Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs Using ${}^{75}\mathrm{As}$-Nuclear Quadrupole Resonance

Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure $P_c$ between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate $1/T_1$ reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in $1/T_1$ in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor.

Figure 1

Zero-field NMR spectra for the ${}^{75}\mathrm{As}$ nucleus at ambient pressure. The complicated spectrum at 5 K can be almost reproduced by a model based on the HM state reported by previous neutron scattering measurements, as shown by the red curve. The crystal and magnetic structures are shown. The As site is surrounded by six neighboring Cr sites composed of three inequivalent sites for the As site. The inset shows the temperature dependence of $H_{\text{int}}$ at the As site, which decreased gradually with increasing temperature and remained large at $T_N$ due to the first-order transition.

Figure 2

Pressure dependence of spectra at 5 K. The spectrum is broadened by applying pressure, but its structure is similar up to 0.53 GPa. At 0.74 GPa close to $P_c$, the signal from the HM phase is suppressed and the new signal corresponding to the PM phase appears, indicating that the phase separation between the HM and PM phases occurs in the vicinity of $P_c$. The intensity of the HM phase at 0.74 GPa is $\sim 5$ times smaller than that at 0.53 GPa. The inset shows the pressure dependence of $H_{\text{int}}$, which has a large value on the border of the HM-PM transition as well as the case of the temperature dependence shown in the inset in Fig. 1. The dotted line represents $P_c$, as determined by resistivity measurement [2].

Figure 3

Temperature dependence of $1/T_{1}T$ in the PM state, where superconductivity occurs. The increase in $1/T_{1}T$ toward low temperatures demonstrates the development of magnetic fluctuations. The inset shows the same data between 0.81 and 2.53 GPa in a logarithmic scale. Even at 0.81 GPa, $1/T_{1}T$ deviates from the behavior expected at a QCP below $\sim 20\mathrm{K}$. The characteristic temperature $T^{*}$ was determined from an intersection point of each extrapolation from the low-temperature and high-temperature sides, as shown for the data at 1.59 GPa.

Figure 4

Pressure-temperature phase diagram of CrAs. The $T_N$ and $T_c$ are taken from Ref. [2] and Supplemental Material [20]. The closed (open) squares represent $T_N$ obtained during the cooling (warming) process. These observations of hysteresis and the present NQR spectrum evidence that the HM-PM transition is of the first order in all pressure-temperature ranges. Even in the vicinity of $P_c$, the system is close to the FL at low temperatures.

Figure 5

(a) Temperature dependence of $1/T_{1}T$ below 5 K measured under a zero field. The clear reduction in $1/T_{1}T$ is observed below $T_c=1.85\mathrm{K}$, which is determined by the change in the resonance frequency shown in Fig. 4. The green curve indicates a calculation based on a conventional BCS, whereas the red curve indicates one based on a line-node model (polar type). $1/T_{1}T$ is clearly reproduced by the unconventional model. (c) Temperature dependence of $1/T_1$ at 0.81 GPa. The red curve based on the line-node model is also plotted. The temperature dependence of $1/T_1$ is close to $T^3$ below $T_c$.