156 research outputs found
Vortex-loops and solid nucleation in superfluid He and He
We propose a new model for the nature of the nucleation of solid from the
superfluid phases of He and He. Unique to the superfluid phases the
solid nucleation involves an extremely fast solidification front. This results
in a local release of pressure and a velocity field in the superfluid. The
superfluid velocity in turn facilitates the nucleation of vortex-loops. The
kinetic energy gain of this process balances the surface tension, as the solid
surface is quickly covered by many vortex-loops ("hairy snow-ball"). We show
that this scenario gives good agreement with experiments on heterogeneous
nucleation, which differ with the classical theory of homogeneous nucleation by
8 orders of magnitude. We propose several experiments that could show the
involvement of vortices with solid nucleation.Comment: 7 pages, 7 figure
Non-adiabatic dissociation of molecules and BEC loss due to shock-waves
Recent experiments have shown the likely appearance of coherent BEC
atom-molecule oscillations in the vicinity of a Feshbach resonance. In
addition, a new loss mechanism was observed, whereby the loss of atoms from the
BEC is inversely dependent on the rate of change of the applied magnetic field.
We present here a phenomenological model which gives a good description of the
scaling properties of this new decay process, by attributing it to
non-adiabatic dissociation of molecules by a propagating shock-wave. The model
has only two free parameters, which specify the size of the "shocked-region",
and can be readily tested by future experiments.Comment: 5 pages, 3 figure
Quantum nature of dislocations in pure bcc Helium
Recent experiments show the thermal growth of dislocation lines in unlta-pure
bcc He. The activation energy for the growth of the dislocation lines is
found to agree with the activation energy of mass diffusion. We propose that
these dislocations are topological defects in the phase of the complex
order-parameter which describes the dynamic zero-point atomic correlations,
unique to the bcc phase. These is a shear strain field associated with these
topological defects. We show that the smallest topological defect is a
localized excitation, a loop-defect, which leads to the exponential growth of
the dislocation lines with temperature.Comment: 7 pages, 4 figure
Coherent dipolar correlations in the ground-state of Kagome frustrated antiferromagnets
We propose a new model for the nature of the low temperature phase of a
geometrically frustrated antiferromgnet (AFM) with a Kagome lattice,
SrCrGaO. We propose that the long-range dipolar
interaction between the magnetic Cr ions introduces correlations in
their dynamics. The dipolar ground-state has the spins performing correlated
zero-point oscillations in a coherent state with a well defined global phase
and a complex order-parameter (i.e. Off-Diagonal Long Range Order). We
calculate the magnon excitations of such a dipolar array and we find good
agreement with the spin-wave velocities infered from measurements of the
specific-heat. Various experimental properties of these materials are naturally
explained by such a model.Comment: 7 pages, 6 figure
Bcc ^4He as a Coherent Quantum Solid: "Super-Solid" ?
In this work we investigate the quantum nature of bcc He. We show that
it is a solid phase with an Off-Diagonal Long Range Order of coherently
oscillating local electric dipole moments. These dipoles arise from the
correlated zero-point motion of the atoms in the crystal potential, which
oscillate in synchrony so that the dipolar interaction energy is minimized.
This coherent state has a three-component complex order parameter. The
condensation energy of these dipoles in the bcc phase further stabilizes it
over the hcp phase at finite temperatures. This condensation of the dipoles is
not a 'super-solid'. We further show that there can be fermionic excitations of
this ground-state and predict that they form an optic-like branch in the (110)
direction.Comment: 6 pages, 2 figures, QFS2000 conference to appear in Physica
Rychtmyer-Meshkov instability and solid He melting driven by acoustic pulse
Recent experiments have shown remarkable dynamics of solid He melting and
growth, driven by the normal incidence of an acoustic pulse on the solid-liquid
interface. The theory of solid growth/melting, driven by the radiation pressure
of the acoustic pulse, accounts well for the temperature dependence of the
measured data. There is however an observed source of extra,
temperature-independent, melting. We here propose that this extra melting is
due to solid-liquid mixing (and consequent melting) at the interface, in a
process similar to the Richtmyer-Meshkov instability: Initial undulations of
the rough interface, grow when accelerated by the acoustic pressure
oscillations. This model predicts a temperature-independent extra melting and
its dependence on the acoustic power, which is in agreement with the measured
data.Comment: 5 pages, 3 figure
Traffic Jams and Shocks of Molecular Motors inside Cellular Protrusions
Molecular motors are involved in key transport processes inside actin-based
cellular protrusions. The motors carry cargo proteins to the protrusion tip
which participate in regulating the actin polymerization, and play a key role
in facilitating the growth and formation of such protrusions. It is observed
that the motors accumulate at the tips of cellular protrusions, and in addition
form aggregates that are found to drift towards the protrusion base at the rate
of actin treadmilling. We present a one-dimensional driven lattice model, where
motors become inactive after delivering their cargo at the tip, or by loosing
their cargo to a cargo-less neighbor. The results suggest that the experimental
observations may be explained by the formation of traffic jams that form at the
tip. The model is solved using a novel application of mean-field and shock
analysis. We find a new class of shocks that undergo intermittent collapses,
and on average do not obey the Rankine-Hugoniot relation.Comment: 5 pages, 5 figure
Dipolar corrections to the static magnetic susceptibility of condensed He
We examine the consequences of a recent model describing correlated
zero-point polarization of the electronic cloud in solid He. This
polarization arises from the highly anisotropic and correlated dynamic mixing
of the and electronic levels (%). The magnetic polarization
introduces a small paramagnetic correction, of %, to the static
susceptibility of condensed He. This correction could explain recent
measurements in liquid He.Comment: 3 pages, 1 figur
Nonequilibrium mode-coupling theory for dense active systems of self-propelled particles
The physics of active systems of self-propelled particles, in the regime of a
dense liquid state, is an open puzzle of great current interest, both for
statistical physics and because such systems appear in many biological
contexts. We develop a nonequilibrium mode-coupling theory (MCT) for such
systems, where activity is included as a colored noise with the particles
having a self-propulsion foce and persistence time . Using the
extended MCT and a generalized fluctuation-dissipation theorem, we calculate
the effective temperature of the active fluid. The nonequilibrium
nature of the systems is manifested through a time-dependent that
approaches a constant in the long-time limit, which depends on the activity
parameters and . We find, phenomenologically, that this long-time
limit is captured by the potential energy of a single, trapped active particle
(STAP). Through a scaling analysis close to the MCT glass transition point, we
show that , the -relaxation time, behaves as
, where is the MCT exponent for
the passive system. may increase or decrease as a function of
depending on the type of active force correlations, but the behavior
is always governed by the same value of the exponent . Comparison with
numerical solution of the nonequilibrium MCT as well as simulation results give
excellent agreement with the scaling analysis
Theory of epithelial cell shape transitions induced by mechanoactive chemical gradients
Cell shape is determined by a balance of intrinsic properties of the cell as
well as its mechanochemical environment. Inhomogeneous shape changes underly
many morphogenetic events and involve spatial gradients in active cellular
forces induced by complex chemical signaling. Here, we introduce a
mechanochemical model based on the notion that cell shape changes may be
induced by external diffusible biomolecules that influence cellular
contractility (or equivalently, adhesions) in a concentration-dependent manner
-- and whose spatial profile in turn is affected by cell shape. We map out
theoretically the possible interplay between chemical concentration and
cellular structure. Besides providing a direct route to spatial gradients in
cell shape profiles in tissues, our results indicate that the dependence on
cell shape helps create robust mechanochemical gradients.Comment: 10 pages, 4 figures, Supplementary: 5 pages, 5 figure
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