Polarized XANES study of the importance of inter-block vis-à-vis intra-block coupling in evolution of Tc in halide-molecule-intercalated Bi2Sr2CaCu2O8-δ single crystals

In addition to doping in the lattice that affects the intra-block coupling, intercalated molecules sit in between the consecutive basal planes, thereby increasing the effective length of the c-axis. This, in turn, must lead to a decrease in inter-block coupling. Both the doping and intercalation have been reported to affect the evolution of Tc in a system, implying the inherent importance of both types of coupling. In the latter case, the resulting depression of Tc is ascribed to transfer of charge between the intercalate and the host CuO2 plane. Most interesting studies in this regard pertain to use of I2, HgI2 and HgBr2 molecules as intercalates for the Bi2Sr2CaCu2O8+y system. Earlier reports mostly claim that the host CuO2 plane in Bi2Sr2CaCu2O8+y invariably becomes overdoped whichever the intercalate, thereby leading to a fall in Tc. In this paper, we examine these claims in the case of Bi2Sr2CaCu2O8+y single crystals by measuring the number of itinerant holes before and after intercalation by making polarization-dependent soft-x-ray absorption measurements at the O K and the Cu L3 edges. Our results do support the earlier claims on overdoing of the CuO2 plane in the case of iodine intercalate but are not in agreement with those in the case of the HgI2 intercalate.