SEEBECK-EFFECT IN THE MIXED STATE OF Y-BA-CU-O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of a c-axis oriented epitaxial film of Y-Ba-Cu-O. In contrast to conventional superconductors we find a large Seebeck-coefficient S, which is comparable in magnitude to the Nernst-effect. The broadening of the superconducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to "normal" quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude of S. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T(c) superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.