Tuning the distance to a possible ferromagnetic quantum critical point in A₂Cr₃As₃

Although superconductivity in the vicinity of antiferromagnetic (AFM) instability has been extensively explored in the last three decades or so, superconductivity in compounds with a background of ferromagnetic (FM) spin fluctuations is still rare. We report ⁷⁵As nuclear quadrupole resonance measurements on the A₂Cr₃As₃ family, which is the first group of Cr-based superconductors at ambient pressure, with A being alkali elements. From the temperature dependence of the spin-lattice relaxation rate (1/T₁), we find that by changing A in the order of A=Na, Na_0.75K_0.25, K, and Rb, the system is tuned to approach a possible FM quantum critical point (QCP). This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90^◦, which enhances the FM interaction via the Cr2-As2-Cr2 path. Upon moving away from the QCP, the superconducting transition temperature T_sc increases progressively up to 8.0 K in Na₂Cr₃As₃, which is in sharp contrast to the AFM case where T_sc usually shows a maximum around a QCP. The 1/T₁ decreases rapidly below T_sc with no Hebel-Slichter peak, and ubiquitously follows a T⁵ variation below a characteristic temperature T^∗≈0.6 T_sc, which indicates the existence of point nodes in the superconducting gap function commonly in the family. These results suggest that the A₂Cr₃As₃ family is a possible solid-state analog of superfluid ³He.