Finite-temperature dynamics and the role of disorder in nearly critical Ni(Cl1−xBrx)2 · 4SC(NH2)2

Abstract

Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet Ni(Cl1−xBrx)2⋅4SC(NH2)2, x=0.14(1), tuned close to the chemical-composition induced soft-mode transition. The local dynamic structure factor shows ℏω/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. © 2020 American Physical Society.