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

TL;DR

This study investigates how doping levels affect the upper critical field, flux flow activation energy, and critical current in CeFeAsO1-xFx superconductors, revealing optimal doping regions and the interplay of limiting effects.

Contribution

It provides new insights into the doping dependence of critical superconducting parameters and the flux flow activation energy in polycrystalline CeFeAsO1-xFx, highlighting the role of the two fluid model.

Findings

Orbital limiting field decreases with doping.

U0 and Jc peak at mid-doping levels.

U0 correlates with Jc and follows the two fluid model.

Abstract

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.