Spatially Heterogeneous Dynamics in the Density Scaling Regime: Time and Length Scales of Molecular Dynamics near the Glass Transition

A fundamental problem of glass transition physics is to find a proper relation between length and time scales of molecular dynamics near the glass transition. Until now, this relation has been usually expected as a single variable function, for instance, as a consequence of the suggested direct relation between the structural relaxation time tau and the correlation volume defined by the maximum of the four-point correlation function chi(max)(4). Based on high pressure data analyses, we show that it is not the case, because chi(max)(4) evaluated from its estimate based on the enthalpy fluctuations cannot be, in general, a single variable function of tau. For a wide class of real and model supercooled liquids, the molecular dynamics of which obeys a density scaling law at least to a good approximation, we argue that the important relation between the length and time scales that characterize molecular motions near the glass transition is controlled by a density factor, the exponent of which is a measure of the observed decoupling between tau and chi(max)(4). This finding substantially changes our understanding of molecular dynamics near the glass transition.