Anisotropic thermopower in the a-b plane of an untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystal

We have measured the thermopower of an untwinned crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in both the a and b directions. In the a direction the thermopower is about +0.7 μV/K just above ${\mathit{T}}_{\mathit{c}}$, while in the b direction the thermopower is opposite in sign and -6 μV/K. The sharp peak observed at ${\mathit{T}}_{\mathit{c}}$ in twinned crystals is only clearly present in the a-axis measurement. At temperatures close to room temperature the a-axis thermopower becomes temperature independent and very small. It is argued that the peak is evidence of superconducting fluctuation effects in the thermopower.