XANES study of oxycarbonates Hg 1−xM xSr 4Cu 2(CO 3)O 6+δ (M=Mo, Cr)

Abstract

X-ray absorption measurements have been made on CuL 3 and OK edges for two transition metal substituted Hg based oxycarbonates Hg 0.5Mo 0.5Sr 4Cu 2(CO 3)O 6+ δ and Hg 0.4Cr 0.6Sr 4Cu 2(CO 3)O 6+ δ . Despite a much lower T c (37 K) the Cr sample has a much higher diamagnetic fraction than the Mo sample ( T c∼74 K). CuL 3 and OK edge spectra clearly indicate the presence of substantial number of doping holes in both the samples characteristic of their superconducting properties. But interestingly, the density of holes (DOH) are almost the same for the two samples. If the DOH correlated with the T c solely, one would expect it be nearly double in Mo sample than the Cr one. Thus the present study presents a strong evidence for the dependence of DOH upon diamagnetic fraction over and above its dependence upon their transition temperature in the cuprate perovskites. Also the results of resistivity and susceptibility studies made earlier on these samples have been mentioned briefly.