$s$-wave superconductivity in the noncentrosymmetric W${}_3$Al${}_2$C superconductor: An NMR study

Abstract

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. 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 superconductivity. 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.

8 pages, 6 figures, accepted by J. Phys.: Condens. Matter