Superconducting gap structure and impurity effect in tetragonal FeSe_1-xS_x

FeSe-based materials have attracted much interest because of several unconventional superconducting features, such as the FFLO state, the BCS-BEC crossover, and the topological superconductivity. Furthermore, recent muon measurements revealed the time reversal symmetry breaking in orthorhombic and tetragonal phases of FeSe_1-xS_x. This result inspired theoretical studies on the ultranodal superconducting state with Bogoliubov Fermi surfaces, which is consistent with the large residual quasiparticle density of states experimentally observed in the tetragonal FeSe_1-xS_x. However, the nature of the superconducting gap structure in FeSe_1-xS_x is still elusive. Thus, we performed magnetic penetration depth measurements and electron irradiation to study the superconducting state in tetragonal FeSe_1-xS_x. We found a successive decrease in superconducting transition temperature and changes of the low-energy quasiparticle excitations with increasing the irradiation doses. From the experimental results, we will discuss the possibility of the ultranodal state in tetragonal FeSe_1-xS_x.