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 (τ) scale with the viscosity η according to τ(T)∼η(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 {Mathematical expression} behaviour yielding also the Q-dependence of the critical parameters f_Q^c and h_Q. Above T_c the α-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.