Showing papers in "EPL in 1999"
TL;DR: In this article, Qian et al. derived entropy functions whose local equilibria are suitable to recover the Navier-Stokes equations in the framework of the Lattice Boltzmann method.
Abstract: In this letter, we derive entropy functions whose local equilibria are suitable to recover the Navier-Stokes equations in the framework of the Lattice Boltzmann method. For the two-dimensional nine-velocity lattice we demonstrate that such an entropy function is unique, and that the expansion of the corresponding local equilibrium is the well-known local equilibrium of Y. H. Qian et al. (Europhys. Lett., 17 (1992) 479). Based on the knowledge of entropy functions, we introduce a new version of the Lattice Boltzmann method with an H-theorem built in.
317 citations
TL;DR: In this article, the effect of pulsed currents on magnetization reversal was studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length, and the injected current triggered the magnetisation reversal at a value of the applied field distant from the switching field by as much as 20%.
Abstract: The effect of pulsed currents on magnetization reversal were studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length. The magnetization reversal in these wires occurs with a jump of the magnetization at the switching field Hsw, which corresponds to unstable states of the magnetization. A pulsed current of about 107 A/cm2 was injected at different values of the applied field close to Hsw. The injected current triggered the magnetization reversal at a value of the applied field distant from the switching field by as much as 20%. This effect of current-induced magnetization reversal is interpreted in terms of the action of the spin-polarized conduction electrons on the magnetization.
298 citations
TL;DR: In this article, a generalised Fokker-Planck master equation is constructed from a non-homogeneous random walk scheme, which is used to describe anomalous diffusion in external fields.
Abstract: A generalised master equation is constructed from a non-homogeneous random walk scheme. It is shown how fractional Fokker-Planck equations for the description of anomalous diffusion in external fields, recently proposed in the literature, can be derived from this framework. Long-tailed waiting time distributions which cause slowly decaying memory effects, are demonstrated to give rise to a time-fractional Fokker-Planck equation that describes systems close to thermal equilibrium. An extension to include also Levy flights leads to a generalised Laplacian in the corresponding fractional Fokker-Planck equation.
257 citations
TL;DR: In this paper, the authors describe a simulation method based on combining the ideas behind Andersen's thermostat and dissipative particle dynamics (DPD), which is a Galilean invariant thermostate that conserves momentum and enhances viscosity.
Abstract: We describe a simulation method based on combining the ideas behind Andersen's thermostat and dissipative particle dynamics (DPD). The result is a Galilean invariant thermostat that conserves momentum and enhances viscosity. It therefore displays the same characteristics as DPD. Our method differs primarily in that it satisfies detailed balance by construction. If a simple scheme is used to solve the equations of motion the thermostat does not disturb the equilibrium properties of the system, regardless of the time step. We illustrate the properties of the model by describing the results of various tests on the analogous system to the dissipative ideal gas. We show that with an appropriate choice of parameters it is practical to make the viscosity orders of magnitude greater than the diffusion coefficient. This is a criterion that should be satisfied if one is interested in studying the dynamics of mesoscopic systems.
246 citations
TL;DR: Comparing scaling properties of the cardiac dynamics during sleep and wake periods for healthy individuals, cosmonauts during orbital flight, and subjects with severe heart disease finds a greater degree of anticorrelation in the heartbeat fluctuations during sleep compared to wake periods.
Abstract: We compare scaling properties of the cardiac dynamics during sleep and wake periods for healthy individuals, cosmonauts during orbital flight, and subjects with severe heart disease. For all three groups, we find a greater degree of anticorrelation in the heartbeat fluctuations during sleep compared to wake periods. The sleep-wake difference in the scaling exponents for the three groups is comparable to the difference between healthy and diseased individuals. The observed scaling differences are not accounted for simply by different levels of activity, but appear related to intrinsic changes in the neuroautonomic control of the heartbeat.
227 citations
TL;DR: In this article, it is shown that a droplet rolls instead of sliding, which leads to a surprising law for the velocity as a function of the drop radius: the smaller the droplet, the larger the running velocity.
Abstract: A viscous liquid drop sliding down an inclined solid that it partially wets runs all the faster since it is large. Here we examine what happens in the limit of very high contact angles, on a so-called super-hydrophobic surface. It is shown that a droplet rolls instead of sliding, which leads to a surprising law for the velocity as a function of the drop radius: the smaller the droplet, the larger the running velocity. A recent model of Mahadevan and Pomeau allows us to propose an explanation for this paradoxical behaviour.
221 citations
TL;DR: In this paper, the authors investigated the stationary states of one-dimensional driven diffusive systems coupled to boundary reservoirs with fixed particle densities and showed that the generic phase diagram is governed by an extremal principle for the macroscopic current irrespective of the local dynamics.
Abstract: We investigate the stationary states of one-dimensional driven diffusive systems, coupled to boundary reservoirs with fixed particle densities. We argue that the generic phase diagram is governed by an extremal principle for the macroscopic current irrespective of the local dynamics. In particular, we predict a minimal current phase for systems with local minimum in the current density relation. This phase is explained by a dynamical phenomenon, the branching and coalescence of shocks; Monte Carlo simulations confirm the theoretical scenario.
204 citations
TL;DR: During the aging of a colloidal glass, which is obtained for extremely low volume fractions due to strong electrostatic repulsions, leading to the formation of a Wigner glass, a new crossover between a complete and incomplete decay of the correlation function is observed, accompanied by an increase in the non-ergodicity parameter.
Abstract: We study the aging of a colloidal glass, which is obtained for extremely low volume fractions due to strong electrostatic repulsions, leading to the formation of a "Wigner glass". During the aging, a new crossover between a complete and incomplete decay of the correlation function is observed, accompanied by an increase in the non-ergodicity parameter. The dynamics can be described as a cage-diffusion process. For short times, the escape of the particles from "cages" formed by neighbouring particles dominates; for long times the particles cannot escape anymore and the system becomes strongly non-ergodic.
192 citations
TL;DR: In this article, a new proximity interferential effect is predicted for ferromagnetic-superconductor-ferromagnet (F/S/F) systems, which may result in the fabrication of the so-called spin-valve sandwiches revealing huge magnetoresistance.
Abstract: A new proximity interferential effect is predicted for ferromagnet-superconductor-ferromagnet (F/S/F) systems. When the superconducting layer thickness is smaller or of the order of the superconducting coherence length, its critical temperature and other superconducting characteristics strongly depend on the mutual orientation of the magnetization in F layers. Such a behaviour may result in the fabrication of the so-called spin-valve sandwiches revealing huge magnetoresistance.
173 citations
TL;DR: In this paper, the authors presented a new measurement of the 87Rb hyperfine frequency with a 1.3 × 10−14 relative accuracy, by comparison with a Cs fountain primary standard.
Abstract: We describe the operation of a laser-cooled rubidium 87Rb frequency standard. We present a new measurement of the 87Rb hyperfine frequency with a 1.3 × 10−14 relative accuracy, by comparison with a Cs fountain primary standard. The measured 87Rb ground-state hyperfine splitting is ν87 = 6 834 682 610.90429(9) Hz. This value differs from previously published values (see Essen L., Hope E. G. and Sutcliffe D., Nature 189 1961 298; Penselin S., Moran T., Cohen W. and Wscinkler G., Phys. Rev. 127 1962 524; Arditi M. and Cerez P. IEEE Trans. Instrum. Meas. IM-21 1972 391) by about 2 − 3 Hz and is 104 times more accurate. Because of the low collisional shift in 87Rb, future improvements may lead to a stability of 1 × 10−14τ−1/2 and a relative accuracy in the 10−17 range.
164 citations
TL;DR: In this article, the authors characterized the locked-to-running transition of a Brownian particle in a tilted washboard potential by looking at its transport properties in the vicinity of the transition threshold.
Abstract: We characterize the locked-to-running transition of a Brownian particle in a tilted washboard potential by looking at its transport properties in the vicinity of the transition threshold. At low temperatures the (normal) spatial diffusion of the particle is enhanced as a consequence of the unlocking mechanism; in the overdamped regime an analytic expression is obtained that relates the particle diffusion constant to its mobility; in the underdamped regime an unusually large diffusion constant is revealed through numerical simulation. The latter regime is analyzed in terms of multiple jump statistics.
TL;DR: In this article, the authors show that spontaneous overcharging in polyelectrolyte-colloid complexes can be explained in the Poisson-Boltzmann approximation, and that overcharging is an entropic effect associated with release of the counterions of the highly charged polyelectron chain as it "overwinds" onto the rigid colloidal particle.
Abstract: Oppositely charged macro-ions—one of which is flexible and carries a larger total charge than the other—are expected on an energetic basis to assemble into neutral complexes which can aggregate due to attractive (e.g., van der Waals) interactions. We treat in this paper the counter-intuitive appearance of spontaneous overcharging in polyelectrolyte-colloid complexes, a phenomenon which has been observed in experiments and simulations involving flexible chains adsorbing on oppositely charged cores. We show that this overcharging can be accounted for in the Poisson-Boltzmann approximation; within this theory, overcharging is an entropic effect associated with release of the counterions of the highly charged polyelectrolyte chain as it "overwinds" onto the rigid colloidal particle. Increasing the rigidity of the polyelectrolyte chain leads eventually to undercharging of the complex.
TL;DR: In this article, a consistent classical statistical mechanical theory of non-Hamiltonian dynamical systems is presented, where it is shown that compressible phase space flows generate coordinate transformations with a nonunit Jacobian, leading to a metric on the phase space manifold which is nontrivial.
Abstract: A consistent classical statistical mechanical theory of non-Hamiltonian dynamical systems is presented. It is shown that compressible phase space flows generate coordinate transformations with a nonunit Jacobian, leading to a metric on the phase space manifold which is nontrivial. Thus, the phase space of a non-Hamiltonian system should be regarded as a general curved Riemannian manifold. An invariant measure on the phase space manifold is then derived. It is further shown that a proper generalization of the Liouville equation must incorporate the metric determinant, and a geometric derivation of such a continuity equation is presented. The manifestations of the nontrivial nature of the phase space geometry on thermodynamic quantities is explored.
TL;DR: In this paper, the influence of screw dislocation on the energy spectrum of an electron in the magnetic field was studied in a continuum theory of defects that is isomorphic to three-dimensional gravity.
Abstract: We study electrons moving in a magnetic field in the presence of a screw dislocation. We focus on the influence of the screw dislocation on the energy spectrum of the electron (or hole) in the magnetic field, in a continuum theory of defects that is isomorphic to three-dimensional gravity. We find exact expressions for the eigenfunctions and eigenvalues of the energy and verify that the infinite degeneracy of the Landau levels is broken by the unusual boundary conditions imposed by the defect.
TL;DR: In this paper, the critical current density of grain boundaries in YBa2Cu3O7 − δ films is increased beyond the hitherto established limit by overdoping the superconductor.
Abstract: The critical current density of grain boundaries in YBa2Cu3O7 − δ films is increased beyond the hitherto established limit by overdoping the superconductor. Using Ca-doping, values for the critical current density are obtained, that exceed the highest values reported ever for the undoped material by more than a factor of seven. Further, it is found that the overdoping strongly reduces the grain boundary normal-state resistivity. This systematic improvement of the grain boundary transport properties by doping is of great importance for applications of the high-Tc cuprates and gives insight into the mechanisms controlling the grain boundary behavior.
TL;DR: In this paper, the electrostatic interaction between a spherical macroion of charge Qq and an oppositely, highly charged polyelectrolyte of charge − Nq (N,Q > 0) is considered.
Abstract: We consider the electrostatic interaction between a spherical macroion of charge Qq and an oppositely, highly charged polyelectrolyte of charge − Nq (N,Q > 0). For N ≤ Q the polyelectrolyte fully collapses on the macroion, while for N > Q only a partial collapse (up to neutralization of the macroion) is expected. We show, however, that for N > Q, the amount of collapsed polyelectrolyte can be bigger than that required to neutralize the macroion, i.e. the macroion can be overcharged. The overcharging increases with the diameter of the macroion (continuously or through multiple first-order transitions), until a total collapse of the polyelectrolyte takes place. The predictions of this model are then compared with Monte Carlo simulations and a possible mechanism is suggested for the overcharging of the octameric histone in the nucleosome.
TL;DR: In this article, the zero-temperature phase diagram of the disordered Bose-Hubbard model was calculated in one dimension using the density matrix renormalization group, where the Mott insulator is always separated from the superfluid by a Bose glass phase.
Abstract: We calculate the zero-temperature phase diagram of the disordered Bose-Hubbard model in one dimension using the density matrix renormalization group. For integer filling the Mott insulator is always separated from the superfluid by a Bose glass phase. There is a re-entrance of the Bose glass both as a function of the repulsive interaction and of disorder. At half-filling where no Mott insulator exists, the superfluid density has a maximum where the kinetic and repulsive energies are about the same. Superfluidity is suppressed both for small and very strong repulsion but is always monotonic in disorder.
TL;DR: In this article, the glass transition temperature Tg and the temperature Tα corresponding to the peak in the dielectric loss due to the α-process were simultaneously determined as functions of the film thickness d through dielectrics measurements for thin films of polystyrene.
Abstract: The glass transition temperature Tg and the temperature Tα corresponding to the peak in the dielectric loss due to the α-process have been simultaneously determined as functions of the film thickness d through dielectric measurements for thin films of polystyrene. A decrease of Tg was observed with decreasing film thickness, while Tα was found to remain almost constant for d > dc and decrease drastically for d < dc. Here, dc is a critical thickness dependent on molecular weight. The thickness dependence of Tg is related to the distribution of the relaxation times of the α-process, not to the relaxation time itself.
TL;DR: By adding a simple bookkeeping step to the Metropolis algorithm, statistical estimators of canonical macrostate probabilities enable a natural accumulation of statistics from simulations having different importance weights, enable temperature extrapolation without using energy to define macrostate labels, improve parallelization, and reduce variance.
Abstract: We examine non-Boltzmann Monte Carlo algorithms used to study slowly relaxing systems. By adding a simple bookkeeping step to the Metropolis algorithm, we obtain statistical estimators of canonical macrostate probabilities. These estimators enable a natural accumulation of statistics from simulations having different importance weights, enable temperature extrapolation without using energy to define macrostate labels, improve parallelization, and reduce variance. We illustrate with an Ising model example.
TL;DR: In this paper, the authors introduce a model of interacting Brownian particles in a symmetric, periodic potential that undergoes a noise-induced non-equilibrium phase transition The associated spontaneous symmetry breaking entails a ratchet-like transport mechanism.
Abstract: We introduce a model of interacting Brownian particles in a symmetric, periodic potential that undergoes a noise-induced non-equilibrium phase transition The associated spontaneous symmetry breaking entails a ratchet-like transport mechanism In response to an external force we identify several novel features; among the most prominent being a zero-bias negative conductance and a prima facie counterintuitive, anomalous hysteresis
TL;DR: In this article, the low-shear viscosity of polyelectrolyte microgels is studied as a function of concentration, crosslink density and ionic strength.
Abstract: The low-shear viscosity of polyelectrolyte microgels is studied as a function of concentration, crosslink density and ionic strength. The variation of the viscosity with volume fraction is very similar to that found in hard-sphere colloidal suspensions. In salt-free dispersions, osmotic effects are responsible for the de-swelling of microgels at high polymer concentration. This softening effect modifies the viscosity near the overlap concentration.
TL;DR: This work investigates the performance of error-correcting codes, where the code word comprises products of K bits selected from the original message and decoding is carried out utilizing a connectivity tensor with C connections per index, and examines the finite-temperature case.
Abstract: We investigate the performance of error-correcting codes, where the code word comprises products of K bits selected from the original message and decoding is carried out utilizing a connectivity tensor with C connections per index. Shannon's bound for the channel capacity is recovered for large K and zero temperature when the code rate K/C is finite. Close to optimal error-correcting capability is obtained for finite K and C. We examine the finite-temperature case to assess the use of simulated annealing for decoding and extend the analysis to accommodate other types of noisy channels.
TL;DR: In this article, the authors recover the classical Debye-Huckel (DH) theory, which is found to exhibit the full hierarchy of multibody correlations determined by pair-distribution functions given by the screened DH potential.
Abstract: Charged systems are considered using field theoretic methods which include fluctuations and correlations via a systematic expansion in powers of the local electrostatic potential. At the Gaussian level we recover the classical Debye-Huckel (DH) theory, which is found to exhibit the full hierarchy of multibody correlations determined by pair-distribution functions given by the screened DH potential. Higher-order corrections can lead to attractive pair interactions between similarly charged particles in asymmetric ionic environments. The phase diagram for an ionic fluid shows a critical point of demixing. The critical temperature and critical density exhibit pronounced deviations from DH theory for highly asymmetric ionic systems.
TL;DR: In this paper, the electrical transport properties of grain boundaries in epitaxial La2/3Ca1/3MnO3 − δ thin films have been studied as a function of temperature and applied magnetic field below the Curie temperature TC.
Abstract: The electrical transport properties of grain boundaries in the epitaxial La2/3Ca1/3MnO3 − δ thin films have been studied as a function of temperature and applied magnetic field Below the Curie temperature TC an additional grain boundary resistance, highly non-linear current-voltage curves, and a large magnetoresistive effect in the whole temperature regime below TC are found The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below TC due to an increase of the work function of the ferromagnetic grain material adjacent to this layer The related band bending and space charge effects are important for the physics of grain boundaries in the manganites
TL;DR: In this paper, the authors studied the electromagnetic coupling and concomitant heating of a particle in a miniaturized trap close to a solid surface, and two dominant heating mechanisms were identified: proximity fields generated by thermally excited currents in the absorbing solid and time-dependent image potentials due to elastic surface distortions (Rayleigh phonons).
Abstract: We study the electromagnetic coupling and concomitant heating of a particle in a miniaturized trap close to a solid surface. Two dominant heating mechanisms are identified: proximity fields generated by thermally excited currents in the absorbing solid and time-dependent image potentials due to elastic surface distortions (Rayleigh phonons). Estimates for the lifetime of the trap ground state are given. Ions are particularly sensitive to electric proximity fields: for a silver substrate, we find a lifetime below one second at distances closer than some ten μm to the surface. Neutral atoms may approach the surface more closely: if they have a magnetic moment, a minimum distance of one μm is estimated in tight traps, the heat being transferred via magnetic proximity fields. For spinless atoms, heat is transferred by inelastic scattering of virtual photons off surface phonons. The corresponding lifetime, however, is estimated to be extremely long compared to the timescale of typical experiments.
TL;DR: In this paper, the authors describe an experiment in which a laser beam is sent into a high-finesse optical cavity with a mirror coated on a mechanical resonator and show that the reflected light is very sensitive to small mirror displacements.
Abstract: We describe an experiment in which a laser beam is sent into a high-finesse optical cavity with a mirror coated on a mechanical resonator. We show that the reflected light is very sensitive to small mirror displacements. We have observed the Brownian motion of the resonator with a very high sensitivity corresponding to a minimum observable displacement of 2 × 10−19 m/√Hz.
TL;DR: In this paper, a new constitutive law relating the stress tensor to non-local functions of the velocity field and structural parameters of the material was proposed, which can be used to describe the diphasic behavior of granular medium under stress.
Abstract: The stationary dense granular flow along an inclined plane has been studied in the present work. Several experimental results of the velocity profile published previously can not be described using conventional constitutive laws of continuum mechanics. Considering recent results revealing the diphasic behavior of granular medium under stress, we propose a new constitutive law relating the stress tensor to non-local functions of the velocity field and structural parameters of the material.
TL;DR: In this paper, the authors proposed a non-equilibrium drying transition model for the boiling crisis, which is based on the spreading of the dry spot under a vapor bubble. But the model is not suitable for the case of large-scale nuclear power stations.
Abstract: Boiling crisis is the rapid formation of the quasi-continuous vapor film between the heater and the liquid when the heat supply exceeds a critical value. We propose a mechanism for the boiling crisis that is based on the spreading of the dry spot under a vapor bubble. The spreading is initiated by the vapor recoil force, a force coming from the liquid evaporation into the bubble. Since the evaporation intensity increases sharply near the triple contact line, the influence of the vapor recoil can be described as a change of the apparent contact angle. Therefore, for the most usual case of complete wetting of the heating surface by the liquid, the boiling crisis can be understood as a drying transition from complete to partial wetting. The state of nucleate boiling, which is boiling in its usual sense, is characterized by a very large rate of heat transfer from the heating surface to the bulk because the superheated liquid is carried away from the heating surface by the departing vapor bubbles. If the heating power is increased, the temperature of the heating surface increases with the heat flux. When the heat flux from the heater reaches a threshold value q CHF (the critical heat flux, CHF), the vapor bubbles suddenly form a film which covers the heating surface and insulates the latter from the bulk of the liquid. The temperature of the heating surface grows so rapidly that the heater can fuse unless its power is controlled. This phenomenon is known under the names of " boiling crisis, " " burnout, " or " Departure from Nucleate Boiling " (DNB) [1]. The final state of this transition is called film boiling. This problem has become very important since the 1940's, with the beginning of the industrial exploitation of heat exchangers with large heat fluxes (as with nuclear power stations). Since then a huge amount of research has been done for the various conditions of pool boiling (boiling without imposed external flow) and flow boiling (boiling of the flowing water) [2]. Numerous empirical correlations have been proposed, each describing the dependence of the CHF on the physical parameters of the liquid and of the heater more or less correctly for a particular geometry and particular conditions of boiling [2]. A strong dependence of the threshold on the details of the experimental setup coupled with difficulties in separating the consequences of DNB from its causes is at the origin of a large number of frequently controversial hypotheses [2]. The violence of boiling makes observations quite difficult. Good quality photographic experiments are presented in only a few articles (see e.g. [3] – [6]). Despite an increasing interest in the physical aspect of the problem during recent years [7,8] and numerous empirical approaches, the underlying physics still remains obscure. In this Letter, we propose a model based on a non-equilibrium drying transition.
TL;DR: In this paper, the authors used Small-Angle Neutron Scattering (SAS) to analyze the anisotropic patterns of the soft and hard particles in aqueous dispersions.
Abstract: Nanocomposites have been made by mixing soft particles (polymer latex) with hard particles (silica) in aqueous dispersions and extracting water to produce a dense film. Segregation between the two kinds of particles can be controlled, and even suppressed. The elongational modulus is strongly increased by such fillers at low deformations, and remains important at large deformations, which the samples can stand without breaking. Since the silica particles are small (200 A), we can follow their relative displacements under stretching, by Small-Angle Neutron Scattering, through analysis and simulation of the anisotropic patterns. The latter show a crossover from affine displacements to a set of shear displacements that let the particles avoid each other at large deformations. The shear could release the localized stresses (due to polymer confinement) and dissipate more energy. In this way it may contribute to the toughness of the composite against crack propagation.
TL;DR: In this article, a new approach for calculating the depletion potential in a hard-sphere mixture is presented, which is valid for any number of components and for arbitrary densities.
Abstract: A versatile new approach for calculating the depletion potential in a hard-sphere mixture is presented. This is valid for any number of components and for arbitrary densities. We describe two different routes to the depletion potential for the limit in which the density of one component goes to zero. Both routes can be implemented within density functional theory and simulation. We illustrate the approach by calculating the depletion potential for a big hard sphere in a fluid of small spheres near a planar hard wall. The density functional results are in excellent agreement with simulations.