Dynamics of a bond-disordered $S=1$ quantum magnet near $z=1$ criticality

TL;DR

This study uses neutron scattering to explore how bond disorder affects the magnetic excitations in an $S=1$ quantum magnet near criticality, revealing renormalized dispersion, decreased excitation lifetimes, and localized states.

Contribution

It provides detailed experimental insights into the effects of bond disorder on the excitation spectrum of a gapped $S=1$ antiferromagnetic quantum spin system near criticality.

Findings

Renormalized spin excitation dispersion compared to the parent compound

Decreased lifetime of excitations near the band bottom

Detection of localized states above the top of the band

Abstract

Neutron scattering is used to study NiCl$_{2-2x}$Br$_{2x}\cdot$4SC(NH$_2$)$_2$, $x=0.06$, a bond-disordered modification of the well-known gapped $S=1$ antiferromagnetic quantum spin system NiCl$_{2}\cdot$4SC(NH$_2$)$_2$. The magnetic excitation spectrum throughout Brillouin zone is mapped out at $T=60$ mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy $\Delta\simeq0.2$ meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.