Anomalous peak in the thermopower of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals: A possible fluctuation effect

We present results for the thermopower of five crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ from the transition temperature $T_c$ to 250 K and in magnetic fields to 2 T. The thermopower is approximately -4 μV/K at 250 K for all samples, with a broad phonon drag peak around 150 K. Sharp peaks in the thermopower are observed in the vicinity of $T_c$. The thermopower is qualitatively the same for all crystals except one that was annealed at 650 °C in oxygen. That crystal had a positive thermopower and exhibited no peak at $T_c$. After a reanneal in oxygen at 400 °C, the negative thermopower and peak at $T_c$ appeared. We discuss the possibility that the anomalous peak at $T_c$ is a fluctuation effect and suggest a simple kinetic argument for the presence of a divergent fluctuation contribution.