London penetration depth in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni) superconductors irradiated with heavy ions

TL;DR

This study investigates how heavy ion irradiation-induced disorder affects the London penetration depth in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ superconductors, revealing a nodeless $s^$ pairing state influenced by impurity scattering.

Contribution

It provides experimental evidence linking disorder to the suppression of $T_c$ and the change in penetration depth behavior, supporting the $s^$ pairing symmetry with intermediate impurity scattering.

Findings

Irradiation decreases $T_c$ without broadening the transition.

Power-law behavior of $mbda(T)$ with decreasing exponent $n$ as disorder increases.

Supports nodeless $s^$ pairing with interband impurity scattering.

Abstract

Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, $\lambda(T)$, in single crystals of Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni). An increase of the irradiation dose results in a monotonic decrease of the superconducting transition temperature, $T_c$, without affecting much the transition width. In both Co and Ni doped systems we find a power-law behavior, $\Delta\lambda(T) \propto T^n$, with the exponent $n$ systematically decreasing with the increase of disorder. This observation, supported by the theoretical analysis, conclusively points to a nodeless $s^\pm$ state with pairbreaking impurity scattering (interband) with strength being intermediate between Born and unitary limits.