Ferromagnetic Spin Fluctuation and Unconventional Superconductivity in ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ Revealed by ${}^{75}\mathrm{As}$ NMR and NQR

We report ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ with a quasi-one-dimensional crystal structure. Below $T\sim 100\mathrm{K}$, the spin-lattice relaxation rate ($1/T_1$) divided by temperature, $1/T_{1}T$, increases upon cooling down to $T_c=4.8\mathrm{K}$, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, $1/T_1$ decreases rapidly below $T_c$ without a Hebel-Slichter peak, and follows a $T^5$ variation below $T\sim 3\mathrm{K}$, which points to unconventional superconductivity with point nodes in the gap function.

Figure 1

(a) ${}^{75}\mathrm{As}$ NQR spectra of ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. The solid curves are Lorentzian function fittings with area ratio 1:1 of the two peaks. (b) The crystal structure viewed from $c$-axis direction. (c) The $T$ dependence of the ${}^{75}\mathrm{As}$ NQR frequency ${\nu }_Q$. The solid curves are guides for the eyes.

Figure 2

The temperature dependence of the ${}^{75}\mathrm{As}$ NQR spin-lattice relaxation rate divided by temperature ($1/T_{1}T$).

Figure 3

(a) ${}^{75}\mathrm{As}$ NMR spectra at 50 and 3.1 K. Each site of As has two peaks with $\theta =42°$ (around 81 MHz) and $\theta =90°$ (around 91 MHz). (b) The enlarged part of the peak with $\theta =90°$.

Figure 4

The $T$ dependence of the ${}^{75}\mathrm{As}$ Knight shift $K$ at the As(1) site in the normal state. The error bar was estimated from the frequency interval in measuring the NMR spectra.

Figure 5

(a) An enlarged part of $1/T_{1}T$ for $T_c\le T<80\mathrm{K}$. The solid curves are fittings by the Curie-Weiss law, $1/T_{1}T=a+b/(T+{\theta }_C)$, with ${\theta }_C\sim 0\mathrm{K}$ (see text). (b) A plot of the same data as (a) to emphasize the low-$T$ part.

Figure 6

The $T$ dependence of $1/T_1$ at zero magnetic field. The arrow indicates $T_c$ and the straight line indicates $1/T_1\propto T^5$. The curve below $T_c$ is a calculation of the ABM gap with ${\mathrm{\Delta }}_0=2.1k_B{T}_c$ and $\delta ={\mathrm{\Delta }}_0/15$, where $\delta$ is the broadening width of the energy level proposed by Hebel [26] to account for the effects such as impurity scattering [27]. The dotted curve above $T_c$ is the same result as the curve in Fig. 5. A slight deviation from the data points is seen at high $T$, which may indicate that the 1D Fermi sheets can play some role at high $T$ [21].