Doping states and irreversibility fields of high-pressure-synthesized (Tl,Cu)Sr2Ca2Cu3Oy superconductors

The effects of Cu substitution for Tl in TlSr 2 Ca 2 Cu 3 O y superconductor were investigated. Nearly single-phase samples of Tl 1− x Cu x Sr 2 Ca 2 Cu 3 O y ( x =0.2−0.6) were successfully synthesized using a high pressure technique. The observed shrinkage in the lattice parameter a , concave ρ – T curve above T c and negative thermoelectric power near room temperature indicated that the Cu-substituted samples were over-doped. T c for the sample with x =0.4 increased from 73 to 94 K after annealing treatment in flowing argon gas. The Cu-substituted samples showed lower irreversibility field B irr as a function of reduced temperature than that of the pristine TlSr 2 Ca 2 Cu 3 O y [Physica C 324 (1999) 15].