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Roles of icosahedral and crystal-like order in hard spheres glass transition
A link between local structural ordering and slow dynamics has
recently attracted much attention from the context of the origin of
glassy slow dynamics. There have been a few candidates for such
structural order, icosahedral order, exotic amorphous order,
and crystal-like order. Each type of order is linked to a different
scenario of glass transition. Thus, revealing the order responsible
for slow dynamics is crucial for our understanding of the glass
transition. Here we experimentally access local structural order in
polydisperse hard spheres by its particle-level observation with
confocal microscopy. We identify the key structures as icosahedral and
face-centred-cubic(fcc)-like order, excluding any other simple local
symmetry. We find that both types of order are statistically
associated with slow particles. However, when approaching the glass
transition, the icosahedral order does not grow in size whereas
crystal-like structures grow. It is the latter that governs the
dynamics and is linked to dynamic heterogeneity. This questions the
direct roles of the icosahedral ordering in glassy slow dynamics and
stresses the importance of the structural order compatible with the
avoided first order transition, crystallization. Our finding also
suggests that the growing lengthscale of structural order is essential
for the slowing down of dynamics and the nonlocal cooperativity in
particle motion.