Van Hove excitons and high-Tcsuperconductivity. I. Excitonic superconductivity

Presents a new variant of an excitonic model to explain the superconductivity in the high-T c oxides. The carriers in the CuO 2 layers are divided into two groups: heavy and light holes. The heavy holes localise into a (charge) density wave, and it is virtual excitations across the density-wave gap which pair the light holes in a superconducting state. Such a mechanism can lead to strong deviations from BCS form for the temperature dependence of the gap. Comparison with experiment suggests that the density wave is present as two-dimensional fluctuations with long-range order setting in only near or below T c . In La 2-x Sr x CuO 4 it is suggested that materials with x not=0 or 0.15 are two-phase mixtures.