Single crystal studies of the pairing mechanism in Tl₂Ba₂CuO₆ superconductors

Reasonably large crystals (several mm size) of Tl₂Ba₂CuO₆ (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{variant}_ab (along ab-plane) and ρ{variant}_c (along c-axis) were measured on these crystals by Montgomery method. ρ{variant}_c was found to be 2-3 orders of magnitude greater than ρ{variant}_ab. The anisotropy ratio (ρ{variant}_c/ρ{variant}_ab) increases with T_c. For higher T_c crystals, the superconducting and the normal state properties of ρ{variant}_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 T²-dependence of the in-plane resistivities. Unlike YBa₂Cu₃O₇, Bi-2212, and other copper oxide superconductors, ρ{variant}_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.