T{sub c}-variation of (Y{sub 1-y}Ca{sub y})Ba{sub 2}Cu{sub 3}O{sub x} single crystals under hydrostatic pressure

Former pressure experiments on YBa{sub 2}Cu{sub 3}O{sub x} single crystals showed only small dT{sub c}/dp values at high oxygen contents (6.9{le}x{le}7.0), which corresponds to an almost optimal hole-doping n{sub h}. By Ca-doping these investigations can be extended to the heavily overdoped region. Reducing then the oxygen content allows to return to the optimal hole concentration n{sub h,opt}, which gives the maximum transition temperature T{sub c,max} for this Ca-content. Oxygen and Ca-doping of YBa{sub 2}Cu{sub 3}O{sub x} single crystals are used to determine the parabolic T{sub c}(n{sub h}) dependency by ac-susceptibility measurements under hydrostatic pressure conditions up to 0.6 GPa. In order to avoid pressure induced oxygen ordering processes the samples were held below 100 K during the experiment including all pressure changes. The Ca-doped highly oxygenated single crystals show large negative dT{sub c}/dp values. Reducing the oxygen content increases dT{sub c}/dp to zero, as expected. A further reduction of the oxygen content increases the dT{sub c}/dp values much faster than expected from the overdoped region. These findings are interpreted to arise from the anisotropic pressure effect on T{sub c} with a pressure effect in c-axis direction that is mainly due to hole doping and with additional structural effects when pressuremore » is applied in a- and b-axis direction.« less