Stripe order and spin dynamics in the triangular-lattice antiferromagnet KErSe2 : A single-crystal study with a theoretical description

The rare-earth triangular-lattice chalcogenide is a great platform for exploring both spin liquids and novel magnetic orders with anisotropic spin interactions and magnetic frustrations. Here, we report the thermodynamic and neutron-scattering measurements of rare-earth triangular-lattice chalcogenide ${\mathrm{KErSe}}_{2}$, using single-crystal samples. Our experiments revealed a long-range stripe order below 0.2 K. Although the magnetic order was three dimensional, magnetic excitations exhibited negligible modulation along the $z$ direction, indicating very weak interlayer coupling. Furthermore, magnetic excitation developed a well-defined spin-wave dispersion with a gap of $\ensuremath{\sim}0.03\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ at $M$ points. Both the stripe order and spin-wave excitations could be quantitatively understood from the anisotropic spin interactions of the ${\mathrm{Er}}^{3+}$ Kramers doublets.