UNIAXIAL PRESSURE-DEPENDENCE OF T-C IN (Y1-YCAY)(1)BA2CU3OX SINGLE-CRYSTALS VIA HIGH-RESOLUTION THERMAL-EXPANSION

The uniaxial pressure dependence of T-c, dT(c)/dp(i) (i = a-b, c), of twinned (Y0.89Ca0.11)(1)Ba2Cu3Ox single crystals has been investigated as a function of O doping, with special emphasis given to the over-do ped regime. The dT(C)/dp(i) values are not measured directly, but are determined from high-resolution thermal expansion data (obtained with a capacitance dilatometer) using the thermodynamic Ehrenfest relations hip. Both dT(c)/dp(c) and dT(c)/dp(a,b) become negative in the over-do ped region. The effect is larger along the c-axis. The c-axis results can be described by a simple pressure-induced charge-transfer model, w hereas the a,b-axis results are more complicated.