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 rho(ab) (along ab-plane) and rho(c) (along c-axis) were measured on these crystals by Montgomery method. Rho(c) was found to be 2-3 orders of magnitude greater than rho(ab). The anisotropy ratio (rho(c)/rho(ab)) increases with T(c). For higher T(c) crystals, the superconducting and the normal state properties of rho(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, rho(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.