Penetration depth and gap structure in the antiperovskite oxide superconductor Sr3 -xSnO revealed by μ SR

We report a μ SR study on the antiperovskite oxide superconductor Sr_{3 -x}SnO . By using transverse-field μ SR , we observed an increase of the muon relaxation rate upon cooling below the superconducting transition temperature T_c=5.4 K, evidencing bulk superconductivity. The exponential temperature dependence of the relaxation rate σ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth λ (0 ) ∝1 /√{σ (0 ) } to be around 320-1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal. Moreover, we established that the ratio T_c/λ (0^{) -2} is larger than those of ordinary superconductors and is comparable to those of unconventional superconductors. The relatively high T_c for small carrier density may hint at an unconventional pairing mechanism beyond the ordinary phonon-mediated pairing. In addition, zero-field μ SR did not provide evidence of broken time-reversal symmetry in the superconducting state. These features are consistent with the theoretically proposed topological superconducting state in Sr_{3 -x}SnO , as well as with ordinary s -wave superconductivity.