Dependence of the superconducting transition temperature on the type and number of CuO2 layers in Tl2Ba2Can-1CunO2n+4-y

The CuO2 layer dependence of the superconducting transition temperature T-c at ambient pressure on the intrinsic transition temperature T-c (i) of the type-I CuO2 plane in which the copper atom has fivefold pyramid coordinati on of oxygen and the fourfold square coordinated type-II plane is studied i n terms of the generalized Lawrence-Doniach theory. Calculations show that the increase of T-c with the number of CuO2 layers benefits from the differ ence of the intrinsic T-c (i) of the two types of CuO2 layers and that inte rlayer coupling between the neighboring CuO2 layers can enhance T-c for the multilayer cuprates. The upper limit of T-c is predicted to be 146 K for t he bilayer thallium-based series. We present an extended pressure-induced c harge transfer model for layered cuprate superconductors, assuming that the charge distribution among the crystallographically inequivalent CuO2 layer s is nonhomogeneous, which enables us to investigate the pressure effect on the intrinsic T-c (i). The intrinsic T-c (i) of the two types of CuO2 laye rs is predicted to behave with pressure in a paraboliclike manner. For the optimally doped single, double, and triple CuO2 sheets compounds, the satur ation values of T-c (I) of the type-I CuO2 plane of 91.1, 119.6, and 133.9 K are obtained when P = 2.3, 2.9, and 6.0 GPa, respectively. For the underd oped T1-2234 compound with T-c = 113 K, the calculated T-c (I) of 118.5 K i s obtained at P = 6.0 GPa. Under the application of pressure, the intrinsic T-c (II) of the type-II CuO2 plane in T1-2234 increases strongly compared with a modest increase of T-c (II) in T1-2223, possibly resulting from its underdoped nature. We suggest that at low pressure the T-c is the intrinsic T-c (i) of the type-I CuO2 plane, and at relatively high pressures the int rinsic effect of the type-II plane dominates. Our theoretical results are i n agreement with experiments.