Lifting of Gap Nodes by Disorder in Tetragonal FeSe1-x Sx Superconductors

The observation of time-reversal symmetry breaking and large residual density of states in tetragonal FeSe1-xSx suggests a novel type of ultranodal superconducting state with Bogoliubov Fermi surfaces (BFSs). Although such BFSs in centrosymmetric superconductors are expected to be topologically protected, the impurity effect of this exotic superconducting state remains elusive experimentally. Here, we investigate the impact of controlled defects introduced by electron irradiation on the superconducting state of tetragonal FeSe1-xSx (0.18≤x≤0.25). The temperature dependence of magnetic penetration depth is initially consistent with a model with BFSs in the pristine sample. After irradiation, we observe a nonmonotonic evolution of low-energy excitations with impurity concentrations. This nonmonotonic change indicates a transition from nodal to nodeless, culminating in gapless with Andreev bound states, reminiscent of the nodal s± case. This points to the accidental nature of the possible BFSs in tetragonal FeSe1-xSx, which are susceptible to disruption by the disorder.