DYNAMIC ANOMALIES AND THEIR RELATION TO THE GLASS-TRANSITION - A NEUTRON-SCATTERING STUDY OF THE GLASS FORMING VANDERWAALS LIQUID ORTHO-TERPHENYL

Neutron scattering experiments on the molecular glass former ortho-terphenyl reveal a dynamic anomaly at a temperature T(c) almost-equal-to 290 K well above the calorimetric glass temperature T(g) = 243 K. Close above T(c) the density autocorrelation function PHI-Q(t) shows a two step decay over 4-5 decades in time. The slower component obeys the time-temperature superposition principle. Its line shape can be well parametrized by a Kohlrausch law and is strongly temperature dependent as its relaxation time scales with the shear viscosity. Thus this component is identified with the structural relaxation (alpha-process). The faster component (beta-process) is much less temperature dependent. Its line shape factorizes in a purely Q-dependent and a purely time-dependent part and its time scale follows a power law with a critical exponent a. The crossover region between the alpha- and beta-process follows a power law (von Schweidler law) with a critical exponent b. Approaching T(c) from below the plateau value separating the alpha and the beta process (= Debye-Waller factor) shows a critical decrease and bends over to a weak temperature dependence at T(c). These findings support several predictions made by the mode coupling theory of the glass transition which relates these dynamic anomalies to remnants of an ideal glass transition.