An X-ray absorption study of the dependence of the density of holes on T-c and/or the diamagnetic fraction in the oxycarbonate superconducting system Hg1-xMxSr4Cu2(CO3)O6+delta (M = Mo, Cr)

摘要

X-ray absorption near edge structure (XANES) measurements have been made on Cu L-3 and O 1s edges for two transition metal substituted Hg based superconducting oxycarbonates Hg0.5Mo0.5Sr4Cu2(CO3)O6+delta and Hg0.4Cr0.6Sr4Cu2(CO3)O6+delta. The diamagnetic (DM) fraction for the Cr substituted system is nearly double (similar to 60%) that for the Mo substituted system (similar to 35%). On the other hand, the superconducting transition temperature (T-c) for the Cr sample (37 K) is only about half that for the Mo sample (similar to 74 K). Thus it was very interesting to look at the density of holes in these two cases. Cu L-3 and O 1s edge XANES spectra clearly indicate the presence of an adequate number of doping holes in both the samples, characteristic of their superconducting properties. But surprisingly, the densities of the holes (doh) are almost the same (similar to 0.15) for the two samples. If it was solely correlated with the T-c, one would expect the doh of the Mo sample to be nearly double the Cr one. But the present finding presents evidence indicating that the doh must have a dependence upon the diamagnetic fraction over and above its dependence upon the transition temperature in the cuprate perovskites. This important basic question/fact needs to be explored for some other systems too, to put the conclusion on a firm footing. The results of resistivity and susceptibility measurements have also been presented briefly. (c) 2006 Elsevier Ltd. All rights reserved.