Anomalous pressure dependence of the superconductivity in noncentrosymmetric $\mathrm{LaNi}{\mathrm{C}}_2$: Evidence of strong electronic correlations

The superconductivity of the noncentrosymmetric system $\mathrm{LaNi}{\mathrm{C}}_2$ has been studied under high pressures up to 30 GPa with electrical resistivity measurements. For this superconducting state, the breaking of time-reversal symmetry was shown recently in a muon spin relaxation $\left(\mu \mathrm{SR}\right)$ experiment, which leads to nonunitary spin-triplet superconductive pairings. The present experiments on this superconductor reveal that the transition temperature $T_{\mathrm{c}}$ greatly increases at the rate of 0.25 (±0.01) K/GPa up to 3 GPa. However, above this pressure, $T_{\mathrm{c}}$ gradually decreases; and at the pressures over 8 GPa, the superconductivity disappears completely. With this disappearance of the superconductivity, a different state with a high-energy scale dramatically emerges. These results indicate that the system is not a simple normal metal, but is rather highly correlated with strong electronic interactions which would contribute to its superconductivity.

Figure 1

Crystal structure of $\mathrm{LaNi}{\mathrm{C}}_2$. There is no centrosymmetry along the $c$ axis.

Figure 2

Electrical resistivity under high pressures up to about 3 GPa obtained with the piston-cylinder cell. Inset: The pressure dependence of the superconducting transition temperature $T_{\mathrm{c}}$.

Figure 3

Electrical resistivity under high pressures up to 8 GPa obtained with the cubic-anvil cell. Inset: The resistivity under high pressures at high temperatures.

Figure 4

Electrical resistivity under high pressures up to about 30 GPa obtained with the diamond-anvil cell.

Figure 5

$P\text{−}T$ phase diagram of $\mathrm{LaNi}{\mathrm{C}}_2$. The mark of M denotes metal, S shows superconductivity, and PI indicates a pressure induced state with a high energy scale. The PI is characterized by large and broad anomalies appearing in the resistivity. Inset: The data up to 250 K. There the superconducting transition temperature is multiplied by the factor of 10.