Charge Transfer-T_cRelation in the Superconducting Intercalates IBi₂Sr₂CaCu₂O_y

Two different kinds of stage-1 iodine intercalates of Bi₂Sr₂CaCu₂O_ywith dissimilarT_chave been prepared in order to understand the relationship between superconducting properties and electronic structures. According to the XRD analysis and the magnetic susceptibility measurements, it is found that, in spite of the same basal increments for both intercalates (Δd=~3.6 Å), the iodine intercalate prepared in a vacuum-sealed tube exhibits a largerT_cdepression (13 K) compared to the iodine intercalate synthesized in air (ΔT_c=6 K). The origin of such differentT_cshifts has been investigated by performing the systematic XANES/EXAFS analyses at the I L₁-, Cu K-, and Bi L₃-edges, where the charge transfer between host and guest is found to be strongly dependent on the reaction condition. From the BVS calculation based on the Cu K-edge EXAFS results, it is clarified that there is an excellent correlation between the amount of hole doping andT_cdepression for both iodine intercalates. In this respect, we conclude that, regardless of lattice expansion along thec-axis, theT_cof Bi-based cuprate is wholly dependent on the hole concentration of the CuO₂layer.