SYSTEMATIC THERMOPOWER BEHAVIOR IN SUPERCONDUCTORS

The in-plane thermopower of most of the cuprate superconductors (including the new Hg-based materials) shows a systematic pattern as the hole concentration is increased and conductivity increases, namely an approximately temperature-independent shift to more negative values. This pattern is intimately related to the value of T-c, and so possibly to the mechanism of the superconductivity. In the overdoped limit where T-c is reduced, the thermopower is often approximately proportional to temperature, as for conventional metallic diffusion thermopower, and as seen in fullerene superconductors. A remarkably similar pattern of thermopower shifts (but of smaller magnitude) is seen in NbN films. The similarity of cuprate and non-cuprate thermopower suggests a qualitatively similar mechanism.