The in-plane thermopower of most of the cuprate superconductors (inclu
ding the new Hg-based materials) shows a systematic pattern as the hol
e concentration is increased and conductivity increases, namely an app
roximately 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. Th
e similarity of cuprate and non-cuprate thermopower suggests a qualita
tively similar mechanism.