Pressure-Induced Charge Transfer and Pressure Dependence of theSuperconducting Transition Temperature in HgBa₂CuO_4+δ

The pressureinduced changes of hole concentration and pressure effects on T_c in HgBa₂CuO_4+δ(δ = 0.07 - 0.39) have been investigated on the basis of the pressureinduced charge transfer model. Detailed calculations show that large enhancements in T_c under high pressure can be obtained for materials which are underdoped, and not for compounds which have nearly optimal hole concentration. On the other hand, T_c enhancements in compounds which are heavily overdoped are found to be quite modest. The theoretical prediction is in agreement with experiments. The possible intrinsic factors which are responsible for the pressure dependence of the maximum T_c are discussed within the van Hove singularity scenario.