  Inelastic neutron scattering is used to investigate the temperature
dependence of spin correlations in the 3-dimensional XY antiferromagnet
Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $ x = 0.14(1)$, tuned close to
the chemical-composition-induced soft-mode transition. The local dynamic
structure factor shows $\hbar\omega/T$ scaling behavior characteristic of a
quantum critical point. The deviation of the measured critical exponent from
spin wave theoretical expectations are 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.
