Thermopower anisotropy of highly underdoped and slightly overdopedRBa2Cu3O

Abstract

The thermoelectric power (S) for a series of RBa 2 Cu 3 O 7Ϫ␦ (RϭY, Gd, Dy) single crystals were measured in the ab plane and along the c direction. The doping level of these crystals changes from highly underdoped (T c ϭ44 K), through optimally doped (T c ϭ90 K) to slightly overdoped (T c ϭ88 K). Both S ab and S c change linearly with the doping level, but S ab reverses its sign near the optimal doping stoichiometry, as it was observed for polycrystalline materials, whereas S c remains positive for the whole series. In contrast to the results of electrical resistivity, the anisotropy of S was found to decrease with both increasing oxygen deficiency and decreasing temperature. A brief overview of thermopower results for single crystals of various high-temperature superconducting families is also presented. ͓S0163-1829͑98͒06102-5͔

Lorenz number in the optimally doped and underdoped superconductorEuBa2Cu3Oy

“…It was twinned, therefore the coefficients measured along the ab-plane should represent values averaged between the a and b crystallographic directions. The magnetically measured superconducting transition of the sample had an onset of ≈ 94.5 K and a midpoint of 94.0 K. The oxygen content y was estimated to be close to 7 (on the basis of the T c value and the in-plane thermoelectric power value at room temperature S ab (300 K) ≈ -2 µV/K 13,14,15 ).…”

“…After the reduction treatment the critical temperature (T c ) dropped to about 61 K (measured magnetically) and its thermoelectric power was equal to S ab (300 K) ≈ 30 µV/K. Both values suggest 13,14,15 that the oxygen content has dropped to ≈ 6.65.…”

“…The magnetically measured superconducting transition of the sample had an onset of Ϸ94.5 K and a midpoint of 94.0 K. The oxygen content y was estimated to be close to 7 ͓on the basis of the T c value and the in-plane thermoelectric power value at room temperature S ab ͑300 K͒Ϸ−2V/K͔. [13][14][15] As described below a second set of measurements was performed on the same specimen annealed in air for 12 days at T = 500°C to reduce its oxygen content. After the reduction treatment the critical temperature ͑T c ͒ dropped to about 61 K ͑measured magnetically͒ and its thermoelectric power was equal to S ab ͑300 K͒Ϸ30 V / K. Both values suggest 13-15 that the oxygen content has dropped to Ϸ6.65.…”