We compute analytically and numerically the four-point correlation function that characterizes nontrivial cooperative dynamics in glassy systems within several models of glasses: elastoplastic deformations, mode-coupling theory (MCT), collectively rearranging regions (CRR's), diffusing defects, and kinetically constrained models (KCM's). Some features of the four-point susceptibility ξ 4(t) are expected to be universal: at short times we expect a power-law increase in time as t 4 due to ballistic motion (t 2 if the dynamics is Brownian) followed by an elastic regime (most relevant deep in the glass phase) characterized by a t or √t growth, depending on whether phonons are propagative or diffusive. We find in both the β and early α regime that ξ 4∼t μ, where μ is directly related to the mechanism responsible for relaxation. This regime ends when a maximum of ξ 4 is reached at a time t=t* of the order of the relaxation time of the system. This maximum is followed by a fast decay to zero at large times. The height of the maximum also follows a power law ξ 4(t*) ∼t* λ The value of the exponents μ and λ allows one to distinguish between different mechanisms. For example, freely diffusing defects in d=3 lead to μ=2 and λ= 1, whereas the CRR scenario rather predicts either μ,=1 or a logarithmic behavior depending on the nature of the nucleation events and a logarithmic behavior of ξ 4(t*). MCT leads to μ=b and λ=1/γ, where b and γ are the standard MCT exponents. We compare our theoretical results with numerical simulations on a Lennard-Jones and a soft-sphere system. Within the limited time scales accessible to numerical simulations, we find that the exponent μ is rather small, μ<1, with a value in reasonable agreement with the MCT predictions, but not with the prediction of simple diffusive defect models, KCM's with noncooperative defects, and CRR's. Experimental and numerical determination of ξ 4(t) for longer time scales and lower temperatures would yield highly valuable information on the glass formation mechanism.
|Original language||English (US)|
|Journal||Physical Review E - Statistical, Nonlinear, and Soft Matter Physics|
|State||Published - Apr 2005|
ASJC Scopus subject areas
- Statistical and Nonlinear Physics
- Statistics and Probability
- Condensed Matter Physics