of the oxygen content in the Y compounds. Also, superhigh T c in the metastable states becomes approachable.