Publication : t06/120

Abstract:
We study theoretically and numerically a new family of multi-point dynamic susceptibilities that quantify the strength and characteristic lengthscales of dynamic heterogeneities in glass-forming materials. We use general theoretical arguments (fluctuation-dissipation relations and symmetries of relevant dynamical field theories) to relate the sensitivity of average two-time dynamic correlations to temperature, density, etc. to spontaneous fluctuations of the local dynamics. We discuss some subtle issues associated to the choice of microscopic dynamics and of statistical ensemble through conserved quantities (energy, density, etc.) which are found to play a major role in determining dynamic fluctuations. Our theoretical results are then compared to molecular dynamics simulations of the Newtonian, Brownian and stochastic Monte-Carlo dynamics of two representative glass-forming liquids, a fragile binary Lennard-Jones mixture and a model for the strong glass-former silica. Our main result is that dynamical correlations, in the glassy regime, are mostly driven by structural (most notably energy) fluctuations. Correspondingly, these correlations are found to be different for Newtonian dynamics (energy conserving) and for Brownian or Monte-Carlo dynamics for which energy is not conserved.

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