T-C-VARIATION OF (Y1-YCAY)BA2CU3OX SINGLE-CRYSTALS UNDER HYDROSTATIC-PRESSURE

Former pressure experiments on YBa2Cu3Ox single crystals showed only s mall dT(c)/dp values at high oxygen contents (6.9 less than or equal t o x less than or equal to 7.0), which corresponds to an almost optimal hole-doping n(h). By Ca-doping these investigations can be extended t o the heavily overdoped region. Reducing then the oxygen content allow s to return to the optimal hole concentration n(h.opt), which gives th e maximum transition temperature T-c,T-max for this Ca-content. Oxygen and Ca-doping of YBa2Cu3Ox single crystals are used to determine the parabolic T-c(n(h)) dependency by ac-susceptibility measurements under hydrostatic pressure conditions tip to 0.6 GPa. In order to avoid pre ssure induced oxygen ordering processes the samples were held below 10 0 K during the experiment including all pressure changes. The Ca-doped highly oxygenated single crystals show large negative dT(c)/dp values . Reducing the oxygen content increases dT(c)/dp to zero, as expected. A further reduction of the oxygen content increases the dT(c)/dp valu es much faster than expected from the overdoped region. These findings are interpreted to arise front the anisotropic pressure effect on T-c with a pressure effect in c-axis direction that is mainly due to hole doping and with additional structural effects when pressure is applie d in a- and b-axis direction.