Interband Effects in High-Tc Oxide Superconductors

Interband effects on superconductivity have been theoretically investigated in a two-band model. One of the bands has a dispersion relation of a simple square lattice with nearest-neighbor transfer integrals, while the other has that of a simple chain with nearest-neighbor transfer integrals. It has been shown that the transition temperature is significantly enhanced by interband transfer integrals, if the two bands are situated in such a way that van Hove singularities of both bands might be energetically degenerate. It has been argued that interband effects should explain why transition temperatures of Y–Ba–Cu–O systems are higher than those of La–Cu–O systems.