Time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Ti

We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0K) using a muon-spin rotation/relaxation technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse-field muon measurements suggest nodeless s-wave superconductivity. Similar results were also observed for Re6X (X = Zr, Hf) family of materials which indicates that the pairing symmetry does not depend on the spin-orbital coupling. Altogether, these studies suggest an unconventional nature (TRSB) of superconductivity is intrinsic to the Re6X family of compounds and paves the way for further studies of this family of materials.