Evidence for Unconventional Superconductivity in the Non-Oxide Perovskite $mathrm{MgCNi_3}$ from Penetration Depth Measurements

The London penetration depth, $lambda(T)$, was measured in polycrystalline powders of the non-oxide perovskite superconductor $mathrm{MgCNi_3}$ 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.