Consecutive proton irradiation of iron pnictides reduces the superconducting transition temperature to decrease monotonically. Our systematic optical THz investigations of Ba(Fe0.9Co0.1)2As2 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± to s++ symmetry. Thereby we solve the longstanding puzzle of the iron pnictides gap symmetry in favor of s±-symmetry and demonstrate THz spectroscopy as a tool to determine the symmetry of the superconducting order parameter.
@article{arxiv.1511.05017,
title = {Tracing the $s_\pm$-symmetry in iron pnictides by controlled disorder},
author = {Micha Benjamin Schilling and Andreas Baumgartner and Boris Gorshunov and Elena Zhukova and Valery Dravin and Kiril Mitsen and Dmitri Efremov and Oleg Dolgov and Kazumasa Iida and Martin Dressel and Sina Zapf},
journal= {arXiv preprint arXiv:1511.05017},
year = {2016}
}