Disorder effects and current percolation in FeAs based superconductors

TL;DR

This study investigates how atomic disorder from neutron irradiation affects superconducting properties, flux pinning, and grain connectivity in FeAs-based superconductors, revealing complex impacts on critical temperature, pinning, and current pathways.

Contribution

It provides new insights into how neutron-induced disorder influences superconducting properties and grain connectivity in FeAs-based superconductors.

Findings

Neutron irradiation decreases the transition temperature and upper critical field.

Pinning is significantly improved at optimal defect concentrations.

Weak link behavior and percolation thresholds are identified in grain connectivity.

Abstract

We report the influence of atomic disorder introduced by sequential neutron irradiation on the basic superconducting properties, flux pinning and grain connectivity. Two different polycrystalline Sm-1111 samples (SmFeAsO_{1-x}F_x) and two Ba-122 single crystals (BaFe_{1.8}Co_{0.2}As_2) were investigated. The monotonous decrease of the transition temperature with neutron fluence degrades the upper critical field, at least in the investigated temperature region. Pinning on the other hand is largely improved, with a different optimal defect concentration (fluence) in the two materials. The analysis of the current flow in the polycrystalline samples reveals weak link behaviour in the majority of grain connections and the existence of stronger grain connections. The density of the latter seems to be close to the percolation threshold (i.e. the minimum fraction for a continuous current path). Both types of connections are sensitive to disorder and degrade upon neutron irradiation.