materials*

Quasi-one-dimensional A_xCr₃As₃ (with A = K, Cs, Rb and x = 1,2) is an intriguing new family of superconductors which exhibit many similar features to the high temperature 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 K₂Cr₃As₃ (T_c~7K) which probed the low temperature dynamics near Tc. Neutron diffraction data evidence a strong response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals highly dispersive scattering at the wavevector k=(001/2) indicative of short-range magnetic fluctuations.  Furthermore, ab initio calculations predict and neutron total scattering confirms a structural instability. These observations suggest that K₂Cr₃As₃ is in close proximity to multiple instabilities - in direct analogy with the iron-based and cuprate superconductors.