Origin of the metallization of c-axis resistivity upon iodine intercalation into Bi₂Sr₂CaCu₂O_8+δ

The origin of the metallization of c-axis resistivity upon intercalation of iodine into Bi₂Sr₂CaCu₂O_8+δ has been studied by performing I L₁-edge angle-resolved X-ray absorption spectroscopic (XAS) analysis and the tight binding band calculation with the extended Hückel method. According to the polarized I L_I-edge XAS analysis, it has become clear that there is a significant anisotropy in the I 5p hole distribution of the intercalated iodine layer. Compared to the E⊥c spectrum, the E//c spectrum shows weaker intensity and higher energy for the white line feature corresponding to the 2s → 5p_z transition, indicative of a strong interaction between the BiO layer and the intercalated iodine molecule along the c-axis. Such an interpretation is further supported by the band calculation results showing a significant hybridization between the Bi 6s orbital and the I 5p_z one. On the basis of these findings, it is concluded that the orbital overlap between Bi 6s and I 5p_z opens a conduction channel along the c-axis, which leads to the metallization of out-of-plane resistivity upon iodine intercalation.