s -wave superconductivity in the noncentrosymmetric W 3 Al 2 C superconductor: an NMR study.

We report on a microscopic study of the noncentrosymmetric superconductor W 3 Al 2 C (with T c = 7.6 K), mostly by means of 27 Al- and 13 C nuclear magnetic resonance (NMR). Since in this material the density of states at the Fermi level is dominated by the tungsten's 5 d orbitals, we expect a sizeable spin-orbit coupling (SOC) effect. The normal-state electronic properties of W 3 Al 2 C resemble those of a standard metal, but with a Korringa product 1/( T 1 T ) significantly smaller than that of metallic Al, reflecting the marginal role played by s -electrons. In the superconducting state, we observe a reduction of the Knight shift and an exponential decrease of the NMR relaxation rate 1/ T 1 , typical of s -wave superconductivity (SC). This is further supported by the observation of a small but distinct coherence peak just below T c in the 13 C NMR relaxation-rate, in agreement with the fully-gapped superconducting state inferred from the electronic specific-heat data well below T c . The above features are compared to those of members of the same family, in particular, Mo 3 Al 2 C, often claimed to exhibit unconventional SC. We discuss why, despite the enhanced SOC, W 3 Al 2 C does not show spin-triplet features in its superconducting state and consider the broader consequences of our results for noncentrosymmetric superconductors in general.