Quasi-one-dimensional spin excitations in the iron pnictide NaFe$_{0.53}$Cu$_{0.47}$As

Abstract

Spectroscopic measurements in model one-dimensional (1D) correlated systems offer insights for understanding their two-dimensional counterparts, which include the cuprate and iron pnictide/chalcogenide superconductors. A major challenge is the identification of such correlated systems with dominantly 1D physics. In this work, inelastic neutron scattering measurements on NaFe$_{0.53}$Cu$_{0.47}$As single crystal directly reveal quasi-1D spin excitations, resulting from atomic order that lead to magnetic Fe and nonmagnetic Cu chains. The dominant exchange interaction is antiferromagnetic along the chain ($SJ_{\rm \parallel}\approx90.1(3)$~meV), whereas the inter-chain couplings are much weaker ($SJ_{\rm \perp}\approx-2.4(1)$~meV and $SJ_{\rm c}\approx0.15(5)$~meV). The quasi-1D spin excitations in NaFe$_{0.53}$Cu$_{0.47}$As stem from both the Néel and stripe vectors, with Néel excitations sensitive to Fe impurities on the Cu site. The spin excitations in quasi-1D NaFe$_{0.53}$Cu$_{0.47}$As and quasi-2D FeSe exhibit a striking resemblance, suggesting a common origin for their coexistent stripe and Néel excitations. Our findings demonstrate magnetic dilution in NaFeAs leads to dimension reduction of its magnetic degree of freedom, presenting a strategy for discovering low-dimensional quantum materials.

Figures (4)

• Figure 1: (a) Schematic crystal and magnetic structures of NFCA50. In-plane magnetic structures for (b) NaFeAs and (c) NFCA50. These structures (blue symbols) and their 90$^{\circ}$-rotated twins (red symbols) lead to magnetic and superstructure peaks in (d) and (e), where the main Bragg peaks are not shown. Elastic scattering data for NFCA47 in the (f) $L=0.5$ and the (g) $L=0$ planes. The data are folded into a single quadrant and reproduced in other quadrants. (h) Temperature dependence of the normalized elastic magnetic scattering in NFCA. The $x=0.47$ data are measured with polarized neutrons SI. The $x=0.39$ and 0.44 data are from Ref. Song2016, measured with unpolarized neutrons. (i) The local susceptibility $\chi(\omega)"$ of NFCA47, compared with that of NaFeAs at 5 K CZhang2014. The square, circle and triangle symbols are for $E_{\rm i}=50$ meV, 81 meV, and 245 meV, respectively. The solid lines are guides-to-the-eye.
• Figure 2: Constant-energy $HK$-maps for NFCA47 at $T=5$ K, shown in the upper half of each panel. (a)-(d) are from $E_{\rm i}=50$ meV sample rotations scans. (e)-(j) are from $k_{\rm i}\parallel c$ data. The intensities are color-encoded, and are in units of mbarn meV$^{-1}$ sr$^{-1}$ f.u.$^{-1}$, where f.u. stands for formula unit. The data are folded into a single quadrant and reproduced in other quadrants. Simulated intensities are shown in the bottom half, for NFCA50 on the left, and for NFCA47 on the right, based on a Heisenberg model for NFCA50 SI.
• Figure 3: $E-\bf{Q}$ slices for NFCA47 with $\bf{Q}$ along $[0,K]$ at (a) 5 K and (b) 300 K, (c) along $[H,1]$ at 5 K, and (d) along $[0,1,L]$ at 5 K. (a)-(c) are from $E_{\rm i}$=245 meV $k_{\rm i}\parallel c$ measurements, by binning $H=[-0.2,0.2]$ in (a) and (b), and $K=[0.9,1.1]$ in (c). Data in (d) are from $E_{\rm i}=50$ meV sample rotation scans, by binning $H=[-0.1,0.1]$ and $K=[0.9,1.1]$. The red circles are extracted from fits to constant-$\bf{Q}$ cuts, with the horizontal and vertical bars corresponding to the integration range and fit uncertainties. The curves in (a), (c) and (d) are the spin wave dispersion for a Heisenberg model SI. A similar figure showing intensity$\times$energy is reproduced in the Supplemental Material SI.
• Figure 4: (a) Schematic configurations for the two twins of NFCA50. (b) Schematics for two kinds of spin correlations proposed for FeSe. In both NFCA50 and FeSe, the spin correlations are dominantly quasi-1D, represented by the shaded stripe-like areas. The NNN interchain correlations are stronger than the NN interchain correlations in magnitude ($|\langle S \cdot S_{\rm NNN}\rangle|>|\langle S \cdot S_{\rm NN}\rangle|$). (c) Schematic representation of excitations from (a) or (b), with the two colors corresponding to scattering patterns arising from the two structural twins in (a) or the two kinds of dynamic spin correlations in (b).