Coupling/decoupling between translational and rotational dynamics in a supercooled molecular liquid

Abstract

We use molecular dynamics computer simulations to investigate the coupling/decoupling between translational and rotational dynamics in a glass-forming liquid of dumbbells. This is done via a careful analysis of the $\alpha$-relaxation time ${\tau }_{q^{\ast }}^{\mathrm{C}}$ of the incoherent center-of-mass density correlator at the structure factor peak, the $\alpha$-relaxation time ${\tau }_2$ of the reorientational correlator, and the translational ($D_t$) and rotational ($D_r$) diffusion constants. We find that the coupling between the relaxation times ${\tau }_{q^{\ast }}^{\mathrm{C}}$ and ${\tau }_2$ increases with decreasing temperature $T$, whereas the coupling decreases between the diffusivities $D_t$ and $D_r$. In addition, the $T$-dependence of $D_t$ decouples from that of $1/{\tau }_2$, which is consistent with previous experiments and has been interpreted as a signature of the "translation-rotation decoupling." We trace back these apparently contradicting observations to the dynamical heterogeneities in the system. We show that the decreasing coupling in the diffusivities $D_t$ and $D_r$ is only apparent due to the inadequacy of the concept of the rotational diffusion constant for describing the reorientational dynamics in the supercooled state. We also argue that the coupling between ${\tau }_{q^{\ast }}^{\mathrm{C}}$ and ${\tau }_2$ and the decoupling between $D_t$ and $1/{\tau }_2$, both of which strengthen upon cooling, can be consistently understood in terms of the growing dynamic length scale.

revised manuscript, to appear in Phys. Rev. Lett

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