PRESSURE-INDUCED CHARGE-TRANSFER AND PRESSURE-DEPENDENCE OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE IN HGBA2CUO4+DELTA

The pressure-induced changes of hole concentration and pressure effect s on T-c in HgBa2 CuO4+delta (delta = 0.07 - 0.39) have been investiga ted on the basis of the pressure-induced charge transfer model. Detail ed calculations show that large enhancements in T-c under high pressur e can be obtained for materials which are underdoped, and not for comp ounds which have nearly optimal hole concentration. On the other hand, T-c enhancements in compounds which are heavily overdoped are found t o be quite modest. The theoretical prediction is in agreement with exp eriments. The possible intrinsic factors which are responsible for the pressure dependence of the maximum T-c are discussed within the van H ove singularity scenario.