High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$, a distorted-triangular-lattice antiferromagnet with nearly $XY$-type anisotropy. What was previously thought of as a broad excitation continuum [L. Facheris et al., Phys. Rev. Lett. 129, 087201 (2022)] is shown to be a series of dispersive bound states reminiscent of “Zeeman ladders” in quasi-one-dimensional Ising systems. At wave vectors where interchain interactions cancel at the mean field level, they can indeed be interpreted as bound finite-width kinks in individual chains. Elsewhere in the Brillouin zone their true two-dimensional structure and propagation are revealed.

• Received 31 January 2023
• Revised 13 April 2023
• Accepted 17 May 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.256702