Local structural excitations in model glasses

Structural excitations of model Lennard-Jones glass systems are investigated using the activation-relaxation technique (ART$n$), which explores the potential energy landscape around a local minimum energy configuration by converging to a nearby saddle-point configuration. Performing ART$n$ results in a distribution of barrier energies that is single-peaked for well-relaxed samples. The present work characterizes such atomic-scale excitations in terms of their local structure and environment. It is found that, at zero applied stress, many of the identified events consist of chainlike excitations that can either be extended or ringlike in their geometry. The location and barrier energy of these saddle-point structures are found to correlate with the type of atom involved, and with spatial regions that have low Kelvin eigenshear moduli and are close to the excess free volume within the configuration. Such correlations are, however, weak and more generally the identified local structural excitations are seen to exist throughout the model glass sample. The work concludes with a discussion within the framework of $\alpha$ and $\beta$ relaxation processes that are known to occur in the undercooled liquid regime.

Figure 1

Red balls represent the center of positions of all identified LSEs and the green balls represent regions containing free-volume within the simulation cell of the model glass.

Figure 2

(a) Normalized distribution of barrier energies of LSEs in a 3D model glass system. (b) Histogram of participation number.

Figure 3

Normalized distribution of local (a) energy, (b) Voronoi volume, and (c) eigenshear modulus 1. The red vertical lines represent the corresponding mean values derived from the total sample.

Figure 4

(a) Normalized distribution of LSE-weighted atom types for the activated and final state configurations. (b) Scatter plot of participation number vs LSE-weighted atom type. (c) Scatter plot of barrier energy vs LSE-weighted atom type.

Figure 5

Scatter plots of LSE-weighted local (a) energy, (b) volume, and (c) lowest eigenshear modulus vs barrier energy.

Figure 6

(a) Plot of vibrational density of states and (b) participation number vs eigenfrequency (in LJ units of inverse $\tau =\sqrt{m{\sigma }^2/\varepsilon }$). (c) Plot of maximum and average overlap per eigenfrequency.

Figure 7

Six examples of LSEs identified by ART$n$. In each case, the initial atomic positions are visualized by green balls and the final ones by orange balls. The atomic displacements from the initial to activated and activated to final states are visualized by red and blue arrows, respectively.

Figure 8

Distribution of minimum distances between the center-of-position of an LSE and free volume in the sample. Also shown for comparison are histograms of four random realizations of LSE center-of-positions derived from a uniform distribution within the simulation cell.

Figure 9

(a) Normalized distribution of the change in energy between the initial and final state configurations and (b) a scatter plot of this energy difference with the corresponding barrier energy.