3,141 research outputs found
Barrier Softening near the onset of Non-Activated Transport in Supercooled Liquids: Implications for Establishing Detailed Connection between Thermodynamic and Kinetic Anomalies in Supercooled Liquids
According to the Random First Order Transition (RFOT) theory of glasses, the
barriers for activated dynamics in supercooled liquids vanish as the
temperature of a viscous liquid approaches the dynamical transition temperature
from below. This occurs due to a decrease of the surface tension between local
meta-stable molecular arrangements much like at a spinodal. The dynamical
transition thus represents a crossover from the low activated bevavior to a
collisional transport regime at high . This barrier softening explains the
deviation of the relaxation times, as a function of temperature, from the
simple dependence at the high viscosity to a
mode-mode coupling dominated result at lower viscosity. By calculating the
barrier softening effects, the RFOT theory provides a {\em unified} microscopic
way to interpret structural relaxation data for many distinct classes of
structural glass formers over the measured temperature range. The theory also
provides an unambiguous procedure to determine the size of dynamically
cooperative regions in the presence of barrier renormalization effects using
the experimental temperature dependence of the relaxation times and the
configurational entropy data. We use the RFOT theory framework to discuss data
for tri-naphthyl benzene, salol, propanol and silica as representative systems.Comment: Submitted to J. Chem. Phy
Crystallization in a dense suspension of self-propelled particles
Using Brownian dynamics computer simulations we show that a two-dimensional
suspension of self-propelled ("active") colloidal particles crystallizes at
sufficiently high densities. Compared to the equilibrium freezing of passive
particles the freezing density is both significantly shifted and depends on the
structural or dynamical criterion employed. In non-equilibrium the transition
is accompanied by pronounced structural heterogeneities. This leads to a
transition region between liquid and solid in which the suspension is globally
ordered but unordered liquid-like "bubbles" still persist
A Criterion for the Critical Number of Fermions and Chiral Symmetry Breaking in Anisotropic QED(2+1)
By analyzing the strength of a photon-fermion coupling using basic scattering
processes we calculate the effect of a velocity anisotropy on the critical
number of fermions at which mass is dynamically generated in planar QED. This
gives a quantitative criterion which can be used to locate a quantum critical
point at which fermions are gapped and confined out of the physical spectrum in
a phase diagram of various condensed matter systems. We also discuss the
mechanism of relativity restoration within the symmetric, quantum-critical
phase of the theory.Comment: To appear in Physical Review
Melting and Rippling Phenomenan in Two Dimensional Crystals with localized bonding
We calculate Root Mean Square (RMS) deviations from equilibrium for atoms in
a two dimensional crystal with local (e.g. covalent) bonding between close
neighbors. Large scale Monte Carlo calculations are in good agreement with
analytical results obtained in the harmonic approximation. When motion is
restricted to the plane, we find a slow (logarithmic) increase in fluctuations
of the atoms about their equilibrium positions as the crystals are made larger
and larger. We take into account fluctuations perpendicular to the lattice
plane, manifest as undulating ripples, by examining dual layer systems with
coupling between the layers to impart local rigidly (i.e. as in sheets of
graphene made stiff by their finite thickness). Surprisingly, we find a rapid
divergence with increasing system size in the vertical mean square deviations,
independent of the strength of the interplanar coupling. We consider an
attractive coupling to a flat substrate, finding that even a weak attraction
significantly limits the amplitude and average wavelength of the ripples. We
verify our results are generic by examining a variety of distinct geometries,
obtaining the same phenomena in each case.Comment: 17 pages, 28 figure
The calculation of molecular Eigen-frequencies
A method of determining molecular eigen-frequencies based on the function of Einstein expressing the variation of the atomic heat of various elements is proposed. It is shown that the same equation can be utilized to calculate both atomic heat and optically identifiably eigen-frequencies - at least to an order of magnitude - suggesting that in both cases the same oscillating structure is responsible
Observation of metastable hcp solid helium
We have produced and observed metastable solid helium-4 below its melting
pressure between 1.1 K and 1.4 K. This is achieved by an intense pressure wave
carefully focused inside a crystal of known orientation. An accurate density
map of the focal zone is provided by an optical interferometric technique.
Depending on the sample, minimum density achieved at focus corresponds to
pressures between 2 and 4 bar below the static melting pressure. Beyond, the
crystal undergoes an unexpected instability much earlier than the predicted
spinodal limit. This opens a novel opportunity to study this quantum crystal in
an expanded metastable state and its stability limits.Comment: deuxi\`eme versio
Freezing of parallel hard cubes with rounded edges
The freezing transition in a classical three-dimensional system of parallel
hard cubes with rounded edges is studied by computer simulation and
fundamental-measure density functional theory. By switching the rounding
parameter s from zero to one, one can smoothly interpolate between cubes with
sharp edges and hard spheres. The equilibrium phase diagram of rounded parallel
hard cubes is computed as a function of their volume fraction and the rounding
parameter s. The second order freezing transition known for oriented cubes at s
= 0 is found to be persistent up to s = 0.65. The fluid freezes into a
simple-cubic crystal which exhibits a large vacancy concentration. Upon a
further increase of s, the continuous freezing is replaced by a first-order
transition into either a sheared simple cubic lattice or a deformed
face-centered cubic lattice with two possible unit cells: body-centered
orthorhombic or base-centered monoclinic. In principle, a system of parallel
cubes could be realized in experiments on colloids using advanced synthesis
techniques and a combination of external fields.Comment: Submitted to JC
Scaling Approach to the Phase Diagram of Quantum Hall Systems
We present a simple classification of the different liquid and solid phases
of quantum Hall systems in the limit where the Coulomb interaction between the
electrons is significant, i.e. away from integral filling factors. This
classification, and a criterion for the validity of the mean-field
approximation in the charge-density-wave phase, is based on scaling arguments
concerning the effective interaction potential of electrons restricted to an
arbitrary Landau level. Finite-temperature effects are investigated within the
same formalism, and a good agreement with recent experiments is obtained.Comment: 4 pages, 3 figures; to be published in Europhys. Lett.; new version
contains more detailed description of finite-temperature effect
Effect of inelasticity on the phase transitions of a thin vibrated granular layer
We describe an experimental and computational investigation of the ordered
and disordered phases of a vibrating thin, dense granular layer composed of
identical metal spheres. We compare the results from spheres with different
amounts of inelasticity and show that inelasticity has a strong effect on the
phase diagram. We also report the melting of an ordered phase to a homogeneous
disordered liquid phase at high vibration amplitude or at large inelasticities.
Our results show that dissipation has a strong effect on ordering and that in
this system ordered phases are absent entirely in highly inelastic materials.Comment: 5 pages, 5 figures, published in Physical Review E. Title of first
version slightly change
Percolation of Immobile Domains in Supercooled Thin Polymeric Films
We present an analysis of heterogeneous dynamics in molecular dynamics
simulations of a thin polymeric film, supported by an absorbing structured
surface. Near the glass transition "immobile" domains occur throughout the
film, yet the probability of their occurrence decreasing with larger distance
from the surface. Still, enough immobile domains are located near the free
surface to cause them to percolate in the direction perpendicular to surface,
at a temperature near the glass transition temperature. This result is in
agreement with a recent theoretical model of glass transition
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