S-wave superconductivity in the noncentrosymmetric W₃Al₂C superconductor: An NMR study

We report on a microscopic study of the noncentrosymmetric superconductor W3Al2C (with T c = 7.6 K), mostly by means of 27Al-and 13C nuclear magnetic resonance (NMR). Since in this material the density of states at the Fermi level is dominated by the tungsten's 5d orbitals, 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/(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 13C 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, 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.