Tunneling into high Tc superconductors—the effect of a two-dimensional van Hove singularity

Abstract

In the analysis of tunneling experiments on high Tc superconductors it is generally assumed that the tunneling matrix elements are energy independent and isotropic in the ab‐plane. This assumption breaks down if the Fermi energy is close to a van Hove singularity in the density of states, related to a saddlepoint in the bandstructure. The vanishing group velocity near such a saddlepoint leads to a strongly reduced tunneling probability. This bandstructure effect may provide a coherent explanation for many of the anomalies found in tunneling experiments on high Tc superconductors. Examples include the linear background and its dependence on the barrier height, suppressed gap singularities, weak link behaviour of grain boundaries, and low proximity coupling with normal metals.