Confinement of fractional excitations in a triangular lattice antiferromagnet

TL;DR

This study reveals that what was previously considered a broad excitation continuum in a triangular lattice antiferromagnet is actually a series of dispersive bound states, providing new insights into fractional excitations.

Contribution

The paper demonstrates that the broad continuum is composed of bound states, challenging prior interpretations and revealing the true two-dimensional nature of excitations in Cs$_2$CoBr$_4$.

Findings

Bound states form a Zeeman ladder-like structure.

Inter-chain interactions cancel at specific wave vectors.

Revealed the two-dimensional propagation of excitations.

Abstract

High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs$_2$CoBr$_4$, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [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 inter-chain 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.