Microscopic dynamics of molecular liquids and glasses: Role of orientations and translation-rotation coupling

摘要

We investigate the dynamics of a fluid of dipolar hard spheres in its liquidand glassy phase, with emphasis on the microscopic time or frequency regime.This system shows rather different glass transition scenarios related to itsrich equilibrium behavior which ranges from a simple hard sphere fluid to along range ferroelectric orientational order. In the liquid phase close to theideal glass transition line and in the glassy regime a medium rangeorientational order occurs leading to a softening of an orientational mode. Toinvestigate the role of this mode we use the molecular mode-coupling equationsto calculate the spectra $\phi_{lm}^{\prime \prime}(q,\omega)$ and $\chi_{lm}''(q,\omega)$. In the center of mass spectra $ \phi_{00}''(q,\omega)$ and$\chi_{00}''(q,\omega)$ we found besides a high frequency peak at $\omega_{hf}$ a peak at $\omega_{op}$, about one decade below $\omega_{hf}$. $\omega_{op}$ has almost no $q$-dependence and exhibits an ``isotope'' effect $\omega_{op}\propto I^{-1/2}$, with $I$ the moment of inertia. We give evidence thatthe existence of this peak is related to the occurrence of the medium rangedorientational order. It is shown that some of these feature also exist forschematic mode coupling models.