Quasi-one-dimensional $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ (with $A=\text{K}$, Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet, in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism—an absence which defies predictions of first-principles models. We report the results of neutron scattering experiments on polycrystalline ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($T_c\sim 7\text{K}$) which probed the low-temperature dynamics near $T_c$. Neutron diffraction data evidence a subtle response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wave vector $q=\left(00\frac{1}{2}\right)$ which loses intensity beneath $T_c$—indicative of short-range magnetic fluctuations. Using linear spin-wave theory, we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity, in direct analogy with the iron-based superconductors.

• Magnetism
• Superconductivity
• 1-dimensional systems
• Inelastic neutron scattering
• Neutron diffraction