THERMOPOWER ANISOTROPY OF HIGHLY UNDERDOPED AND SLIGHTLY OVERDOPED RBA2CU3O7-DELTA SINGLE-CRYSTALS

The thermoelectric power (S) for a series of RBa2Cu3O7-delta (R = Y, G d, Dy) single crystals were measured in the ab plane and along the c d irection. The doping level of these crystals changes from highly under doped (T-c=44 K), through optimally doped (T-c=90 K) to slightly overd oped (T-c=88 K). Both S-ab and S-c change linearly with the doping lev el but S-ab reverses its sign near the optimal doping stoichiometry, a s it was observed for polycrystalline materials, whereas S-c remains p ositive for the whole series. In contrast to the results of electrical resistivity, the anisotropy of S was found to decrease with both incr easing oxygen deficiency and decreasing temperature. A brief overview of thermopower results for single crystals of various high-temperature superconducting families is also presented.