Dynamics of supercooled liquids: excess wings, β peaks, and rotation–translation coupling

Dielectric susceptibility spectra of liquids cooled towards the liquid-glass transition often exhibit secondary structure in the frequency region between the a peak and the susceptibility minimum, in the form of either an 'excess wing' or a secondary peak-the Johari-Goldstein beta peak. Recently, Gotze and Sperl (2004 Phys. Rev. Lett. 92 105701) showed that a simple schematic mode coupling theory model, which incorporates rotationtranslation (RT) coupling, successfully describes the nearly logarithmic decay observed in optical Kerr effect data. This model also exhibits both excess wing and beta peak features, qualitatively resembling experimental dielectric data. It also predicts that the excess wing slope decreases with decreasing temperature and gradually evolves into a beta peak with increasing RT coupling. We therefore suggest that these features and their observed evolution with temperature may be consequences of RT coupling.