Locally favoured structures and dynamic length scales in a simple glass-former

Abstract

We investigate the static and dynamic properties of a weakly polydisperse hard sphere system in the deeply supercooled state, i.e. at densities higher than that corresponding to the mode-coupling transition. The structural analysis reveals the emergence of icosahedral locally favoured structures, previously only found in trace quantities. We present a new approach to probe the shape of dynamically heterogeneous regions, which is insensitive to particle packing eects that can hamper such analysis. Our results indicate that the shape of the dynamically heterogeneous regions changes only weakly and moreover hint that the often-used four-point correlation length may exhibit a growth in excess of that which our method identifies. The growth of the size of the dynamically heterogeneous regions appears instead to be in line with the one of structural and dynamic propensity correlations.

The race to the bottom: approaching the ideal glass?

“…Data for SiO 2 and orthoterphenyl (OTP) are quoted from Angell [48] and Berthier and Witten [49]. The other data concern model systems: KA denotes Kob-Andersen and Wahn denotes Wahnström binary Lennard-Jones systems respectively while HS denotes hard spheres where the control parameter is the compressibility factor Z [50,51]. CuZr and CuZrAl denote numerical calculations of metallic glassformers by ourselves based on an embedded atom model for Cu 65 Zr 27.5 Al 7.5 [52].…”

“…Probability distributions of populations (filled symbols) of particles in locally favoured structures in trajectories sampled from experimental data. The locally favoured structure is the defective icosahedron [51], indicated top left. Data is then reweighted according to a dynamical chemical potential µ (open symbols) to reveal a phase transition at µ * between a local-structure-rich state (right) and a normal liquid.…”

“…17 holds. Some attempt to identify defects in the form of "locally unfavoured structures" has been made [51,90], but much remains to be done. Order agnostic approaches such as overlap may provide crucial insight here [230].…”

Structural-dynamical transition in the Wahnström mixture

“…According to previous studies [49], we do not expect this motif to be a good predictor of structural-dynamical heterogeneity for the Wahnström mixture. However, in the case of other simple liquids dominated by five-fold symmetric local order, such as moderately polydisperse hardsphere, 9A motifs have been shown to be complementary to local icosahedral order, becoming less frequent when the packing fraction (and the population of icosahedra) increase [59].…”

Structure in sheared supercooled liquids: Dynamical rearrangements of an effective system of icosahedra

“…It has been proposed that icosahedral arrangements of the constituent atoms may form in some supercooled systems [3] and that dynamic arrest may be related to a (geometrically frustrated) transition to a phase of such icosahedra [4,5]. Geometric motifs such as icosahedra and other locally favoured structures (LFS) can be identified in particleresolved colloidal experiments [6][7][8][9][10][11][12] and computer simulations [13][14][15][16][17][18][19][20]. In particular, it has been shown that the onset of slow dynamics in simulated Lennard-Jones systems is closely coupled to the local structure, characterized by the LFS [18,21,22].…”