Intertwined Magnetic and Nematic Orders in Semiconducting ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$

Superconductivity in the iron pnictides emerges from metallic parent compounds exhibiting intertwined stripe-type magnetic order and nematic order, with itinerant electrons suggested to be essential for both. Here we use x-ray and neutron scattering to show that a similar intertwined state is realized in semiconducting ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ (${\mathrm{K}}_5{\mathrm{Fe}}_4{\mathrm{Ag}}_6{\mathrm{Te}}_{10}$) without itinerant electrons. We find that Fe atoms in ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ form isolated $2\times 2$ blocks, separated by nonmagnetic Ag atoms. Long-range magnetic order sets in below $T_N\approx 35\mathrm{K}$, with magnetic moments within the $2\times 2$ Fe blocks ordering into the stripe-type configuration. A nematic order accompanies the magnetic transition, manifest as a structural distortion that breaks the fourfold rotational symmetry of the lattice. The nematic orders in ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ and iron pnictide parent compounds are similar in magnitude and in how they relate to the magnetic order, indicating a common origin. Since ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ is a semiconductor without itinerant electrons, this indicates that local-moment magnetic interactions are integral to its magnetic and nematic orders, and such interactions may play a key role in iron-based superconductivity.

Figure 1

(a) Crystal structure of ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$, with the unit cell expanded by $\sqrt{5}\times \sqrt{5}$ in the $ab$ plane relative to the $I4/mmm$ unit cell. (b) Magnetic structure of ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$, with only Fe atoms shown. (c) Schematic of Fe plane in ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$. (d) Schematic of Fe-Ag plane in ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$. The solid-line boxes in (c) and (d) represent the $I4/mmm$ unit cells, and the dashed-line boxes are the $\sqrt{5}\times \sqrt{5}$ superstructure unit cells. The squares shaded in red highlight the $2\times 2$ Fe blocks.

Figure 2

Ordering of Fe and Ag for superstructure (a) domain 1 and (b) domain 2. The presence of both domains results in superstructure peaks in reciprocal space, as shown for the (c) $[H,K,0]$ and (d) $[H,K,1]$ planes. The superstructure peaks due to the two domains are well separated. X-ray diffraction data of ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ in the (e) $[H,K,0]$ and (f) $[H,K,1]$ scattering planes is measured at 260 K. The red horizontal and vertical lines represent the reciprocal lattice of the $I4/mmm$ unit cell.

Figure 3

(a) Temperature dependence of magnetic intensity at $\mathbf{Q}=(1,1,0{)}_S+{\mathbf{q}}_m=\mathbf{(}1.29,0.74,0{\mathbf{)}}_S$. (b) Schematic of stripe-type pattern within the $2\times 2$ block associated with ${\mathbf{q}}_m=(0.29,-0.26,0{)}_S$. Error bars represent one standard deviation.

Figure 4

Schematic of the (a) real and (b) reciprocal space lattices of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ in the tetragonal phase. Upon cooling below the tetragonal-to-orthorhombic transition temperature, four structural domains as shown in (c) form in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, resulting in the splitting of Bragg peaks in reciprocal space shown in (d). The diagrams in (a)–(d) are adapted from Ref. [48]. $dQ/|\mathbf{Q}|$ for ${\mathrm{KFe}}_{0.8}{\mathrm{Ag}}_{1.2}{\mathrm{Te}}_2$ for (e) transverse and (f) longitudinal scans at $(2,2,0{)}_T$, and similarly, for (g) transverse and (h) longitudinal scans at $(2,0,0{)}_T$. For $T\ge 35\mathrm{K}$, we do not observe broadening within our resolution (shaded gray area); therefore for these points $dQ/|\mathbf{Q}|$ are set to be zero (open symbols). The insets in (e)–(h) compare corresponding scans at 6 K and 40 K; the solid lines are fits to the data as described in the text. Error bars represent one standard deviation.