Out-of-plane thermopower in the mixed state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.92}$ single crystals

Transport properties in a temperature gradient perpendicular to the ${\mathrm{CuO}}_2$ planes were investigated in the mixed state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.92},$ where the magnetic field $\mathrm{}\left(H\right)\mathrm{}$ was parallel to the ${\mathrm{CuO}}_2$ planes. A clear difference from the in-plane data is that the ratio of the Seebeck coefficient to the resistivity is essentially independent of $H,$ which indicates that the mean free path across the ${\mathrm{CuO}}_2$ planes is very short in the mixed state. The measured Nernst voltage is as small as about 2% of that for $H\perp {\mathrm{CuO}}_2.$ This small Nernst voltage is consistently explained by the anisotropic transport entropy of a vortex, which seems to obey the scaling relation to the anisotropic resistivity.