Negative thermopower of ${\mathrm{YbBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$

We have measured the thermopower of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ 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 ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{YbBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ 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.