Full Gap Superconductivity in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Muon Spin Rotation

TL;DR

This study uses muon spin rotation to reveal that Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ exhibits full-gap superconductivity consistent with BCS s-wave pairing, showing strong coupling and possible single or double-gap structures.

Contribution

It provides direct experimental evidence for full-gap s-wave superconductivity in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, including detailed gap parameters and strong coupling indications.

Findings

Superfluid density matches BCS s-wave model.

Evidence for single-gap or double-gap structure.

Large gap ratios indicate strong coupling.

Abstract

Superfluid density ($n_s$) in the mixed state of an iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin rotation for a sample with optimal doping ($x=0.4$). The temperature dependence of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap with $\Delta=8.35(6)$ meV [$2\Delta/k_BT_c=5.09(4)$], or double-gap structure with $\Delta_1=12$ meV (fixed) [$2\Delta_1/k_BT_c=7.3$] and $\Delta_2=6.8(3)$ meV [$2\Delta_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of angle-resolved photo-emssion spectroscopy. The large gap parameters ($2\Delta/k_BT_c$) indicate extremely strong coupling of carriers to bosons that mediate the Cooper pairing.