Time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Ti

We have investigated the superconducting state of the noncentrosymmetric superconductor ${\mathrm{Re}}_{6}\mathrm{Ti}$ (${T}_{c}=6.0$ K) 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 ${\mathrm{Re}}_{6}X$ ($X=\text{Zr}$, 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 ${\mathrm{Re}}_{6}X$ family of compounds and paves the way for further studies of this family of materials.