411 research outputs found
Spatial structure of quasi-localized vibrations in nearly jammed amorphous solids
The low-temperature properties of amorphous solids are widely believed to be
controlled by low-frequency quasi-localized modes. What governs their spatial
structure and density is however debated. We study these questions numerically
in very large systems as the jamming transition is approached and the pressure
p vanishes. We find that these modes consist of an unstable core in which
particles undergo the buckling motions and decrease the energy, and a stable
far-field component which increases the energy and prevents the buckling of the
core. The size of the core diverges as and its characteristic volume
as These features are precisely those of the anomalous modes known
to cause the Boson peak in the vibrational spectrum of weakly-coordinated
materials. From this correspondence we deduce that the density of
quasi-localized modes must go as ,
in agreement with previous observations. Our analysis thus unravels the nature
of quasi-localized modes in a class of amorphous materials.Comment: 5 pages, 4 figure
Arrhenius temperature dependence of the crystallization time of deeply supercooled liquids
Usually, supercooled liquids and glasses are thermodynamically unstable
against crystallization. Classical nucleation theory (CNT) has been used to
describe the crystallization dynamics of supercooled liquids. However, recent
studies on overcompressed hard spheres show that their crystallization dynamics
are intermittent and mediated by avalanche-like rearrangements of particles,
which largely differ from the CNT. These observations suggest that the
crystallization times of deeply supercooled liquids or glasses cannot be
described by the CNT, but this point has not yet been studied in detail. In
this paper, we use molecular dynamics simulations to study the crystallization
dynamics of soft spheres just after an instantaneous quench. We show that
although the equilibrium relaxation time increases in a super-Arrhenius manner
with decreasing temperature, the crystallization time shows an Arrhenius
temperature dependence at very low temperatures. This is contrary to the
conventional formula based on the CNT. Furthermore, the estimated energy
barrier for the crystallization is surprisingly small compared to that for the
equilibrium dynamics. By comparing the crystallization and aging dynamics
quantitatively, we show that a coupling between aging and crystallization is
the key for understanding the rapid crystallization of deeply supercooled
liquids or glasses.Comment: 8 pages, 4 figure
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