  Electron-doping of the semimetal (CeOFeAs) by either fluorine (max Tc ~ 43
K)or cobalt (max Tc ~ 11 K) leads to superconductivity. Here we show the effect
of transition metal (Co) substitution at the iron site on the superconducting
properties of CeO0.9F0.1FeAs (Tc ~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 Tc of 9.8 K
which is lower than the Tc obtained when either the conducting layer (CeAs) or
charge reservoir layer (CeO) is individually doped. This suggests a clear
interplay between the two layers to control the superconductivity. The
resistivity shows a T2 dependence (T >>Tc) which indicates strong
electron-electron correlation. Hall coefficient and thermoelectric power
indicate increased carrier concentration with cobalt doping in CeO0.9F0.1FeAs.
The rf penetration depth both for CeO0.9F0.1Fe0.95Co0.05As and CeO0.9F0.1FeAs
show an exponential temperature dependence with a gap value of ~ 1.6 and 1.9
meV. A resistance minimum is observed in the normal state near Tc which also
shows negative magnetoresistance and provides evidence for the onset of weak
localization.
