Thermopower of zinc-doped Y-Ba-Cu-O

Absolute thermopower measurements on well oxygen annealed ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathrm{x}}$ ${\mathrm{Zn}}_{\mathrm{x}}$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ (x=0.025, 0.05, 0.075, 0.1, 0.15, 0.2, 0.3, and 0.4) were made in the temperature range 250 K to the superconducting transition temperature, $T_c$ (zero). X-ray analysis showed the specimens to be single phase. The $T_c$ decreased drastically with increasing zinc concentration. The thermopower was found to be positive and an enhancement relative to the linear electron-diffusion contribution was observed over a range of temperatures from 125 K down to the onset of the superconducting transition temperature, $T_c$ (onset). The enhancement showed a steep exponential drop as the temperature increases from $T_c$ (onset). There exists unambiguous evidence that phonon drag is not responsible for the peak. The enhancement of thermopower in all these specimens varies more steeply with temperature than what one would expect from electron-phonon interaction. Hence, the cause of the enhancement must be other than the electron-phonon interaction.