Spin-singlet superconductivity with a full gap in locally non-centrosymmetric SrPtAs
Abstract
We report 195Pt-NMR and 75As-NQR measurements for the locally non-centrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin lattice relaxation rate 1/T1 shows a well-defined coherence peak below Tc and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below Tc down to T<Tc/6. These data together with the penetration depth obtained from the NMR spectra can be consistently explained by assuming a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-breaking is not large enough to bring about an exotic superconducting state, or the inter-layer hopping interaction is larger than the spin-orbit coupling.