Viscous flow and jump dynamics in molecular supercooled liquids. II. Rotations

The rotational dynamics of a supercooled model liquid of rigid A-B dumbbells interacting via a Lennard-Jones potential is investigated along one single isobar. The time-temperature superposition principle, one key prediction of mode-couplings theory (MCT), was studied for the orientational correlation functions C-iota. In agreement with previous studies we found that the scaling of C-iota in a narrow region at longs times is better at high-iota values. However, on a wider time interval the scaling works fairly better at low-iota values. Consistently, we observed the remarkable temperature dependence of the rotational correlation time tau (1) as a power law in T - T-c over more than three orders of magnitude and the increasing deviations from that law on increasing iota (T-c is the MCT critical temperature). For 0.7<T<2, good agreement with the diffusion model is found. For lower temperatures the agreement becomes poorer, and the results are also only partially accounted for by the jump-rotation model. The angular Van Hove function shows that in this region a meaningful fraction of the sample reorientates by jumps of about 180 degrees. The distribution of the waiting times in the angular sites cuts exponentially at long times. At lower temperatures it decays at short times as t(xi -1), with xi = 0.34 +/- 0.01 at T = 0.5, in analogy with the translational case. The breakdown of the Debye-Stokes-Einstein relation is observed at lower temperatures, where the rotational correlation times diverge more weakly than the viscosity.