Low temperature magnetic structure and lattice response in SmCuAs${}_2$

We investigated the structural and magnetic properties of single-crystalline SmCuAs${}_2$ using high-resolution synchrotron X-ray diffraction and X-ray resonant magnetic scattering (XRMS) at the Sm $L_2$ and $L_3$ edges. Temperature-dependent diffraction measurements confirm that SmCuAs${}_2$ maintains its tetragonal symmetry from room temperature down to 8 K, with lattice parameters showing anomalous behavior below the resistivity minimum ($T\approx$ 30 K). Notably, the $\mathbf{\text{c}}$-axis lattice parameter exhibits a plateau and subsequent increase near the Néel temperature, indicating magnetoelastic coupling. XRMS measurements reveal a commensurate antiferromagnetic structure with a propagation vector $\mathit{\text{q}}$ = (0, 0, 0.5). Our measurement shows that the Sm moments are aligned within the $\mathit{\text{ab}}$ plane and arranged in a $++--$ stacking along the $\mathit{\text{c}}$-axis. Comparison with related $\mathit{\text{RE}}$CuAs${}_2$ compounds ($\mathit{\text{RE}}$ = Pr, Nd, and Gd) suggests that in-plane moment orientation and associated magnetic frustration play a key role in the emergence of the resistivity minimum. Differences in spin-orbit and magnetoelastic coupling across the series highlight their importance in governing low-temperature transport behavior.