Oxygen-deficiency dependence of the thermopower ofYBa2Cu3O7−y

We have measured the thermopower of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-y}$ as a function of oxygen deficiency. We found that both the magnitude and the temperature dependence of the thermopower strongly depend upon the oxygen deficiency. As the oxygen deficiency increases, the magnitude at 273 K increases by more than a factor of 90 and the large positive peak observed at the superconducting transition temperature moves toward a higher-temperature region with its width broadening. We found that there exist some discrepancies between our observation and the models suggested by others. As a plausible explanation for the experimental results, a magnon-drag effect is considered.