Superconducting properties of a new oxysulfate superconductor

We report the effects of thermal treatment and La doping on the Sr site in the newly discovered intergrowth superconductor TlSr4Cu2Oz(SO4), whose zero-resistivity temperature is found to be above 70 K. This new phase crystallizes in the tetragonal system (p4/mmm, a = 3.83 angstrom, c =16.6 angstrom). Resistivity and room-temperature thermoelectric measurements for the TlSr4Cu2Oz(SO4) samples heat-treated at temperatures below 500 degrees C in Ar atmosphere suggest that the as-prepared sample is under a nearly optimal hole-doped state. In the Tl (Sr4-xLax)Cu2Oz(SO4) (x = 0-1.0) system, superconductivity is suppressed as the La doping content increases, and the decrease in transition temperature with the increase in La doping content above 0.1 is accompanied by increased formation of the TISr2CuOz phase and decomposition of the TlSr4Cu2Oz(SO4) phase, suggesting that the formation of the TlSr4Cu2Oz(SO4) phase is strongly affected by La doping on the Sr site compared with that of the TlSr2CuOz phase.