Neutron spin echo study of dynamic correlations near the liquid-glass transition

Neutron spin-echo measurements have been performed on the ionic system ${\mathrm{Ca}}_{0.4}$ ${\mathrm{K}}_{0.6}$(${\mathrm{NO}}_3$ ${)}_{1.4}$ around the glass transition temperature ${\mathrm{T}}_0$, in order to determine the time dependence of the density correlation function ${\mathrm{\phi }}_{\mathrm{q}}$(t), which plays a central role in recent theories. Above ${\mathrm{T}}_0$ the results reveal that ${\mathrm{\phi }}_{\mathrm{q}}$(t) contains two distinct ‘‘slow’’ components, the slower of which (a) has a stretched exponential form exp[-(t/τ${)}^{\mathrm{\beta }}$], (b) shows a slowing down as T approaches ${\mathrm{T}}_0$ which scales with the Stokes-Einstein diffusion constant, and (c) tends to become the nonergodic fraction of the structure factor at ${\mathrm{T}}_0$. .AE