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Doping dependence of the upper critical field, superconducting current density and thermally activated flux flow activation energy in polycrystalline CeFeAsO1-xFx superconductors

We report the results from resistivity and magnetic measurements on polycrystalline Ce oxypnictide (CeFeAsO1-xFx) samples where x spans from 0.13 to 0.25. We find that the orbital limiting field is as high as 150 T and it systematically decreases with increasing doping. The Maki parameter is greater than one across the phase diagram and the large Maki parameter suggests that orbital and Pauli limiting effects contribute to the upper critical field. The broadening of the superconducting transition in the resistivity data was interpreted using the thermally activated flux flow (TAFF) model where we find that the TAFF activation energy, U0(B), is proportional to B^{-(gamma)} from 1 T to high fields, and (gamma) does not significantly change with doping. However, U0 and the superconducting critical current, Jc, are peaked in the mid-doping region (x = 0.15 to x = 0.20), and not in the low (x < 0.15) or high doping (x > 0.20) regions. Furthermore, U0 is correlated with Jc and follows the two fluid model for granular samples.