Tracing the $s_\pm$-symmetry in iron pnictides by controlled disorder

TL;DR

This study uses controlled disorder via proton irradiation and THz spectroscopy to investigate the superconducting gap symmetry in iron pnictides, providing evidence for a disorder-induced transition from $s_{}$ to $s_{++}$ symmetry.

Contribution

It demonstrates that controlled disorder can induce a transition in the superconducting order parameter symmetry in iron pnictides, confirming the $s_{}$-symmetry hypothesis.

Findings

Superconducting gap initially suppressed then recovers with irradiation.

Superfluid density decrease halts at higher disorder levels.

Results agree with theoretical predictions of $s_{}$ to $s_{++}$ transition.

Abstract

Consecutive proton irradiation of iron pnictides reduces the superconducting transition temperature to decrease monotonically. Our systematic optical THz investigations of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin films reveal, however, that the low-energy superconducting gap is first suppressed, but recovers for higher irradiation doses. At the same time the decrease of the superfluid density with disorder comes to a halt. These observations are in excellent agreement with theoretical calculations, which predict a disorder-induced transition of the order parameter from $s_{\pm}$ to $s_{++}$ symmetry. Thereby we solve the longstanding puzzle of the iron pnictides gap symmetry in favor of $s_{\pm}$-symmetry and demonstrate THz spectroscopy as a tool to determine the symmetry of the superconducting order parameter.