Effect of iodine intercalation on superconductivity in the high-TC series Bi2Sr2CaN−1CuNO2N+4+δ, N=1–3, and in the yttrium doped N=2 compound. A quantitative analysis on the basis of indirect exchange pairing

This paper deals with a quantitative, non-empirical, analysis of the effect on critical temperatures of iodine intercalation in (Bi–O)2 bilayers of the high-TC series Bi2Sr2CaN−1CuNO2N+4+δ (abbreviated Bi(N)) with N=1–3 CuO2 layers per molecular unit. In addition, we consider the dependence of TC on yttrium doping in pristine and iodine-intercalated Bi(N=2). The approach is based on a first-principle indirect-exchange pairing mechanism between conduction electrons (quasiparticles) via diamagnetic oxygen anions (valence-band states). The coupling W is a function of three parameters α,β, and α refers to the oxygen anions, β is a parameter in the wave function for the conduction quasiparticles, and κ is linearly related to the Fermi-vector length kF. A similar analysis was given earlier [Physica A 198 (1993) 551]; the present paper constitutes an extension and reconsideration of this work in connection with recent experimental results. In contrast to all existing (semi-) empirical treatments in the literature, we conclude that iodine intercalation leads to removal of oxygen anions from inside (Bi–O)2 bilayers and migration of these oxygens to the CuO2 layers as the only effect, i.e., neither a proposed hole transport nor weakening of interlayer coupling takes place. The results are found to be in quantitative agreement with experiments without adapting parameter values. We critically assess developments in this field resulting from recent experimental information and novel theoretical interpretations.