Viscous flow and jump dynamics in molecular supercooled liquids. II. Rotations - art. no. 036702

The rotational dynamics of a supercooled model liquid of rigid A-B dumbbell s interacting via a Lennard-Jones potential is investigated along one singl e isobar. The time-temperature superposition principle, one key prediction of mode-couplings theory (MCT), was studied for the orientational correlati on functions C-iota. In agreement with previous studies we found that the s caling of C-iota in a narrow region at longs times is better at high-iota v alues. However, on a wider time interval the scaling works fairly better at low-iota values. Consistently, we observed the remarkable temperature depe ndence 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 fro m 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 temperat ures the agreement becomes poorer, and the results are also only partially accounted for by the jump-rotation model. The angular Van Hove function sho ws 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 an gular sites cuts exponentially at long times. At lower temperatures it deca ys at short times as t(xi -1), with xi = 0.34 +/- 0.01 at T = 0.5, in analo gy with the translational case. The breakdown of the Debye-Stokes-Einstein relation is observed at lower temperatures, where the rotational correlatio n times diverge more weakly than the viscosity.