763 research outputs found
Model Energy Landscapes of Low-Temperature Fluids: Dipolar Hard Spheres
An analytical model of non-Gaussian energy landscape of low-temperature
fluids is developed based on the thermodynamics of the fluid of dipolar hard
spheres. The entire excitation profile of the liquid, from the high
temperatures to the point of ideal-glass transition, has been obtained from the
Monte Carlo simulations. The fluid of dipolar hard spheres loses stability when
reaching the point of ideal-glass transition transforming via a first-order
transition into a columnar liquid phase of dipolar chains locally arranged in a
body-centered tetragonal order.Comment: 4 pages, 3 figure
Cotunneling Transport and Quantum Phase Transitions in Coupled Josephson-Junction Chains with Charge Frustration
We investigate the quantum phase transitions in two capacitively coupled
chains of ultra-small Josephson-junctions, with emphasis on the external charge
effects. The particle-hole symmetry of the system is broken by the gate voltage
applied to each superconducting island, and the resulting induced charge
introduces frustration to the system. Near the maximal-frustration line, where
the system is transformed into a spin-1/2 Heisenberg antiferromagnetic chain,
cotunneling of the particles along the two chains is shown to play a major role
in the transport and to drive a quantum phase transition out of the
charge-density wave insulator, as the Josephson-coupling energy is increased.
We also argue briefly that slightly off the symmetry line, the universality
class of the transition remains the same as that right on the line, still being
driven by the particle-hole pairs.Comment: Final version accepted to Phys. Rev. Lett. (Longer version is
available from http://ctp.snu.ac.kr/~choims/
Numerical Studies of the Two Dimensional XY Model with Symmetry Breaking Fields
We present results of numerical studies of the two dimensional XY model with
four and eight fold symmetry breaking fields. This model has recently been
shown to describe hydrogen induced reconstruction on the W(100) surface. Based
on mean-field and renormalization group arguments,we first show how the
interplay between the anisotropy fields can give rise to different phase
transitions in the model. When the fields are compatible with each other there
is a continuous phase transition when the fourth order field is varied from
negative to positive values. This transition becomes discontinuous at low
temperatures. These two regimes are separated by a multicritical point. In the
case of competing four and eight fold fields, the first order transition at low
temperatures opens up into two Ising transitions. We then use numerical methods
to accurately locate the position of the multicritical point, and to verify the
nature of the transitions. The different techniques used include Monte Carlo
histogram methods combined with finite size scaling analysis, the real space
Monte Carlo Renormalization Group method, and the Monte Carlo Transfer Matrix
method. Our numerical results are in good agreement with the theoretical
arguments.Comment: 29 pages, HU-TFT-94-36, to appear in Phys. Rev. B, Vol 50, November
1, 1994. A LaTeX file with no figure
Dynamical transitions and sliding friction of the phase-field-crystal model with pinning
We study the nonlinear driven response and sliding friction behavior of the
phase-field-crystal (PFC) model with pinning including both thermal
fluctuations and inertial effects. The model provides a continuous description
of adsorbed layers on a substrate under the action of an external driving force
at finite temperatures, allowing for both elastic and plastic deformations. We
derive general stochastic dynamical equations for the particle and momentum
densities including both thermal fluctuations and inertial effects. The
resulting coupled equations for the PFC model are studied numerically. At
sufficiently low temperatures we find that the velocity response of an
initially pinned commensurate layer shows hysteresis with dynamical melting and
freezing transitions for increasing and decreasing applied forces at different
critical values. The main features of the nonlinear response in the PFC model
are similar to the results obtained previously with molecular dynamics
simulations of particle models for adsorbed layers.Comment: 7 pages, 8 figures, to appear in Physcial Review
The role of Helium-3 impurities in the stress induced roughening of superclimbing dislocations in solid Helium-4
We analyze the stress induced and thermally assisted roughening of a forest
of superclimbing dislocations in a Peierls potential in the presence of
Helium-3 impurities and randomly frozen in static stresses. It is shown that
the temperature of the dip in the flow rate observed by Ray and Hallock
(Phys.Rev. Lett. {\bf 105}, 145301 (2010)) is determined by the energy of the
impurity activation from dislocation core. However, it is suppressed by,
essentially, the logarithm of the impurity fraction. The width of the dip is
determined by inhomogeneous fluctuations of the stresses and is shown to be
much smaller than .Comment: Submitted to the LT26-conference proceeding
Cool Core Clusters from Cosmological Simulations
We present results obtained from a set of cosmological hydrodynamic
simulations of galaxy clusters, aimed at comparing predictions with
observational data on the diversity between cool-core (CC) and non-cool-core
(NCC) clusters. Our simulations include the effects of stellar and AGN feedback
and are based on an improved version of the smoothed particle hydrodynamics
code GADGET-3, which ameliorates gas mixing and better captures gas-dynamical
instabilities by including a suitable artificial thermal diffusion. In this
Letter, we focus our analysis on the entropy profiles, the primary diagnostic
we used to classify the degree of cool-coreness of clusters, and on the iron
profiles. In keeping with observations, our simulated clusters display a
variety of behaviors in entropy profiles: they range from steadily decreasing
profiles at small radii, characteristic of cool-core systems, to nearly flat
core isentropic profiles, characteristic of non-cool-core systems. Using
observational criteria to distinguish between the two classes of objects, we
find that they occur in similar proportions in both simulations and in
observations. Furthermore, we also find that simulated cool-core clusters have
profiles of iron abundance that are steeper than those of NCC clusters, which
is also in agreement with observational results. We show that the capability of
our simulations to generate a realistic cool-core structure in the cluster
population is due to AGN feedback and artificial thermal diffusion: their
combined action allows us to naturally distribute the energy extracted from
super-massive black holes and to compensate for the radiative losses of
low-entropy gas with short cooling time residing in the cluster core.Comment: 6 pages, 4 figures, accepted in ApJL, v2 contains some modifications
on the text (results unchanged
Phase Diagram and Commensurate-Incommensurate Transitions in the Phase Field Crystal Model with an External Pinning Potential
We study the phase diagram and the commensurate-incommensurate transitions in
a phase field model of a two-dimensional crystal lattice in the presence of an
external pinning potential. The model allows for both elastic and plastic
deformations and provides a continuum description of lattice systems, such as
for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a
mode expansion analysis is used to determine the ground states and the
commensurate-incommensurate transitions in the model as a function of the
strength of the pinning potential and the lattice mismatch parameter. Numerical
minimization of the corresponding free energy shows good agreement with the
analytical predictions and provides details on the topological defects in the
transition region. We find that for small mismatch the transition is of
first-order, and it remains so for the largest values of mismatch studied here.
Our results are consistent with results of simulations for atomistic models of
adsorbed overlayers
Fingering Instability of Dislocations and Related Defects
We identify a fundamental morphological instability of mobile dislocations in
crystals and related line defects. A positive gradient in the local driving
force along the direction of defect motion destabilizes long-wavelength
vibrational modes, producing a ``fingering'' pattern. The minimum unstable
wavelength scales as the inverse square root of the force gradient. We
demonstrate the instability's onset in simulations of a screw dislocation in Al
(via molecular dynamics) and of a vortex in a 3-d XY ``rotator'' model.Comment: 4 pages, 3 figure
Glassy phases and driven response of the phase-field-crystal model with random pinning
We study the structural correlations and the nonlinear response to a driving
force of a two-dimensional phase-field-crystal model with random pinning. The
model provides an effective continuous description of lattice systems in the
presence of disordered external pinning centers, allowing for both elastic and
plastic deformations. We find that the phase-field crystal with disorder
assumes an amorphous glassy ground state, with only short-ranged positional and
orientational correlations even in the limit of weak disorder. Under increasing
driving force, the pinned amorphous-glass phase evolves into a moving
plastic-flow phase and then finally a moving smectic phase. The transverse
response of the moving smectic phase shows a vanishing transverse critical
force for increasing system sizes
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