Probing time-reversal symmetry breaking in FeSe_1-xTe_x superconductors by muon spin relaxation

The FeSe-based superconductor is an attractive quantum material that exhibits inherently exotic quantum phenomena such as nontrivial topology, superconductivity, nematicity, and magnetism. In particular, FeSe_1-xTe_x superconductors have nontrivial band topology, which provides a promising platform for hosting Majorana particles. In addition, recent reports on FeSe_1-xTe_x have found a nematic quantum critical point (QCP) near x = 0.5. In a sister system FeSe_1-xS_x, which also has a nematic QCP at x ~ 0.17, the time-reversal symmetry breaking (TRSB) in the superconducting state has been reported by the muon spin relaxation (μSR) measurements and a possible ultranodal state for the tetragonal phase at x > 0.17 has been discussed. Therefore, it is important to study how the TRSB superconducting state evolves in the FeSe_1-xTe_x system.

Here, we report the results of zero-field (ZF) and transverse-field (TF) μSR measurements in single crystals of FeSe_1-xTe_x both inside and outside the nematic phase. We will discuss the time-reversal symmetry breaking from the ZF results and the gap structure in the superconducting state through the temperature dependence of penetration depth from the TF results. From these results, we construct a combined superconducting phase diagram of FeSe_1-xS_x and FeSe_1-xTe_x with QCPs, which gives an important input for understanding the relationship between the superconducting state, nematicity, and nontrivial topology.