Pressure dependence of the superconducting critical temperature of Tl2Ba2Ca2Cu3O10+y and Tl2Ba2Ca3Cu4O12+y up to 21 GPa

Using a cryogenic diamond anvil cell (DAC) the pressure dependence of the superconducting transition temperature Tc of Tl2Ba2Ca2Cu3O10+y (Tl-2223) and Tl2Ba2Ca3Cu4O12+y (Tl-2234) has been measured resistively up to 21 GPa. At ambient pressure these compounds have Tc close-quote s of 128.5 K and 113 K. At low pressures, the pressure dependence ∂Tc/∂p is 1.75 K/GPa in Tl-2223 and 2.0 K/GPa in Tl-2234. As pressure is increased Tc continues to increase (although the rate diminishes) until Tc reaches a maximum of 133 K at 4.2 GPa in Tl-2223 and of 120 K at 6.6 GPa in Tl-2234. At higher pressures Tc decreases. In this region a rather abrupt change in ∂Tc/∂p is observed at pc=12.0 GPa in Tl-2223 and at pc=10.5 GPa in Tl-2234. The kink at pc is interpreted as an indication of the presence of inequivalent CuO2 layers: Below pc the Tc of the samples is determined by the intrinsic Tc of the outer CuO2 layers while above pc it is determined by that of the inner CuO2 layers. copyright 1996 The American Physical Society