AC susceptibility investigation of vortex dynamics in nearly-optimally doped REFeAsO${}_{1-x}$F${}_x$ superconductors (RE = La, Ce, Sm)

Abstract

Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO${}_{0.9}$F${}_{0.1}$ and CeFeAsO${}_{0.92}$F${}_{0.08}$ superconductors, complementing earlier results on SmFeAsO${}_{0.8}$F${}_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_c$ in the range of applied fields ($H\le 5$ T). This result indicates that SmFeAsO${}_{0.8}$F${}_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_0$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{cr}\lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth ${\lambda }_{ab}(0)$ and the anisotropy parameter $\gamma$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{\pm }$ two-gap scenario.

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