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

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.

12 pages, 4 figures

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• Recent progress on superconductors with time-reversal symmetry breaking
• Simultaneous Nodal Superconductivity and Time-Reversal Symmetry Breaking in the Noncentrosymmetric Superconductor CaPtAs
• Dirac lines and loop at the Fermi level in the Time-Reversal Symmetry Breaking Superconductor LaNiGa${}_2$
• Structure and superconductivity in the binary Re${}_{1-x}$Mo${}_x$ alloys
• Charge density wave and large non-saturating magnetoresistance in YNiC${}_2$ and LuNiC${}_2$
• Study of Nb${}_{0.18}$Re${}_{0.82}$ non-centrosymmetric superconductor in the normal and superconducting states
• Conventional type-II superconductivity in locally non-centrosymmetric LaRh${}_2$As${}_2$ single crystals
• Electronic structure, electron-phonon coupling and superconductivity in noncentrosymmetric ThCoC2 from ab initio calculations
• Two-Gap Time Reversal Symmetry Breaking Superconductivity in Non-Centrosymmetric LaNiC2
• Strong antisymmetric spin-orbit coupling and superconducting properties: The case of noncentrosymmetric LaPtSi
• Correlation between charge density waves and antiferromagnetism in Nd${}_{1-x}$Gd${}_x$NiC${}_2$ solid solution
• Crossover from charge density wave stabilized antiferromagnetism to superconductivity in Nd${}_{1-x}$La${}_x$NiC${}_2$ compounds