s-wave superconductivity in the noncentrosymmetric W₃Al₂C superconductor: an NMR study

We report on a microscopic study of the noncentrosymmetric superconductor W₃Al₂C (withT_c= 7.6 K), mostly by means of²⁷Al- and¹³C nuclear magnetic resonance (NMR). Since in this material the density of states at the Fermi level is dominated by the tungsten's 5dorbitals, we expect a sizeable spin-orbit coupling (SOC) effect. The normal-state electronic properties of W₃Al₂C resemble those of a standard metal, but with a Korringa product 1/(T₁T) significantly smaller than that of metallic Al, reflecting the marginal role played bys-electrons. In the superconducting state, we observe a reduction of the Knight shift and an exponential decrease of the NMR relaxation rate 1/T₁, typical ofs-wave superconductivity (SC). This is further supported by the observation of a small but distinct coherence peak just belowT_cin the¹³C NMR relaxation-rate, in agreement with the fully-gapped superconducting state inferred from the electronic specific-heat data well belowT_c. The above features are compared to those of members of the same family, in particular, Mo₃Al₂C, often claimed to exhibit unconventional SC. We discuss why, despite the enhanced SOC, W₃Al₂C does not show spin-triplet features in its superconducting state and consider the broader consequences of our results for noncentrosymmetric superconductors in general.