Nodeless superconductivity and time-reversal symmetry breaking in the noncentrosymmetric superconductor Re 24 Ti 5

The noncentrosymmetric superconductor Re24Ti5, a time-reversal symmetry- (TRS-) breaking candidate with Tc=6K, was studied by means of muon-spin rotation/relaxation (μSR) and tunnel-diode oscillator techniques. At the macroscopic level, its bulk superconductivity was investigated via electrical resistivity, magnetic susceptibility, and heat-capacity measurements. The low-temperature penetration depth, superfluid density, and electronic heat capacity all evidence an s-wave coupling with an enhanced superconducting gap. The spontaneous magnetic fields revealed by zero-field μSR below Tc indicate a time-reversal symmetry breaking and thus the unconventional nature of superconductivity in Re24Ti5. The concomitant occurrence of TRS breaking also in the isostructural Re6(Zr,Hf) compounds hints at its common origin in this superconducting family and that an enhanced spin-orbital coupling does not affect pairing symmetry.