Intergrain effects in the AC susceptibility of polycrystalline LaFeAsO_{0.94}F_{0.06}: comparison with cuprate superconductors

TL;DR

This study investigates the intergrain effects in the AC susceptibility of polycrystalline LaFeAsO_{0.94}F_{0.06} superconductor, revealing that intergrain critical current is governed by Josephson critical current rather than vortex pinning.

Contribution

It provides a detailed analysis of intergrain effects in LaFeAsO_{0.94}F_{0.06} and compares these effects with those in cuprate superconductors, highlighting the role of Josephson critical current.

Findings

Intergrain critical current is dominated by Josephson critical current.

AC susceptibility shows a two-step transition related to grains and intergranular medium.

Intergranular susceptibility depends on AC field amplitude and frequency.

Abstract

The AC susceptibility at zero DC magnetic field of a polycrystalline sample of LaFeAsO_{0.94}F_{0.06} (T_c = 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range Hac = 0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The temperature dependence of the AC susceptibility exhibits the typical two-step transition arising from the combined response of superconduncting grains and intergranular weak-coupled medium. The intergranular part of the susceptibility strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T_c down to T = 4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.