EFFECTS OF HELIUM GAS-ADSORPTION ON THE PHONON-DRAG OF GRANULAR YBA2CU3O7-DELTA

We have measured the thermoelectric power, S(T) , of granular YBa2Cu3O 7-delta samples as a function of temperature gradient, delT, at the cr itical temperature region. measurements were conducted with the sample vessel either in vacuum, at a pressure P almost-equal-to 0.01 Torr, o r in a helium gas atmosphere, at a pressure P(He) almost-equal-to 760 Torr. When P(He) almost-equal-to 760 Torr, the phonon-drag suppression with increasing VT was more pronounced than when P almost-equal-to 0. 01 Torr. This observation was attributed to the scattering of phonons by gaseous helium atoms incorporated into the intergrain regions of th e sample. Electrical resistivity however, was independent of the sampl e vessel environment.