Broken time-reversal symmetry in a new non-centrosymmetric superconductor Re8NbTa

Re-based superconductors provide a rich platform for the study of unconventional superconductivity. We have investigated the superconducting properties of Re_8NbTa, a new noncentrosymmetric cubic (α-Mn structure) rhenium-based ternary superconductor using transport, magnetization, specific heat, and muon spin rotation/relaxation (μSR) measurements. Specific heat and transverse field μSR measurements suggest moderately coupled fully gapped superconductivity, well described by BCS theory. However, our zero-field μSR measurements reveal a small internal field onsetting around the superconducting T_c, indicating that the superconducting order parameter breaks the time-reversal symmetry.