Anisotropic superconductivity in high-T_c layered crystals

Abstract

The new high-T_c superconductors have layered structures with anisotropic coherence lengths small compared to the normal state carrier mean free path l. It is shown that the anisotropy of the energy-gap function of the generalized BCS pairing theory for layered materials with nonspherical Fermi surface, plays a crucial role in determining many of their properties in the superconducting state. The usual comparison of experimental results with the one-parameter isotropic BCS model is thus quite misleading in such cases. It is argued that single-crystal results on the low-temperature electronic specific heat and infrared absorption may be understood in terms of possible anisotropies in the gap function and the Fermi surface.