Evidence for Unconventional Superconductivity in the Non-Oxide Perovskite $\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{N}{\mathrm{i}}_3$ from Penetration Depth Measurements

The London penetration depth, $\lambda (T)$, was measured in polycrystalline powders of the non-oxide perovskite superconductor $\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{N}{\mathrm{i}}_3$ by using a sensitive tunnel-diode resonator technique. The penetration depth exhibits distinctly non s-wave BCS low-temperature behavior, instead showing quadratic temperature dependence, suggestive of a nodal order parameter.