465 research outputs found
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Eddy Covariance flux errors due to random and systematic timing errors during data acquisition
Modern eddy covariance (EC) systems collect
high-frequency data (10–20 Hz) via digital outputs of instru ments. This is an important evolution with respect to the tra ditional and widely used mixed analog/digital systems, as
fully digital systems help overcome the traditional limita tions of transmission reliability, data quality, and complete ness of the datasets
Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot
We have performed detailed photoluminescence (PL) and absorption spectroscopy
on the same single self-assembled quantum dot in a charge-tunable device. The
transition from neutral to charged exciton in the PL occurs at a more negative
voltage than the corresponding transition in absorption. We have developed a
model of the Coulomb blockade to account for this observation. At large
negative bias, the absorption broadens as a result of electron and hole
tunneling. We observe resonant features in this regime whenever the quantum dot
hole level is resonant with two-dimensional hole states located at the capping
layer-blocking barrier interface in our structure.Comment: 6 pages, 6 figure
Inevitable Irreversibility Generated by the Glass Transition of the Binary Lattice Gas Model
We numerically investigate the thermodynamic properties of the glass state.
As the object of our study, we employ a binary lattice gas model. Through Monte
Carlo simulations, we find that this model actually experiences a glass
transition. We introduce a potential into the model that represents a piston
with which we compress the glass. By measuring the work performed in this
process, we find that irreversible works exist at the glass state even in the
quasistatic limit. This implies that yield stress is created by the glass
transition.Comment: 4 pages, 5 figure
Dynamic heterogeneities in the out-of-equilibrium dynamics of simple spherical spin models
The response of spherical two-spin interaction models, the spherical
ferromagnet (s-FM) and the spherical Sherrington-Kirkpatrick (s-SK) model, is
calculated for the protocol of the so-called nonresonant hole burning
experiment (NHB) for temperatures below the respective critical temperatures.
It is shown that it is possible to select dynamic features in the
out-of-equilibrium dynamics of both models, one of the hallmarks of dynamic
heterogeneities. The behavior of the s-SK model and the s-FM in three
dimensions is very similar, showing dynamic heterogeneities in the long time
behavior, i.e. in the aging regime. The appearence of dynamic heterogeneities
in the s-SK model explicitly demonstrates that these are not necessarily
related to {\it spatial} heterogeneities. For the s-FM it is shown that the
nature of the dynamic heterogeneities changes as a function of dimensionality.
With incresing dimension the frequency selectivity of the NHB diminishes and
the dynamics in the mean-field limit of the s-FM model becomes homogeneous.Comment: 16 pages, 8 figure
Non-monotonic temperature evolution of dynamic correlations in glass-forming liquids
The viscosity of glass-forming liquids increases by many orders of magnitude
if their temperature is lowered by a mere factor of 2-3 [1,2]. Recent studies
suggest that this widespread phenomenon is accompanied by spatially
heterogeneous dynamics [3,4], and a growing dynamic correlation length
quantifying the extent of correlated particle motion [5-7]. Here we use a novel
numerical method to detect and quantify spatial correlations which reveal a
surprising non-monotonic temperature evolution of spatial dynamical
correlations, accompanied by a second length scale that grows monotonically and
has a very different nature. Our results directly unveil a dramatic qualitative
change in atomic motions near the mode-coupling crossover temperature [8] which
involves no fitting or indirect theoretical interpretation. Our results impose
severe new constraints on the theoretical description of the glass transition,
and open several research perspectives, in particular for experiments, to
confirm and quantify our observations in real materials.Comment: 7 page
Dynamics of supercooled liquids: density fluctuations and Mode Coupling Theory
We write equations of motion for density variables that are equivalent to
Newtons equations. We then propose a set of trial equations parameterised by
two unknown functions to describe the exact equations. These are chosen to best
fit the exact Newtonian equations. Following established ideas, we choose to
separate these trial functions into a set representing integrable motions of
density waves, and a set containing all effects of non-integrability. It
transpires that the static structure factor is fixed by this minimum condition
to be the solution of the Yvon-Born-Green (YBG) equation. The residual
interactions between density waves are explicitly isolated in their Newtonian
representation and expanded by choosing the dominant objects in the phase space
of the system, that can be represented by a dissipative term with memory and a
random noise. This provides a mapping between deterministic and stochastic
dynamics. Imposing the Fluctuation-Dissipation Theorem (FDT) allows us to
calculate the memory kernel. We write exactly the expression for it, following
two different routes, i.e. using explicitly Newtons equations, or instead,
their implicit form, that must be projected onto density pairs, as in the
development of the well-established Mode Coupling Theory (MCT). We compare
these two ways of proceeding, showing the necessity to enforce a new equation
of constraint for the two schemes to be consistent. Thus, while in the first
`Newtonian' representation a simple gaussian approximation for the random
process leads easily to the Mean Spherical Approximation (MSA) for the statics
and to MCT for the dynamics of the system, in the second case higher levels of
approximation are required to have a fully consistent theory
Is there something of the MCT in orientationally disordered crystals ?
Molecular Dynamics simulations have been performed on the orientationally
disordered crystal chloroadamantane: a model system where dynamics are almost
completely controlled by rotations. A critical temperature T_c = 225 K as
predicted by the Mode Coupling Theory can be clearly determined both in the
alpha and beta dynamical regimes. This investigation also shows the existence
of a second remarkable dynamical crossover at the temperature T_x > T_c
consistent with a previous NMR and MD study [1]. This allows us to confirm
clearly the existence of a 'landscape-influenced' regime occurring in the
temperature range [T_c-T_x] as recently proposed [2,3].Comment: 4 pages, 5 figures, REVTEX
Phase transitions in the Potts spin glass model
We have studied the Potts spin glass with 2-state Ising spins and s-state
Potts variables using a cluster Monte Carlo dynamics. The model recovers the +-
J Ising spin glass (SG) for s=1 and exhibits for all s a SG transition at
T_{SG}(s) and a percolation transition at higher temperature T_p(s). We have
shown that for all values of at T_p(s) there is a thermodynamical
transition in the universality class of a ferromagnetic s-state Potts model.
The efficiency of the cluster dynamics is compared with that of standard spin
flip dynamics.Comment: 8 pages, Latex, with 8 EPS fig
Decoupling of diffusion from structural relaxation and spatial heterogeneity in a supercooled simple liquid
We report a molecular dynamics simulation of a supercooled simple monatomic
glass-forming liquid. It is found that the onset of the supercooled regime
results in formation of distinct domains of slow diffusion which are confined
to the long-lived icosahedrally structured clusters associated with deeper
minima in the energy landscape. As these domains, possessing a low-dimensional
geometry, grow with cooling and percolate below , the critical temperature
of the mode coupling theory, a sharp slowing down of the structural relaxation
relative to diffusion is observed. It is concluded that this latter anomaly
cannot be accounted for by the spatial variation in atomic mobility; instead,
we explain it as a direct result of the configuration-space constraints imposed
by the transient structural correlations. We also conjecture that the observed
tendency for low-dimensional clustering may be regarded as a possible mechanism
of fragility.Comment: To be published in PR
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