Field-Induced Magnon Decay, Magnon Shadows, and Roton-like Excitations in the Honeycomb Antiferromagnet YbBr$_3$

TL;DR

This study investigates the complex spin excitation spectra of the unfrustrated honeycomb antiferromagnet YbBr₃, revealing magnon decay, shadows, and roton-like excitations driven by quantum fluctuations, supported by advanced neutron scattering and theoretical calculations.

Contribution

It provides the first detailed experimental and theoretical analysis of magnon decay and roton-like excitations in an unfrustrated quantum magnet on the honeycomb lattice.

Findings

Observation of magnon decay and shadows in YbBr₃

Detection of a roton-like excitation in the spectrum

Quantitative agreement between experiments and tensor network calculations

Abstract

Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr$_3$ is an excellent realization of the $S = 1/2$ antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced ``shadows'' of the single magnons and the spectacular emergence of a roton-like excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.