79 research outputs found
Random Pinning Glass Model
Glass transition where viscosity of liquids increases dramatically upon
decrease of temperature without any major change in structural properties,
remains one of the most challenging problems in condensed matter physics
(Cavagna, 2009; Berthier and Biroli, 2011) in spite of tremendous research
efforts in last decades. On the other hand disordered freezing of spins in a
magnetic materials with decreasing temperature, the so-called spin glass
transition, is relatively better understood (Mezard, Parisi and Virasoro, 1987;
Castellani and Cavagna, 2005). Previously found similarity between some spin
glass models with the structural glasses (Kirkpatrick and Thirumalai, 1987;
Kirkpatrick and Wolynes, 1987; Kirkpatrick and Wolynes, 1987; Franz and Parisi,
1999; Moore and Drossel, 2002) inspired development of theories of structural
glasses (Kirkpatrick, Thirumalai and Wolynes, 1989; Barrat, Franz and Parisi,
1997; M\'ezard and Parisi, 1999; Lubchenko and Wolynes, 2007; Biroli and
Bouchaud, 2012) based on the scenario of spin glass transition. This scenario
though looks very appealing is still far from being well established. One of
the main differences between standard spin systems to molecular systems is the
absence of quenched disorder and the presence of translational invariance: it
often assumed that this difference is not relevant, but this conjecture is
still far from being established. The quantities, which are well defined and
characterized for spin models, are not easily calculable for molecular glasses
due to the lack of quenched disorder which breaks the translational invariance
in the system and the characterization of the similarity between the spin and
the structural glass transition remained an elusive subject still now. In this
study we introduced a model structural glass with built in quenched disorder
which alleviates this main difference between the spin and molecular glasses
thereby helping us to compare these two systems: the possibility of producing a
good thermalization at rather low temperatures is one of the advantages of this
model.Comment: Submitted to PNAS with 7 pages 5 figures and Supplementary Material
Growing length and time scales in glass forming liquids
We study the growing time scales and length scales associated with dynamical
slow down for a realistic glass former, using computer simulations. We perform
finite size scaling to evaluate a length scale associated with dynamical
heterogeneity which grows as temperature decreases. However, relaxation times
which also grow with decreasing temperature, do not show the same kind of
scaling behavior with system size as the dynamical heterogeneity, indicating
that relaxation times are not solely determined by the length scale of
dynamical heterogeneity. We show that relaxation times are instead determined,
for all studied system sizes and temperatures, by configurational entropy, in
accordance with the Adam-Gibbs relation, but in disagreement with theoretical
expectations based on spin-glass models that configurational entropy is not
relevant at temperatures substantially above the critical temperature of mode
coupling theory. The temperature dependence of the heterogeneity length scale
shows significant deviations from theoretical expectations, and the length
scale one may extract from the system size dependence of the configurational
entropy has much weaker temperature dependence compared to the heterogeneity
length scale at all studied temperatures. Our results provide new insights into
the dynamics of glass-forming liquids and pose serious challenges to existing
theoretical descriptions
Short-time -relaxation in glass-forming liquids is cooperative in nature
Temporal relaxation of density fluctuations in supercooled liquids near the
glass transition occurs in multiple steps. The short-time -relaxation is
generally attributed to spatially local processes involving the rattling motion
of a particle in the transient cage formed by its neighbors. Using molecular
dynamics simulations for three model glass-forming liquids, we show that the
-relaxation is actually cooperative in nature. Using finite-size scaling
analysis, we extract a growing length-scale associated with -relaxation
from the observed dependence of the -relaxation time on the system size.
Remarkably, the temperature dependence of this length scale is found to be the
same as that of the length scale that describes the spatial heterogeneity of
local dynamics in the long-time -relaxation regime. These results show
that the conventional interpretation of -relaxation as a local process
is too simplified and provide a clear connection between short-time dynamics
and long-time structural relaxation in glass-forming liquids
Dynamics of Glass Forming Liquids with Randomly Pinned Particles
It is frequently assumed that in the limit of vanishing cooling rate, the
glass transition phenomenon becomes a thermodynamic transition at a temperature
. However, with any finite cooling rate, the system falls out of
equilibrium at temperatures near , implying that the very
existence of the putative thermodynamic phase transition at can be
questioned. Recent studies of systems with randomly pinned particles have
hinted that the thermodynamic glass transition may be observed in simulations
and experiments carried out for liquids with randomly pinned particles. This
expectation is based on the results of approximate calculations that suggest
that the temperature of the thermodynamic glass transition increases as the
concentration of pinned particles is increased and it may be possible to
equilibrate the system at temperatures near the increased transition
temperature. We test the validity of this prediction through extensive
molecular dynamics simulations of two model glass-forming liquids in the
presence of random pinning. We fit the temperature-dependence of the structural
relaxation time to the Vogel-Fulcher-Tammann form that predicts a divergence of
the relaxation time at a temperature and identify this temperature
with the thermodynamic transition temperature . We find that
does not show any sign of increasing with increasing concentration of pinned
particles. The main effect of pinning is found to be a rapid decrease in the
kinetic fragility of the system with increasing pin concentration. Implications
of these observations for current theories of the glass transition are
discussed.Comment: submitted to scientific repor
Universality of the Plastic Instability in Strained Amorphous Solids
By comparing the response to external strains in metallic glasses and in
Lenard-Jones glasses we find a quantitative universality of the fundamental
plastic instabilities in the athermal, quasistatic limit. Microscopically these
two types of glasses are as different as one can imagine, the latter being
determined by binary interactions, whereas the former by multiple interactions
due to the effect of the electron gas that cannot be disregarded. In spite of
this enormous difference the plastic instability is the same saddle-node
bifurcation. As a result the statistics of stress and energy drops in the
elasto-plastic steady state are universal, sharing the same system-size
exponents
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