Effect of iodine intercalation on superconductivity in the high-Tc series Bi2Sr2CaN-1CuNO2N+4+δ, N = 1,2 and 3, and in the yttrium doped N = 2 compound. An indirect exchange analysis

A quantitative analysis is presented of the effect on critical temperatures of iodine intercalation in the (Bi-O)2 bilayers of the bismuth cuprates with N = 1,2 and 3 CuO2 layers per molecular unit, as well as of the N = 2 system Bi2Sr2Ca1-xYxCu2O8+δ with varying yttrium doping. The formalism used is that of indirect exchange pairing between conduction electrons via closed-shell oxygen anions. For the N = 1,2 and 3 undoped systems, quantitative agreement with experiment results from supposing that iodine intercalation leads to a migration of interstitially stored oxygens, implying an increase near CuO2 layers of one oxygen anion per molecular unit. For the yttrium-doped N = 2 system, detailed agreement with experimental results for both the pristine and iodine-intercalated materials is achieved. The results strongly indicate that neither an increase of hole concentration in the CuO2 layers, nor weakened inter-cell coupling between these layers, as a result of intercalation, is the cause of the phenomena observed.