Negative thermopower of YbBa2Cu3O7-y

We have measured the thermopower of RBa2Cu3O7-y for R=Yb, Er, Y, Dy, and Gd as a function of temperature with further details on the Yb compound. We find that the Yb compound has a negative thermopower at all temperatures above Tc unlike the other compounds, whereas its temperature dependence is similar to that of the others. We also find that as the oxygen deficiency increases, the thermopower of YbBa2Cu3O7-y increases and becomes positive, while the whole shape of the thermopower-versus-temperature curve remains nearly unchanged. The observed insensitivity of the thermopower-versus-temperature profile to the sign of the thermopower or to the carrier concentration indicates that the electronic structure of the 1:2:3 compounds cannot be adequately explained within a single-conduction-band framework.