Strength Of Interparticle Interactions Research Articles

Coherent disintegration and stability of vortices in trapped Bose condensates

We consider the intrinsic stability of the vortex states of a pure Bose-Einstein condensate confined in a harmonic potential under the effects of coherent atom-atom interaction. We find that stable vortices can be supported, and that vortex stability can be controlled by changing the inter-particle interaction strength. At unstable regimes, a vortex will spontaneously disintegrate into states with different angular momenta even without external perturbations, with the lifetime determined by its imaginary excitation frequencies.

Canonical Bose-Einstein condensation of interacting bosons in two dimensions

The onset of Bose-Einstein condensation in a 2D gas of bosons in a parabolic confinement potential is investigated. For such a system of non-interacting bosons, both the grand-canonical and the canonical partition function are calculated. The canonical partition function of a system of harmonically interacting bosons in 2D is obtained analytically. Both in the case of interacting and of non-interacting bosons, a distinct peak in the specific heat is found at a critical temperature and the dependence of this critical temperature on the number of particles and on the interparticle interaction strength is investigated.

Fine particle magnetic properties and microstructures of nanocrystalline Fe-Cu-Nb-Si-B alloy at elevated temperatures

Magnetic behavior in nanocrystalline Fe 72Cu 1Nb 4.5Si 13.5B 9 alloy is investigated by measurements of low and high field magnetization and coercivity over the temperature range from 300 K to 800 K. When the measuring temperatures approach the Curie temperature of intergranular amorphous phase T c(amorphous), the nanocrystalline materials consist of ferromagnetic Fe-rich nanocrystals embedded in a weakly magnetic or non-magnetic matrix. Samples with different volume fractions of crystalline phase are prepared to investigate the nature and strength of inter-particle interactions between the nanocrystallites. It is shown that at temperatures above T c(amorphous), the specimens with low fraction of nanocrystalline particles exhibit superparamagnetic behavior. In the case of specimens with increasing volume fraction of crystalline phase, the magnetic interaction between particles suppresses superparamagnetic fluctuations and superferromagnetic ordering of the particle magnetic moments above T c(amorphous) is observed.

Shear-induced enhancement of self-diffusion in interacting colloidal suspensions.

We set up the generalized Langevin equations describing coupled single-particle and collective motion in a suspension of interacting colloidal particles in a shear flow and use these to show that the measured self-diffusion coefficients in these systems should be strongly dependent on shear rate \ensuremath{\epsilon}\ifmmode \dot{}\else \.{}\fi{}. Three regimes are found: (i) an initial const+\ensuremath{\epsilon}${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}^{2}$, followed by (ii) a large regime of \ensuremath{\epsilon}${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}^{1/2}$ behavior, crossing over to an asymptotic power-law approach (iii) ${\mathit{D}}_{0}$-const\ifmmode\times\else\texttimes\fi{}\ensuremath{\epsilon}${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}^{\mathrm{\ensuremath{-}}1/2}$ to the Stokes-Einstein value ${\mathit{D}}_{0}$. The shear dependence is isotropic up to very large shear rates and increases with the interparticle interaction strength. Our results provide a straightforward explanation of recent experiments and simulations on sheared colloids. (c) 1995 The American Physical Society

Brownian-dynamics simulation studies of a charge-stabilized colloidal suspension under shear flow.

We have carried out Brownian-dynamics simulations of a charged colloidal suspension under oscillatory shear flow with both Couette and Poiseuille velocity profiles. We show that in the steady-shear limit, for both of the velocity profiles, the enhancement of the self-diffusion coefficient in directions transverse to the flow shows a crossover from a $\dot\gamma^2$ dependence to a $\dot\gamma^{1/2}$ dependence as the shear rate $\dot\gamma$ increases. The magnitude of the enhancement is a function of the interparticle interaction strength. There is a layering tendency in the direction of the velocity gradient for Couette flow, while such a tendency is not that prominent for Poiseuille flow.

Dynamics of fluid spreading: A solid-on-solid Monte Carlo study.

We study the spreading of both a fluid wedge and a droplet in two dimensions by means of Monte Carlo simulations of a solid-on-solid model. For the case of the fluid wedge, our results confirm those of Abraham, Heinio, and Kaski [J. Phys. A 24, L309 (1991)] who have done analogous simulations. For the case of the droplet, both wetting and partial-wetting behaviors are observed, depending on the relative strength of the fluid's interparticle interactions and the particle-substrate interaction. For the wetting case, a single-layer precursor film is found to spread at the expense of the remainder of the droplet

Anhysteretic susceptibility and static magnetic properties of interacting small particles

It is pointed out that the strength and sign of interparticle interactions largely determine the anhysteretic susceptibility of strongly interacting systems and correlate with the measured switching field distributions. Platelets of doped barium ferrite are shown to display evidence of positive interactions in their static magnetization behavior, and there is a correspondingly large anhysteretic susceptibility. It is concluded that this makes barium ferrite a suitable material for anhysteretic tape duplication but may, via the rerecord mechanism, be deleterious to the overwrite performance. >

Self-diffusion of interacting colloids far from equilibrium.

Forced Rayleigh scattering is used to measure the self-diffusion of charged polyball suspensions driven far from equilibrium by an oscillating shear flow. Aside from the usual enhancement of the diffusion due to Taylor dispersion, we observe an additional isotropic contribution which increases linearly with the shear rate. This anomalous contribution is absent in noninteracting samples and increases monotonically with the interparticle interaction strength.

The split in the second peak in the structure factor of binary colloidal suspensions: glass-like order

Structure factors of dilute binary colloidal suspensions of various compositions, particle diameter ratios and ionic impurity concentrations are investigated using polarised light scattering. For the first time, a split in the second peak of the measured structure factor is observed, suggesting the existence of glass-like order in the binary suspensions. The near-forward-scattered intensity from the colloidal suspension exhibiting a split second peak fluctuates much more slowly than that from the liquid-like ordered phase, corroborating the existence of glass-like order in the binary mixtures. The time evolution of the structure indicates that a liquid-like order continuously evolves into glass-like order as the strength of inter-particle interaction is increased by reducing the impurity concentration. The glass-like order is unstable towards formation of the crystalline state above a critical volume fraction (which depends on the impurity concentration) of the particles.

Solvation Effects on Dilatancy in Concentrated PVC Plastisols

The effect of the strength of particle‐particle interactions on dilatancy was investigated in concentrated poly(vinyl chloride) (PVC) emulsion resin plastisols. Particle interactions were controlled by selection of a series of phthalate ester and mixed solvent suspending media. Solvent effects on the strength of interparticle interactions were interpreted in terms of steric stabilization theory. The flow strength required to effect dilatancy decreased as the solvent quality of the continuous phase was reduced.

Molecular dynamics studies of Lennard-Jones liquid mixtures. VI. Temperature, density, and unlike interaction dependencies in static and dynamic properties of binary equimolar mixtures

Molecular dynamics calculations have been made for pure fluids and binary equimolar mixtures each containing 108 Lennard-Jones particles. The purpose of the calculations is threefold. First, the effect of the attractive term on the dynamic properties of liquids has been examined at fixed temperature and number density with equimolar mixtures of three models in which different combining rules for the energy parameter are assumed. Next the temperature and density dependences in static and dynamic properties of Lorentz–Berthelot mixture have been investigated. The variation of the velocity autocorrelation function with temperature and number density has been examined in detail. The activation energy for self-diffusion at constant number density is almost independent of the strength of interparticle interaction and is estimated to be 0.35 kcal mol−1, which is comparable with that for argon. In spite of remarkable change in the pattern of velocity autocorrelation function in the density range studied, self-diffusion coefficient changes linearly with density. Finally, a preliminary calculation on the microscopic process of mixing has been described.

The one dimensional Ising model with a transverse field: Pierre Pfeuty. Dept. of Theoretical Physics, 12 parks Road, Oxford OX1 3PQ, Great Britain

We present a formal analysis of nonlinear response functions in terms of correlation functions in real- and imaginary-time domains. In particular, we show that causal nonlinear response functions, expressed in terms of nested commutators in real time, can be obtained from the analytic continuation of time-ordered response functions, which are more easily amenable to diagrammatic calculation. This generalizes the well-known result of linear response theory. We then use gauge invariance arguments to derive exact relations between second-order response functions in density and current channels. These identities, which are non-perturbative in the strength of inter-particle interactions, allow us to establish exact connections between nonlinear optics calculations done in different electromagnetic gauges.