Unconventional superconductivity and an ambient-pressure magnetic quantum critical point in single-crystal LaNiC$_2$

TL;DR

This study reveals unconventional superconductivity with point nodes in LaNiC₂, driven by magnetic fluctuations near an ambient-pressure quantum critical point, challenging the idea that such superconductivity is solely electron-phonon mediated.

Contribution

It provides the first evidence of magnetism in LaNiC₂ and demonstrates a pressure-induced antiferromagnetic phase within the superconducting state at ambient pressure.

Findings

Superconductivity in LaNiC₂ exhibits point nodes, indicating unconventional pairing.

Magnetism appears under pressure, with a quantum critical point near ambient pressure.

Unconventional superconductivity is linked to magnetic fluctuations rather than solely electron-phonon interactions.

Abstract

Superconductivity in noncentrosymmetric LaNiC$_2$ 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 LaNiC$_2$ at pressures up to 2.5 GPa and temperatures down to 0.04 K. At ambient pressure and below 0.5$T_c$ the penetration depth goes as $T^4$ for the in-plane and $T^2$ 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 first evidence of magnetism in LaNiC$_2$ 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.