The order-disorder phase transition in ${\mathrm{C}\mathrm{u}}_3\mathrm{A}\mathrm{u}$ has been studied by x-ray intensity fluctuation spectroscopy. Following a quench from the high-temperature, disordered phase, the ordering kinetics is well described by a universal scaling form that can be measured by time-resolved (incoherent) x-ray scattering. By using coherent scattering, we have measured the fluctuations about this universal scaling form. In the late stages of the ordering process, these fluctuations give a two-time correlation function $C(q,t_1,t_2)$ which has a scaling form with natural variables $\delta t=|t_1-t_2|$ and $\overline{t}=\frac{(t_1+t_2)}{2}$. The scaling form crosses over from linear in $\overline{t}$ to ${\overline{t}}^{1/2}$. These present the first such results for a nonconserved system.

• Received 16 September 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.055501