Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet $\mathrm{Ni}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2, x=0.14\left(1\right)$, tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows $\hslash \omega /T$ scaling behavior characteristic of a quantum-critical point. The deviation of the measured critical exponent from spin wave theoretical expectations is attributed to disorder. Another effect of disorder is local excitations above the magnon band. Their energy, structure factor, and temperature dependence are well explained by simple strong-bond dimers associated with Br-impurity sites.

• Received 13 August 2020
• Revised 6 October 2020
• Accepted 17 November 2020

DOI:https://doi.org/10.1103/PhysRevB.102.224405