Effects of simultaneous carrier doping in the charge reservoir and conducting layers of superconducting CeO_0.9F_0.1Fe_1-xCo_xAs

Abstract

Electron-doping of the semimetal (CeOFeAs) by either fluorine (max T_c ~ 43 K) or cobalt (max T_c ~ 11 K) leads to superconductivity. Here we show the effect of transition metal (Co) substitution at the iron site on the superconducting properties of CeO_0.9F_0.1FeAs (T_c ~ 38 K) to understand the interplay of charge carriers in both the rare earth-oxygen and Fe-As layers. Simultaneous doping of equivalent number of charge carriers in both layers leads to a T_c of 9.8 K which is lower than the T_c obtained when either the conducting layer (FeAs) or charge reservoir layer (CeO) is individually doped. This suggests a clear interplay between the two layers to control the superconductivity. Resistivity upturn and negative magnetoresistance are observed with Co doping that is interpreted in the gamut of Kondo effect. Hall coefficient and thermoelectric power indicate increased carrier concentration with cobalt doping in CeO_0.9F_0.1FeAs. The rf penetration depth both for CeO_0.9F_0.1Fe_0.95Co_0.05As and CeO_0.9F_0.1FeAs show an exponential temperature dependence with gap values of ~1.6 and 1.9 meV respectively.