Thermopower sign inversion for the high Tc superconductor YBa2Cu3O7-δ

Thermopower measurements of YBa2Cu3O7-δ samples cooled down in three variants from 950 0C in oxygen atmosphere at a rate of 20 K/h are presented. The oxygen content was determined from sample weight loss after reduction by hydrogen at 900 0C and corresponding δ was found to be 0.17, 0.19, 0.05, and 0.03 with an accuracy ± 0.05. The curves of absolute thermopower as a function of temperature S(T) for samples with different oxygen content show that S(T) inverses sign for δ between 0.17 and 0.05. Results have been discussed in the frame of band parameters