Quantitative analysis of superconductivity in the Hg-cuprates. Comparison with the Tl2-series

A quantitative analysis is developed for the effect of external pressure on critical temperatures in Hg(N)-and Tl2(N)-superconductors with N = 1,2, and 3 CuO2 layers per molecular unit, and with chemical composition HgBa2CaN − 1 CuNO2N + 2 + δ and Tl2Ba2CaN − 1 CuNO2N + 4 + δ, respectively. As in earlier work, Cooper pair formation is assumed to arisefrom indirect exchange pairing between conduction electrons via closed-shell oxygen anions (O2−), in a s-wave BCS formalism. The exchange coupling is obtained as a function of three parameters α, β, and κ, with α and β (Gaussian) parameters in the wavefunction for the oxygen valence band and the conduction electrons, respectively, and κ = bkF/α1/2, with b a materials constant for the cuprates; kF is the length of the Fermi vector. Comparison is also made with experiments in which, for the Tl2 series, pressure is applied (or changed) either just above Tc or at room temperature. Agreement with experimental results is quantitative throughout; in particular, the high Tc values of 154 K (for Hg(N = 2)) and ≈ 160 K (Hg(N = 3)) are quantitatively reproduced, at the pressures observed. They are due to two-dimensional characteristics of superconductivity in these systems. The anomalous behavior of Tc(P) in the Tl2(N) compounds with pressure applied at ambient temperature is found to result from pressure-induced diffusion of oxygen anions from the (Tl-O)2 bilayers towards the CuO2 layers. Neither van Hove singularities nor pressure-induced changes of the hole concentration in the CuO2 layers are found to play a role in superconductivity of these compounds.