Single crystal studies of the pairing mechanism in Tl 2Ba 2CuO 6 superconductors

Abstract

Reasonably large crystals (several mm size) of Tl 2Ba 2CuO 6 (referred as Tl-2201) were grown by a self-flux technique. T c's were altered by annealing the crystals in different atmospheres and at different temperatures. As the oxygen content increased, the T c decreased from 90 K to 0 K. The anisotropic resistivities ϱ ab (along ab-plane) and ϱ c (along c-axis) were measured on these crystals by Montgomery method. ϱ c was found to be 2–3 orders of magnitude greater than ϱ ab. The anisotropy ratio (ϱ c/ϱ ab) increases with T c. For higher T c crystals, the superconducting and the normal state properties of ϱ ab can be explained by the nested Fermi surface behavior with a T-dependence of the in-plane resistivities. For lower T c crystals, the normal Fermi liquid picture emerges from the T 2-dependence of the in-plane resistivities. Unlike YBa 2Cu 3O 7, Bi-2212, and other copper oxide superconductors, ϱ c for Tl-2201 crystals is metal-like. The paraconductivity data shows that Tl-2201 is a two-dimensional superconductor. The c-axis Seebeck coefficients are observed to be p-type for all crystals in the whole temperature range studied whereas some of the corresponding ab-plane Seebeck coefficients change sign from n-type to p-type and some are n-type down to T c. The thermopower behavior is still theoretically unclear.