Hole superconductivity in oxides: A two-band model

The role of the Cu ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ orbitals in the theory of hole superconductivity in oxides is considered. We use a two-band model of the type introduced by Suhl et al., describing a purely oxygen band coupled to a copper-oxygen band. The pure O band is nearly full, and the Cu-O band is nearly half-filled in the parameter range of interest. It is found that many of the features of the single-band hole-pairing description of the oxides survive the generalization to the two-band model. The new feature of the present model is the existence of a second, smaller, superconducting gap associated with the Cu-O band. This can give rise to features in the tunneling characteristics that reproduce certain experimental observations. The effect of the second gap in the specific-heat behavior below ${\mathit{T}}_{\mathit{c}}$ is also examined, and deviations from exponential behavior at intermediate temperatures are found. It is also proposed that this model provides an explanation for the anomalous behavior found in the Hall coefficient just below ${\mathit{T}}_{\mathit{c}}$.