Connection between the high-frequency crossover of the temperature dependence of the relaxation time and the change of intermolecular coupling in glass-forming liquids

In this paper, an interpretation of the high-frequency crossover, observed for glass formers at temperature far above the glass transition, is described in terms of the large change in the heterogeneous character of the dynamics. For five prototypical glass formers, dielectric relaxation data spanning many decades were analyzed. Unlike the temperature behavior of the dielectric (structural) relaxation times τ_α, which deviate from a Vogel-Fulcher-Tamman (VFT) relationship, the non-cooperative relaxation times, τ₀, calculated using the coupling model, are well described by a single VFT over the entire temperature range. Thus, the dynamic crossover evident in τ_α and other relaxation properties is suppressed for τ₀. This result suggests that the crossover is a direct consequence of the strong increase in intermolecular cooperativity (many-body effects), which also causes the dynamics of the system to become heterogeneous and non-exponential. This interpretation is consistent with recent findings concerning the pressure and temperature dependence of the dynamic crossover. Finally, a possible relationship of τ₀ to thermodynamic properties of the glass former is discussed.