Observations on the thermopower of the high-Tc superconductors

Measurements of the thermopower of the high-temperature superconductor ceramics in the normal state have generally shown a positive, almost temperature-independent Seebeck coefficient, which has been interpreted by several authors in terms of the correlated hopping of the carriers as would be found in the strong Coulomb interaction limit of the Hubbard model. In this case the thermopower has a contribution from spin entropy which can be quenched by a large magnetic field. On the contrary we measure a field-independent thermopower to 30 T. Moreover, in single crystals we find that the thermopower in the Cu-O planes is negative, while the perpendicular thermopower is positive.