Thermopower in epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films

Thermopower measurements are reported in several perfect epi- taxial ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films above 80 K. A prominent peak was observed both in the in-plane and out-of-plane thermopower ($S_{\mathrm{ab}}$ and $S_c$) and it is associated with the perfectness of the crystal structure of the sample. Above 140 K, $S_{\mathrm{ab}}$ is almost temperature independent with small positive value (about 0.2 μV/K), and $S_c$ increases with linear temperature dependence. The slope ${\mathrm{dS}}_c$/dT is about 2.6×${10}^{\mathrm{-}2}$ μV/${\mathrm{K}}^2$ which is much greater than expected in a metal.