Thermopower of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and related superconducting oxides

There is some disagreement in the literature concerning both the measured magnitude and the interpretation of the thermoelectric power of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. We report a series of measurements that have guided us to a satisfactory explanation of both our own results and those reported by other groups. Most important is that we have explained the surprising precursor peak just above $T_c$ as resulting from an enhanced phonon mean-free path in the presence of superconducting fluctuations. We develop a model that permits us to separately estimate the values of the diffusion and phonon-drag contributions to the thermopower. At 100 K the diffusion component is estimated to be -14 μV/K, a large negative value that suggests conduction by a narrow electron band. The phonon-drag thermopower is positive with a magnitude that depends on crystallite grain size.