Type-II superconductivity in W5Si3-type Nb5Sn2Al

We report the discovery of superconductivity (SC) in the ternary aluminide Nb5Sn2Al, which crystallizes in the W5Si3-type structure with one-dimensional Nb chains along the c-axis. It is found that the compound has a multiband nature and becomes a weakly coupled, type-II superconductor below 2.0 K. The bulk nature of SC is confirmed by the specific heat jump, whose temperature dependence shows apparent deviation from a single isotropic gap behavior. The lower and upper critical fields are estimated to be 2.0 mT and 0.3 T, respectively. From these values, we derive the penetration depth, coherence length and Ginzburg-Landau parameter to be 516 nm, 32.8 nm and 15.6, respectively. In contrast, the isostructural compound Nb5Sn2Al does not superconduct above 0.5 K. A comparison of these results with other W5Si3-type superconductors suggests that T-c of these compounds correlates with the average number of valence electrons per atom.