Vortex-loops and solid nucleation in superfluid 4^4He and 3^3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of $^4$He and $^3$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 $^{3}$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, SrCr$_{8-x}$Ga$_{4+x}$O$_{19}$. We propose that the long-range dipolar interaction between the magnetic Cr$^{3+}$ 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 $^{4}$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 4^4He melting driven by acoustic pulse

Recent experiments have shown remarkable dynamics of solid $^4$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 3^3He

We examine the consequences of a recent model describing correlated zero-point polarization of the electronic cloud in solid $^3$He. This polarization arises from the highly anisotropic and correlated dynamic mixing of the $s$ and $p$ electronic levels ($\sim1$%). The magnetic polarization introduces a small paramagnetic correction, of $1-0.1$%, to the static susceptibility of condensed $^3$He. This correction could explain recent measurements in liquid $^3$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 $f_0$ and persistence time $\tau_p$. Using the extended MCT and a generalized fluctuation-dissipation theorem, we calculate the effective temperature $T_{eff}$ of the active fluid. The nonequilibrium nature of the systems is manifested through a time-dependent $T_{eff}$ that approaches a constant in the long-time limit, which depends on the activity parameters $f_0$ and $\tau_p$. 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 $\tau_\alpha$, the $\alpha$-relaxation time, behaves as $\tau_\alpha\sim f_0^{-2\gamma}$, where $\gamma=1.74$ is the MCT exponent for the passive system. $\tau_\alpha$ may increase or decrease as a function of $\tau_p$ depending on the type of active force correlations, but the behavior is always governed by the same value of the exponent $\gamma$. 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