Disorder-induced broadening of the spin waves in a triangular-lattice quantum-spin-liquid candidate YbZnGaO$_4$

TL;DR

This study investigates how disorder affects spin dynamics in a triangular-lattice quantum spin liquid candidate, revealing that disorder broadens magnetic excitations and can mimic spin-liquid features, with insights gained from neutron scattering and simulations.

Contribution

It demonstrates the significant role of disorder in broadening spin excitations and reproduces experimental spectra using classical Monte Carlo simulations accounting for disorder.

Findings

Disorder causes broadening of magnetic excitation spectra.

High magnetic fields reveal clear spin-wave excitations with a gap.

Simulations incorporating disorder match experimental results.

Abstract

Disorder is important in the study of quantum spin liquids, but its role on the spin dynamics remains elusive. Here, we explore the disorder effect by investigating the magnetic-field dependence of the low-energy magnetic excitations in a triangular-lattice frustrated magnet YbZnGaO$_4$ with inelastic neutron scattering. With an intermediate field of 2.5 T applied along the $c$-axis, the broad continuum at zero field becomes more smeared both in energy and momentum. With a field up to 10 T, which fully polarizes the magnetic moments, we observe clear spin-wave excitations with a gap of $\sim$1.4 meV comparable to the bandwidth. However, the spectra are significantly broadened. The excitation spectra both at zero and high fields can be reproduced by performing classical Monte Carlo simulations which take into account the disorder effect arising from the random site mixing of nonmagnetic Zn$^{2+}$ and Ga$^{3+}$ ions. These results elucidate the critical role of disorder in broadening the magnetic excitation spectra and mimicking the spin-liquid features in frustrated quantum magnets.