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

The effects of Cu substitution for Tl in TlSr2Ca2Cu3Oy superconductor were investigated. Nearly single-phase samples of Tl1-xCuxSr2Ca2Cu3Oy (x=0.2-0.6) were synthesized using a high pressure technique. The observed shrinkage in the lattice parameter a, concave r-T curve above Tc and negative thermoelectric power near RT indicated that the Cu-substituted samples were over-doped. Tc for the sample with x=0.4 increased from 73 to 94 K after annealing treatment in flowing Ar gas. The Cu-substituted samples showed lower irreversibility field Birr as a function of reduced temperature than that of the pristine TlSr2Ca2Cu3Oy. 17 refs.