Revisiting the slow dynamics of a silica melt using Monte Carlo simulations

Berthier, Ludovic
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We implement a standard Monte Carlo algorithm to study the slow, equilibrium dynamics of a silica melt in a wide temperature regime, from 6100 K down to 2750 K. We find that the average dynamical behaviour of the system is in quantitative agreement with results obtained from molecular dynamics simulations, at least in the long-time regime corresponding to the alpha-relaxation. By contrast, the strong thermal vibrations related to the Boson peak present at short times in molecular dynamics are efficiently suppressed by the Monte Carlo algorithm. This allows us to reconsider silica dynamics in the context of mode-coupling theory, because several shortcomings of the theory were previously attributed to thermal vibrations. A mode-coupling theory analysis of our data is qualitatively correct, but quantitative tests of the theory fail, raising doubts about the very existence of an avoided singularity in this system. We discuss the emergence of dynamic heterogeneity and report detailed measurements of a decoupling between translational diffusion and structural relaxation, and of a growing four-point dynamic susceptibility. Dynamic heterogeneity appears to be less pronounced than in more fragile glass-forming models, but not of a qualitatively different nature.
Comment: 13 pages, 10 figures; to be published in Phys. Rev. E
Condensed Matter - Disordered Systems and Neural Networks, Condensed Matter - Statistical Mechanics