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

## TL;DR

This study investigates how changing alkali elements in A2Cr3As3 compounds tunes the system toward a ferromagnetic quantum critical point, affecting superconductivity and revealing point nodes in the gap function.

## Contribution

It demonstrates the tuning of A2Cr3As3 compounds toward a ferromagnetic QCP and links this to changes in superconducting properties, a novel insight in Cr-based superconductors.

## Key findings

- Approaching FM QCP correlates with increased Tsc in Na2Cr3As3.
- Superconducting gap exhibits point nodes, indicated by T^5 dependence of 1/T1.
- Superconductivity is enhanced away from the QCP, contrasting AFM systems.

## Abstract

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, Na0.75K0.25, K, and Rb, the system is tuned to approach a FM quantum critical point (QCP). This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90 degrees, which enhances the FM interaction via the Cr2-As2-Cr2 path. Upon moving away from the QCP, the superconducting transition temperature Tsc increases progressively up to 8.0 K in Na2Cr3As3, which is in sharp contrast to the AFM case where Tsc usually shows a maximum around a QCP. The 1/T1 decreases rapidly below Tsc with no Hebel-Slichter peak, and ubiquitously follows a T5 variation below a characteristic temperature T*=0.6 Tsc, 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.