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.
Categories: cond-mat.supr-con