Finite-temperature dynamics and the role of disorder in nearly-critical Ni(Cl_1-xBr_x)₂4SC(NH₂)₂

Time-of-flight neutron spectroscopy is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet Ni(Cl_1-xBr_x)₂4SC(NH₂)₂, x = 0.14(1), tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows ω/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, which is also responsible for the localization of excitations above the magnon band. A simple model based on strong-bond dimers associated with Br-impurity sites can account for their energy, structure factor, and measured intensity [1].

[1] L. Facheris et al., arXiv:2008.04037 (2020)