The FeSe_1-xS_x superconductors involving non-magnetic nematic phase and its quantum criticality provide a unique platform to investigate the relationship between nematicity and superconductivity [1]. It has been shown that across the nematic quantum critical point, the superconducting properties change drastically [2,3], and the non-nematic tetragonal FeSe_1-xS_x (x>0.17) exhibits substantial low-energy states despite the high-quality of crystals. Here we report on the muon spin rotation (µSR) measurements on FeSe_1-xS_x (x=0, 0.20, 0.22) and the observation of spontaneous internal field below the superconducting transition temperature T_c, providing strong evidence for time-reversal breaking (TRSB) state in bulk FeSe_1-xS_x [4]. We also find that the superfluid density in the tetragonal crystals is suppressed from the expected value, indicating the presence of non-superconducting carriers. These results in FeSe_1-xS_x are consistent with the recently proposed topological phase transition to a novel ultranodal pair state with Bogoliubov Fermi surface [5].

[1] See, for a review, T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020).
[2] Y. Sato et al., Proc. Natl. Acad. Sci. USA 115, 1227-1231 (2018).
[3] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).
[4] K. Matsuura et al., (unpublished).
[5] C. Setty, S. Bhattacharyya, Y. Cao, A. Kreisel, and P. J. Hirschfeld, Nat. Commun. 11, 523 (2020).

Keywords: Time reversal symmetry breaking, Electronic nematicity, Qunatum critical point, Unconventional superconductivity