1,118 research outputs found
Geometrical properties of the potential energy of the soft-sphere binary mixture
We report a detailed study of the stationary points (zero-force points) of
the potential energy surface (PES) of a model structural glassformer. We
compare stationary points found with two different algorithms (eigenvector
following and square gradient minimization), and show that the mapping between
instantaneous configuration and stationary points defined by those algorithms
is as different as to strongly influence the instability index K vs.
temperature plot, which relevance in analyzing the liquid dynamics is thus
questioned. On the other hand, the plot of K vs. energy is much less sensitive
to the algorithm employed, showing that the energy is the good variable to
discuss geometric properties of the PES. We find new evidence of a geometric
transition between a minima-dominated phase and a saddle-point-dominated one.
We analyze the distances between instantaneous configurations and stationary
points, and find that above the glass transition, the system is closer to
saddle points than to minima
Glass and polycrystal states in a lattice spin model
We numerically study a nondisordered lattice spin system with a first order
liquid-crystal transition, as a model for supercooled liquids and glasses.
Below the melting temperature the system can be kept in the metastable liquid
phase, and it displays a dynamic phenomenology analogous to fragile supercooled
liquids, with stretched exponential relaxation, power law increase of the
relaxation time and high fragility index. At an effective spinodal temperature
Tsp the relaxation time exceeds the crystal nucleation time, and the
supercooled liquid loses stability. Below Tsp liquid properties cannot be
extrapolated, in line with Kauzmann's scenario of a `lower metastability limit'
of supercooled liquids as a solution of Kauzmann's paradox. The off-equilibrium
dynamics below Tsp corresponds to fast nucleation of small, but stable, crystal
droplets, followed by extremely slow growth, due to the presence of pinning
energy barriers. In the early time region, which is longer the lower the
temperature, this crystal-growth phase is indistinguishable from an
off-equilibrium glass, both from a structural and a dynamical point of view:
crystal growth has not advanced enough to be structurally detectable, and a
violation of the fluctuation-dissipation theorem (FDT) typical of structural
glasses is observed. On the other hand, for longer times crystallization
reaches a threshold beyond which crystal domains are easily identified, and FDT
violation becomes compatible with ordinary domain growth.Comment: 25 page
An intermediate state between the kagome-ice and the fully polarized state in DyTiO
DyTiO is at present the cleanest example of a spin-ice material.
Previous theoretical and experimental work on the first-order transition
between the kagome-ice and the fully polarized state has been taken as a
validation for the dipolar spin-ice model. Here we investigate in further depth
this phase transition using ac-susceptibility and dc-magnetization, and compare
this results with Monte-Carlo simulations and previous magnetization and
specific heat measurements. We find signatures of an intermediate state between
the kagome-ice and full polarization. This signatures are absent in current
theoretical models used to describe spin-ice materials.Comment: 7 pages, 4 figure
Specific heat anomaly in a supercooled liquid with amorphous boundary conditions
We study the specific heat of a model supercooled liquid confined in a
spherical cavity with amorphous boundary conditions. We find the equilibrium
specific heat has a cavity-size-dependent peak as a function of temperature.
The cavity allows us to perform a finite-size scaling (FSS) analysis, which
indicates that the peak persists at a finite temperature in the thermodynamic
limit. We attempt to collapse the data onto a FSS curve according to different
theoretical scenarios, obtaining reasonable results in two cases: a
"not-so-simple" liquid with nonstandard values of the exponents {\alpha} and
{\nu}, and random first-order theory, with two different length scales.Comment: Includes Supplemental Materia
Response to "Comment on Static correlations functions and domain walls in glass-forming liquids: The case of a sandwich geometry" [J. Chem. Phys. 144, 227101 (2016)]
The point-to-set correlation function has proved to be a very valuable tool
to probe structural correlations in disordered systems, but more than that, its
detailed behavior has been used to try to draw information on the mechanisms
leading to glassy behavior in supercooled liquids. For this reason it is of
primary importance to discern which of those details are peculiar to glassy
systems, and which are general features of confinement. Within the present
response we provide an answer to the concerns raised in [J. Chem. Phys. 144,
227101 (2016)]
Dynamic relaxation of a liquid cavity under amorphous boundary conditions
The growth of cooperatively rearranging regions was invoked long ago by Adam
and Gibbs to explain the slowing down of glass-forming liquids. The lack of
knowledge about the nature of the growing order, though, complicates the
definition of an appropriate correlation function. One option is the
point-to-set correlation function, which measures the spatial span of the
influence of amorphous boundary conditions on a confined system. By using a
swap Monte Carlo algorithm we measure the equilibration time of a liquid
droplet bounded by amorphous boundary conditions in a model glass-former at low
temperature, and we show that the cavity relaxation time increases with the
size of the droplet, saturating to the bulk value when the droplet outgrows the
point-to-set correlation length. This fact supports the idea that the
point-to-set correlation length is the natural size of the cooperatively
rearranging regions. On the other hand, the cavity relaxation time computed by
a standard, nonswap dynamics, has the opposite behavior, showing a very steep
increase when the cavity size is decreased. We try to reconcile this difference
by discussing the possible hybridization between MCT and activated processes,
and by introducing a new kind of amorphous boundary conditions, inspired by the
concept of frozen external state as an alternative to the commonly used frozen
external configuration.Comment: Completely rewritten version. After the first submission it was
realized that swap and nonswap dynamics results are qualitatively different.
This version reports the results of both dynamics and discusses the different
behaviors. 17 pages, 18 figure
A phase-separation perspective on dynamic heterogeneities in glass-forming liquids
We study dynamic heterogeneities in a model glass-former whose overlap with a
reference configuration is constrained to a fixed value. The system
phase-separates into regions of small and large overlap, so that dynamical
correlations remain strong even for asymptotic times. We calculate an
appropriate thermodynamic potential and find evidence of a Maxwell's
construction consistent with a spinodal decomposition of two phases. Our
results suggest that dynamic heterogeneities are the expression of an ephemeral
phase-separating regime ruled by a finite surface tension
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