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

X-ray absorption near edge structure (XANES) measurements have been made on Cu L3 and O 1s edges for two transition metal substituted Hg based superconducting oxycarbonates Hg0.5Mo0.5Sr4Cu2(CO3)O6+δ and Hg0.4Cr0.6Sr4Cu2(CO3)O6+δ. The diamagnetic (DM) fraction for the Cr substituted system is nearly double (∼60%) that for the Mo substituted system (∼35%). On the other hand, the superconducting transition temperature (Tc)(Tc) for the Cr sample (37 K) is only about half that for the Mo sample (∼74 K). Thus it was very interesting to look at the density of holes in these two cases. Cu L3 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 (∼0.15) for the two samples. If it was solely correlated with the TcTc, 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.