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

TL;DR

This study investigates vortex dynamics in nearly-optimally doped REFeAsO$_{1-x}$F$_{x}$ superconductors using ac susceptibility and magnetization measurements, revealing phase diagrams, depinning energies, and superfluid density suppression.

Contribution

It provides new insights into vortex behavior, phase diagrams, and superfluid density in nearly-optimally doped REFeAsO$_{1-x}$F$_{x}$ superconductors, extending previous work on Sm-based compounds.

Findings

SmFeAsO$_{0.8}$F$_{0.2}$ shows the most promising application potential.

A crossover field separates single and collective depinning regimes.

Superfluid density is significantly suppressed in a two-gap scenario.

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 ($H \leq 5$ 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.