Pressure-Induced Charge Transfer and Pressure Dependence of the Superconducting Transition Temperature in HgBa_(2)CuO_(4+δ)

The pressureinduced changes of hole concentration and pressure effects on Tc in HgBa2CuO4+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 Tc 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,Tc 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 Tc are discussed within the van Hove singularity scenario.