Role of chemical pressure in enhancing the transition temperature (T_c) and upper critical field (H_c2) in the Y-doped Ce-oxyfluoride superconductor

Abstract

Structural and superconducting properties of yttrium substituted Ce_1-xY_x(O/F)FeAs superconductors have been investigated for the first time. All the compounds crystallize in the tetragonal ZrCuSiAs structure type. There is a decrease in both the a and c lattice parameters on increasing yttrium substitution (with fixed F content) along with a substantial enhancement of the superconducting transition temperature (T_c) and upper critical field (H_c2) indicating the influence of chemical pressure. Interestingly the maximum Tc (~48 K) was observed for an intermediate composition (Ce_0.5Y_0.5O_0.9F_0.1FeAs) which is higher than either of the parent Y or Ce-compounds, (YO_0.9F_0.1FeAs (~10 K) and Ce(O/F)FeAs (~42 K)). The transition temperature was also found to be nearly independent of the electron-doping introduced by fluoride substitution (0.1 to 0.2 moles per formula unit) indicating the significance of the charge reservoir layer (Ce-O). The yttrium substituted (fluoride free) compositions of the type, Ce_1-xY_xOFeAs were found to be semimetallic like the parent compound CeOFeAs with the shift in the anomaly temperature towards low temperature on substitution of yttrium ions. Hall coefficient and thermopower measurements show an increase in charge carriers (electrons) through Y-doping in fluorine doped CeOFeAs.