Unconventional superconductivity and an ambient-pressure magnetic quantum critical point in single-crystal LaNiC2

Superconductivity in noncentrosymmetric LaNiC2 is expected to be induced by electron-phonon interactions due to its lack of magnetic instabilities. The non-Bardeen-Cooper-Schrieffer (BCS) behaviors found in this material call into question the long-standing idea that relates unconventional superconductivity with magnetic interactions. Here we report magnetic penetration-depth measurements in a high-purity single crystal of LaNiC2 at pressures up to 2.5 GPa and temperatures down to 0.04 K. At ambient pressure and below 0.5Tc the penetration depth goes as T4 for the in-plane and T2 for the out-of-plane component, firmly implying the existence of point nodes in the energy gap and the unconventional character of this superconductor. The present study also provides evidence of magnetism in LaNiC2 by unraveling a pressure-induced antiferromagnetic phase inside the superconducting state at temperatures below 0.5 K, with a quantum critical point around ambient pressure. The results presented here maintain a solid base for the notion that unconventional superconductivity only arises near magnetic order or fluctuations.