Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$

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.

Figure 1

(a) Temperature dependence of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ lattice parameters $a$, $c$, and unit cell volume $V$ extracted from Rietveld refinements of the NPD patterns normalized to 300 K values. (b)–(d) Low-temperature behavior of $a$, $c$, and $V$. Polynomial fits to the high-temperature data ($20\text{K}<T<300\text{K}$) are plotted as guides to the eye. All $y$-axis ranges show the same relative percent change of the parameters. (e) The crystal structure of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ seen along the $c$ axis and (f) the structure of the DWS with angles As2-Cr2-As2 and As1-Cr1-As1 denoted by $\alpha$ and $\beta$, respectively. In (a)–(d), the error bars are smaller than the individual data point markers.

Figure 2

Temperature dependence of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. (a) As-Cr, (b) Cr-Cr, and (c) As-Cr-As bond lengths and angles as determined from Rietveld refinements. The upper panels focus on $T<20\text{K}$ while the lower panels show all measured temperatures. The inset of (d) shows the degree of noncentrosymmetry as measured by the difference of the two As-Cr-As angles. All scales have been configured to cover similar ranges in percent change of the plotted parameter. Linear fits are provided as guides to the eye.

Figure 3

(a) Neutron spectra intensity map of the dynamic structure factor with the intensity normalized to the monitor. (b) Simulated spin-wave dispersions of the UUDD magnetic structure with arbitrary intensity scale. (c) $Q$ dependence of the scattering intensity for $E$ summed over 2–9 meV for all measured temperatures. Each temperature is offset by an arbitrary $y$ value for visual clarity. Difference curves for the 10 K summed intensity subtracted from the (d) 1.5, (e) 100, and (f) 200 K summed intensities. Lines are added to the plots as guides to the eye. (e) UUDD magnetic structure consistent with $Q\sim 0.75$ Å${}^{-1}$ (magnetic space group $P_{c}3c1$).