We investigate the dynamics of a fluid of dipolar hard spheres in its liquid and glassy phases, with emphasis on the microscopic time or frequency regime. This system shows rather different glass transition scenarios related to its rich equilibrium behavior, which ranges from a simple hard sphere fluid to long range ferroelectric orientational order. In the liquid phase close to the ideal glass transition line and in the glassy regime a medium range orientational order occurs leading to a softening of an orientational mode. To investigate the role of this mode we use the molecular mode-coupling equations to calculate the spectra ${\phi }_{\mathrm{lm}}^{″}(q,\omega )$ and ${\chi }_{\mathrm{lm}}^{″}(q,\omega ).\mathrm{}$ In the center of mass spectra ${\phi }_{00}^{″}(q,\omega )$ and ${\chi }_{00}^{″}(q,\omega )$ we found, besides a high frequency peak at ${\omega }_{\mathrm{hf}},$ a peak at ${\omega }_{\mathrm{op}},$ about one decade below ${\omega }_{\mathrm{hf}}.$ ${\omega }_{\mathrm{op}}$ has almost no q dependence and exhibits an “isotope” effect ${\omega }_{\mathrm{op}}\propto I^{-1/2},$ with I the moment of inertia. We give evidence that the existence of this peak is related to the occurrence of medium range orientational order. It is shown that some of these features also exist for schematic mode coupling models.

• Received 21 May 2001

DOI:https://doi.org/10.1103/PhysRevE.64.051505