TOPOLOGICAL INTERPRETATION OF THE VANHOVE SINGULARITY - ORBITAL SWITCHING

Recent theoretical and (photoemission) experimental results greatly st rengthen the case for the saddle point van Hove singularity (vHs) play ing an important role in the cuprate superconductors. The vHs signals a change in the topology of the Fermi surface-usually from electronlik e to holelike orbits. Thus, near the vHs, there can be tunneling betwe en the different Fermi surface sections. This orbital switching bears some resemblance to magnetic breakdown, but is a distinct phenomenon. In the presence of finite orbit switching, the electronic orbits can f orm a quantum coherent network.