Quantum corrections to the Larmor radiation formula in scalar electrodynamics

We use the semi-classical approximation in perturbative scalar quantum electrodynamics to calculate the quantum correction to the Larmor radiation formula to first order in Planck's constant in the non-relativistic approximation, choosing the initial state of the charged particle to be a momentum eigenstate. We calculate this correction in two cases: in the first case the charged particle is accelerated by a time-dependent but space-independent vector potential whereas in the second case it is accelerated by a time-independent vector potential which is a function of one spatial coordinate. We find that the corrections in these two cases are different even for a charged particle with the same classical motion. The correction in each case turns out to be non-local in time in contrast to the classical approximation.Comment: 19 page

Transport properties in chaotic and non-chaotic many particles systems

Two deterministic models for Brownian motion are investigated by means of numerical simulations and kinetic theory arguments. The first model consists of a heavy hard disk immersed in a rarefied gas of smaller and lighter hard disks acting as a thermal bath. The second is the same except for the shape of the particles, which is now square. The basic difference of these two systems lies in the interaction: hard core elastic collisions make the dynamics of the disks chaotic whereas that of squares is not. Remarkably, this difference is not reflected in the transport properties of the two systems: simulations show that the diffusion coefficients, velocity correlations and response functions of the heavy impurity are in agreement with kinetic theory for both the chaotic and the non-chaotic model. The relaxation to equilibrium, however, is very sensitive to the kind of interaction. These observations are used to reconsider and discuss some issues connected to chaos, statistical mechanics and diffusion.Comment: 23 pgs with 8 Figure

Swapping trajectories: a new wall-induced cross-streamline particle migration mechanism in a dilute suspension of spheres

Binary encounters between spherical particles in shear flow are studied for a system bounded by a single planar wall or two parallel planar walls under creeping flow conditions. We show that wall proximity gives rise to a new class of binary trajectories resulting in cross-streamline migration of the particles. The spheres on these new trajectories do not pass each other (as they would in free space) but instead they swap their cross-streamline positions. To determine the significance of the wall-induced particle migration, we have evaluated the hydrodynamic self-diffusion coefficient associated with a sequence of uncorrelated particle displacements due to binary particle encounters. The results of our calculations quantitatively agree with the experimental value obtained by \cite{Zarraga-Leighton:2002} for the self-diffusivity in a dilute suspension of spheres undergoing shear flow in a Couette device. We thus show that the wall-induced cross-streamline particle migration is the source of the anomalously large self-diffusivity revealed by their experiments.Comment: submited to JF

Literally and figuratively speaking: How concepts and perception influence each other using Stroop paradigms

Metaphorical phrases like ‘seeing red’ in anger or ‘red-hot’ for temperature take abstract ideas and ground them more tangible, physical phenomena, suggesting a relationship between semantic meaning and visual attributes. We tested the dominant direction of influence between abstract semantic processing and visual perception by pairing words and visual attributes in the same stimulus. Semantic categorization was used to test if visual attributes moderate semantic decisions by including both congruent and incongruent pairings with visual attributes. For instance, categorizing ‘scald’ in red font colour as hot (congruent) can be compared to ‘scald’ in blue font colour (incongruent), or ‘freeze’ in red font colour (incongruent) to determine if visual attributes (e.g., colour) automatically affect semantic decisions. Using the same stimuli, visual attribute categorization (e.g., categorizing ‘scald’ as red) was used to test if word meaning automatically affects perceptual decisions. Experiment 1 included mad (red congruent) and sad (blue congruent) emotion words, whereby semantic categorization revealed consistent congruency effects (i.e., shorter RTs and fewer errors with congruent trials than incongruent trials), but not with colour categorization. Experiment 2 extended these effects to the domain of temperature, including hot (red congruent) and cold (blue congruent) temperature words. Semantic categorization revealed consistent congruency effects on RT and errors, but not with colour naming. Experiment 3 extended Experiment 2 by including the neutral colour green. In semantic categorization, congruent pairs showed facilitation relative to neutral, and incongruent pairs showed interference relative to neutral, whereas only facilitation occurred with colour categorization for red-hot pairs. These results support the obligatory processing of visual attributes in semantic tasks, grounding abstract semantic meaning in colour processing. In the reverse direction, colour categorization tasks also showed semantic influences, although smaller and less consistently. Experiment 4 tested the generality of these effects in the visual domain of time processing. Congruent and incongruent pairs were generated by combining short durations and long durations with temporally associated words (e.g., ‘brief,’ ‘eternal’). Congruency effects occurred consistently on RTs and error rates for the duration categorization (revealing semantic influences), but only consistently on errors with semantic categorization. Thus, word meaning serves as the dominant attribute in the domain of time, indicating varying strengths of automaticity between visual attributes. These experiments explore the generality and boundary conditions of how visual attributes, like colour and time perception, and word meaning share representations, whereby asymmetries provide new evidence regarding the automatic direction of processing influences in these domains

Rotation of a spheroid in a simple shear at small Reynolds number

We derive an effective equation of motion for the orientational dynamics of a neutrally buoyant spheroid suspended in a simple shear flow, valid for arbitrary particle aspect ratios and to linear order in the shear Reynolds number. We show how inertial effects lift the degeneracy of the Jeffery orbits and determine the stabilities of the log-rolling and tumbling orbits at infinitesimal shear Reynolds numbers. For prolate spheroids we find stable tumbling in the shear plane, log-rolling is unstable. For oblate particles, by contrast, log-rolling is stable and tumbling is unstable provided that the aspect ratio is larger than a critical value. When the aspect ratio is smaller than this value tumbling turns stable, and an unstable limit cycle is born.Comment: 25 pages, 5 figure

Persistent random walk on a one-dimensional lattice with random asymmetric transmittances

We study the persistent random walk of photons on a one-dimensional lattice of random asymmetric transmittances. Each site is characterized by its intensity transmittance t (t') for photons moving to the right (left) direction. Transmittances at different sites are assumed independent, distributed according to a given probability density Distribution. We use the effective medium approximation and identify two classes of probability density distribution of transmittances which lead to the normal diffusion of photons. Monte Carlo simulations confirm our predictions.Comment: 7 pages, submitted to Phys. Rev.

Transport Properties of the Diluted Lorentz Slab

We study the behavior of a point particle incident from the left on a slab of a randomly diluted triangular array of circular scatterers. Various scattering properties, such as the reflection and transmission probabilities and the scattering time are studied as a function of thickness and dilution. We show that a diffusion model satisfactorily describes the mentioned scattering properties. We also show how some of these quantities can be evaluated exactly and their agreement with numerical experiments. Our results exhibit the dependence of these scattering data on the mean free path. This dependence again shows excellent agreement with the predictions of a Brownian motion model.Comment: 14 pages of text in LaTeX, 7 figures in Postscrip

Persistent correlation of constrained colloidal motion

We have investigated the motion of a single optically trapped colloidal particle close to a limiting wall at time scales where the inertia of the surrounding fluid plays a significant role. The velocity autocorrelation function exhibits a complex interplay due to the momentum relaxation of the particle, the vortex diffusion in the fluid, the obstruction of flow close to the interface, and the harmonic restoring forces due to the optical trap. We show that already a weak trapping force has a significant impact on the velocity autocorrelation function C(t)= at times where the hydrodynamic memory leads to an algebraic decay. The long-time behavior for the motion parallel and perpendicular to the wall is derived analytically and compared to numerical results. Then, we discuss the power spectral densities of the displacement and provide simple interpolation formulas. The theoretical predictions are finally compared to recent experimental observations.Comment: 12 pages, 6 figure

Interaction energies of monosubstituted benzene dimers via nonlocal density functional theory

We present density-functional calculations for the interaction energy of monosubstituted benzene dimers. Our approach utilizes a recently developed fully nonlocal correlation energy functional, which has been applied to the pure benzene dimer and several other systems with promising results. The interaction energy as a function of monomer distance was calculated for four different substituents in a sandwich and two T-shaped configurations. In addition, we considered two methods for dealing with exchange, namely using the revPBE generalized gradient functional as well as full Hartree-Fock. Our results are compared with other methods, such as Moller-Plesset and coupled-cluster calculations, thereby establishing the usefulness of our approach. Since our density-functional based method is considerably faster than other standard methods, it provides a computational inexpensive alternative, which is of particular interest for larger systems where standard calculations are too expensive or infeasible.Comment: submitted to J. Chem. Phy

Signatures of Radiation Reaction in Ultra-Intense Laser Fields

We discuss radiation reaction effects on charges propagating in ultra-intense laser fields. Our analysis is based on an analytic solution of the Landau-Lifshitz equation. We suggest to measure radiation reaction in terms of a symmetry breaking parameter associated with the violation of null translation invariance in the direction opposite to the laser beam. As the Landau-Lifshitz equation is nonlinear the energy transfer within the pulse is rather sensitive to initial conditions. This is elucidated by comparing colliding and fixed target modes in electron laser collisions.Comment: 8 pages, 6 figure