Exotic superconductivity in noncentrosymmetric and magnetic ${\mathrm{CeNiC}}_2$ revealed under high pressure

We have found that magnetically ordered noncentrosymmetric ${\mathrm{CeNiC}}_2$ exhibits superconductivity under very high pressures of near 11 GPa. The transition temperature $T_{\mathrm{c}}$ is 3.5 K, the highest in all Ce heavy-fermion superconductors, implying quite strong electron pairings with a high-energy scale. Several physical quantities show diverging features of a quantum phase transition, however, its criticality appears in a rather narrow range of pressure. The upper critical field ${\mu }_0{H}_{\mathrm{c}2}(T=0)$ is estimated to be 18 T, much higher than the Pauli paramagnetic limiting field of 6.5 T, indicating spin-triplet electron pairings correlated with its noncentrosymmetric structure.

Figure 1

Electrical resistivity of ${\mathrm{CeNiC}}_2$ under high pressures below 50 K. (a) The data from 0 to 8 GPa. (b) Those from 9 to 14 GPa. The black arrows indicate the transition temperatures $T_{\mathrm{AFIC}}$. SC is observed at 3.5 K under pressures around 11 GPa, shown with the (red) arrow. The insets display the resistivity up to 300 K.

Figure 2

Electrical resistivity near 11 GPa and below 8 K. The inset shows the ac magnetic susceptibility, indicating diamagnetic signals.

Figure 3

$P\text{−}T$ phase diagram. Magnetic states: P, AFIC, AFC, and F. The transition temperatures for AFC and F could be obtained only up to 2 GPa. See text. Pressure reveals SC, NFL, FL, and KE. $A$ is the coefficient of the $T^2$ dependence of the resistivity. The temperatures for NFL, FL, and KE are multiplied by a factor of 1/2.

Figure 4

${\mu }_0{H}_{\mathrm{c}2}$ vs $T_{\mathrm{c}}$ determined from the field dependence of the resistivity under 11.4 GPa shown in the inset. ${\mu }_0{H}_{\mathrm{c}2}$ at $T=0$ is estimated to be 18 T. That for ${\mathrm{LaNiC}}_2$, at ambient pressure, is about 0.3 T; the data are multiplied by a factor of 5. The dotted lines show the fits to the WHH theory.