Mode coupling theory of charge fluctuation spectrum in a binary ionic liquid

Abstract

The paper reports a mode coupling calculation of the memory function for the dynamics of long-wave-length charge fluctuations in a symmetric binary ionic liquid, as a simple model for molten alkali halides. Symmetry under charge conjugation is exploited to derive selection rules for the decay of single-mode excitations into multimode excitations. An estimation of the two-mode decay channels indicates that they can be approximately separated into fast processes and slow processes. The former involve a pair of outcoming longitudinal modes, one of acoustic character and the other of optical character, and are the main cause of broadening of the high-frequency peak in the van Hove structure factor for charge density fluctuations. The slow-decay channels involve either a longitudinal-optic and a transverse-acoustic mode or a longitudinal-acoustic and a transverse-optic mode and are dominant in the low-frequency region of the spectrum where Drude-lile conduction is occurring. The calculated shape of the van Hove function is in reasonable agreement with the available computer simulation data, after an empiricla scaling of the spectrum of fluctuating forces.