Glassy Dynamics and Local Crystalline Order in Two-Dimensional Amorphous Silica.

We reassess the modeling of amorphous silica bilayers as a 2D classical system whose particles interact with an effective pairwise potential. We show that it is possible to reparametrize the potential developed by Roy, Heyde, and Heuer to quantitatively match the structural details of the experimental samples. We then study the glassy dynamics of the reparametrized model at low temperatures. Using appropriate cage-relative correlation functions, which suppress the effect of Mermin-Wagner fluctuations, we highlight the presence of two well-defined Arrhenius regimes separated by a narrow crossover region, which we connect to the thermodynamic anomalies and changes in the local structure. We find that the bond-orientational order grows steadily below the crossover temperature and is associated with transient crystalline domains of nanometric size. These findings raise fundamental questions about the nature of the glass structure in two dimensions and provide guidelines to interpret the experimental data.