Impact of iron-site defects on superconductivity in LiFeAs

In iron-based high temperature superconductors the symmetry of the order parameter still remains a controversial topic where for the same compound sign changing and non sign-changing order parameters have been proposed theoretically. Among the iron-based superconductors, LiFeAs takes a special role (together with FeSe) by being a stoichiometric superconductor, minimizing intrinsic scattering. Here, we study engineered iron-site defects in LiFeAs by low temperature scanning tunneling microscopy and spectroscopy (STM/STS). The tunneling spectra obtained on individual defects show signatures of impurity bound states[1]. A detailed comparison of the tunneling spectra measured on impurities with theoretical simulations [2] enables us to draw conclusions about the superconducting order parameter in LiFeAs. Studying Ni, Co, Mn impurities and native defects, we find a continuous evolution from negligible impurity bound states at the smaller gap edge to detectable states as the scattering potential increases. [1] R. Aluru, et al., PRB 94, 134515 (2016) [2] A. Kreisel, et al., arXiv:1610.00619