ON THE RELATIONSHIP BETWEEN DOPING, CRITICAL-TEMPERATURES AND PRESSURE-GRADIENTS OF T(C) IN HIGH-T(C) CUPRATES - THE DOUBLE ROLE OF OXYGEN

We study the effect of hole doping on T(c) and on dln T(c)/dP in two typical high-T(c) cuprates: La2-xSrxCuO4 for varying x and (Yb,Ca) (Ba,Sr)2Cu3Oz for varying oxygen concentration z. Whereas experimentally the T(c)(x) and T(c)z) curves are very similar, their pressure dependence shows a qualitatively different behavior: positive for all x in La-Sr-Cu-O, while changing sign from positive to negative with increasing z in the second system. It is shown that this difference can be quantitatively accounted for on the basis of an indirect exchange pairing mechanism for conduction electrons via oxygen anions. The results illuminate the double role of oxygen in the cuprates: as diamagnetic centers for Cooper pair formation and as hole dopants. On the same basis we discuss the occurrence of negative pressure gradients of T(c) in other hole-doped cuprates, especially in Tl2Ba2CuO6+y. The analysis does not invoke effects due to pressure-induced charge transfer.