DYNAMIC ANOMALY IN THE GLASS-TRANSITION REGION OF ORTHOTERPHENYL - A NEUTRON-SCATTERING STUDY

We report on incoherent and coherent neutron scattering results in the supercooled liquid and the glassy regime of the van der Waals fluid o-terphenyl using the backscattering and spin echo technique, respectively. A critical comparison of both techniques is presented. The data are analysed in the time domain assuming that microscopic correlation times (tau) scale with the viscosity eta according to tau (T) approximately eta (T)/T. With this assumption we obtain an agreement with several predictions of mode coupling theory: the existence of a critical temperature T(c) is shown, independently for both incoherent and coherent data, by a cusp in the temperature dependence of the Debye-Waller factor f(Q)(T). Below T(c) f(Q)(T) follows the predicted square-root T(c) - T behaviour yielding also the Q-dependence of the critical parameters f(Q)c and h(Q). Above T(c) the alpha-process or structural relaxation can be well parametrized by a stretched exponential function. In accordance with theory the structural relaxation curves can also be described by a power law with a critical (von Schweidler) exponent b = 0.525 +/- 0.10.