Quasi-free Motion Research Articles

Soliton motion induced along ferromagnetic skyrmion chains in chiral thin nanotracks

Using atomistic magnetic simulations we investigate the soliton motion along a pinned skyrmion chain containing an interstitial skyrmion. We find that the soliton can exhibit stable motion along the chain without a skyrmion Hall effect for an extended range of drives. Under a constant drive the solitons have a constant velocity. We also measure the skyrmion velocity–current curves and identify the signatures of different phases including a pinned phase, stable soliton motion, and quasi-free motion at higher drives where all of the skyrmions depin from the pinning centers and move along the rigid wall. In the quasi-free motion regime, the velocity is oscillatory due to the motion of the skyrmions over the pinning sites. For increasing pinning strength, the onset of soliton motion shifts to higher values of current density. We also find that for stronger pinning, the characteristic velocity–current shape is affected by the annihilation of single or multiple skyrmions in the drive interval over which the soliton motion occurs. Our results indicate that stable skyrmion soliton motion is possible and that the solitons could be used as information carriers instead of the skyrmions themselves for technological applications.

Disrupting the wall accumulation of human sperm cells by artificial corrugation.

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very different from that observed in the bulk. Swimming along walls is beneficial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells ∼60 μm long, strongly confined to ∼25 μm shallow chambers. We investigate the nature of the cell trajectories between the confining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confirmed by depth resolved confocal microscopy studies. We show that the introduction of artificial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an effect which we hypothesize may be valuable for microfluidic applications in biomedicine.

Observing the Confinement Potential of Bacterial Pore-Forming Toxin Receptors Inside Rafts with Nonblinking Eu3+-Doped Oxide Nanoparticles

We track single toxin receptors on the apical cell membrane of MDCK cells with Eu-doped oxide nanoparticles coupled to two toxins of the pore-forming toxin family: α-toxin of Clostridium septicum and ε-toxin of Clostridium perfringens. These nonblinking and photostable labels do not perturb the motion of the toxin receptors and yield long uninterrupted trajectories with mean localization precision of 30 nm for acquisition times of 51.3 ms. We were thus able to study the toxin-cell interaction at the single-molecule level. Toxins bind to receptors that are confined within zones of mean area 0.40 ± 0.05 μm2. Assuming that the receptors move according to the Langevin equation of motion and using Bayesian inference, we determined mean diffusion coefficients of 0.16 ± 0.01 μm2/s for both toxin receptors. Moreover, application of this approach revealed a force field within the domain generated by a springlike confining potential. Both toxin receptors were found to experience forces characterized by a mean spring constant of 0.30 ± 0.03 pN/μm at 37°C. Furthermore, both toxin receptors showed similar distributions of diffusion coefficient, domain area, and spring constant. Control experiments before and after incubation with cholesterol oxidase and sphingomyelinase show that these two enzymes disrupt the confinement domains and lead to quasi-free motion of the toxin receptors. Our control data showing cholesterol and sphingomyelin dependence as well as independence of actin depolymerization and microtubule disruption lead us to attribute the confinement of both receptors to lipid rafts. These toxins require oligomerization to develop their toxic activity. The confined nature of the toxin receptors leads to a local enhancement of the toxin monomer concentration and may thus explain the virulence of this toxin family.

Interpretation of ultrafast pump-probe terahertz experiments in the time domain: How to exploit two-dimensional correlations

Optical-pump-terahertz-probe spectroscopy has the potential to distinguish between several classes of carrier motion, among others high-frequency oscillatory motion, free motion, and quasifree motion within bounded domains. We present a simplified formalism, applicable to thin samples, which allows identification of these three classes of photoexcited species on basis of the two-dimensional correlations in the time-domain experimental data.

Dynamics of kicked matter-wave solitons in an optical lattice

We investigate effects of the application of a kick to one-dimensional matter-wave solitons in a self-attractive Bose–Einstein condensate trapped in an optical lattice. The resulting soliton’s dynamics is studied within the framework of the time-dependent nonpolynomial Schrödinger equation. The crossover from the pinning to quasi-free motion crucially depends on the size of the kick, strength of the self-attraction, and parameters of the optical lattice.

Infrared spectral profiles in liquids and atom-diatom interactions

Molecular dynamics simulations of the infrared spectrum of a generic simple polar diatomic in a liquid nonpolar solvent allow to reproduce the different prototypical experimental line shapes of this kind of systems. This is feasible by using different solute-solvent anisotropic potentials at fixed thermodynamic conditions. In the limit cases, the rotation of the diatomic is explained in terms of a quasifree motion or a rotational diffusion evolution and the spectra show a doublet structure formed by P and R branches or a unique collapsed branch, respectively. When the profile contains three branches, including an intense Q branch in the vicinity of the center of the band, rotational evolution presents a particular hindering that can be understood by studying the influence on rotational spectral densities of the different time scales involved in rotational relaxation. Cancellation/enhancement effects among spectral density terms arising from intermediate and long times (0.4-1 ps) are essential to understand rotational hindering.

What does the potential energy landscape tell us about the dynamics of supercooled liquids and glasses?

For a model glass former we demonstrate via computer simulations how macroscopic dynamic quantities can be inferred from a potential energy landscape (PEL) analysis. The essential step is to consider whole superstructures of many PEL minima, called metabasins, rather than single minima. We show that two types of metabasins exist: some allowing for quasifree motion on the PEL (liquidlike), and the others acting as traps (solidlike). The activated, multistep escapes from the latter metabasins are found to dictate the slowing down of dynamics upon cooling over a much broader temperature range than is currently assumed.

Thermoelastic and electronic properties of quasi-one-dimensional defects in tellurium

The thermoelastic effect decreases with increasing concentration of linear defects ('a edge dislocations') generated by plastic deformation. Hence, an improvement of binding may be assumed. The authors interpret the linear defects as resulting from interstitial atoms, six per lattice constant c. Binding is increased by additional sigma and pi bonds, with components perpendicular and parallel to the trigonal chains. It is shown experimentally that the linear interstitials defects (LID) are active as acceptors, causing quasi-metallic p-type conduction. This may be realized from the assumption that, in the cylindrical LID potential, a two-dimensional interaction yields equivalent eigenstates and a quasi-free motion parallel to the linear defects. The model, in addition, allows the interpretation of defect-induced cyclotron resonance inactive-mode absorption in an unconstrained manner, as a result of the acceptor effect of the quasi-one-dimensional defects. Furthermore, the angular dependence and fine structure of resonant scattering in the magneto-impurity effects under hot carrier conditions of built-in group V atoms can be explained quantitatively.

Theoretical study of the structure of molecules of alkali metal acetylenides and carbides

1. According to data from nonempirical calculations, the reactivity of alkali metal acetylenide molecules toward electrophilic and nucleophilic reagents increases relative to the acetylene molecule. 2. The vinylidene forms MHC=C rearrange to MC=CH in a barrier-free manner as a result of migration of the alkali metal atom. Acetylenides are structurally nonrigid molecules due to the quasifree motion of the alkali metal atom (but not of the H atom). 3. The nonrigidity of the alkali metal carbide molecules is connected with the possibility of synchronous shift of the metal atoms.

Nuclear magnetic resonance in stereoregular polybutadienes and in polybutadiene-activated carbon black systems

IT IS well known that many finely dispersed solids form with polymers systems with improved mechanical characteristics and stability under the working conditions of articles manufactured from these systems. In technological circles this is called reinforcement and the dispersed solids producing this effect are called active fillers. In the rubber industry finely dispersed carbon (carbon black) is a valuable filler. There is no doubt tha t the reinforcing effect is the result of interaction between the particles of filler (carbon black) and the polymer (rubber). As a result of this interaction the structure and properties of the rubber are altered. I t has been stated that rubbers reinforced with carbon black form quasi-crystalline, strengthened structures [1]. Many problems concerning the nature of the effect of carbon black on the structure of a rubber have, however, not yet been finally solved, and s tudy of these with the aid of modern physical methods, including radiospectroseopy, should be very rewarding. The use of the NMR method for s tudy of rubbers and of rubber-filler systems has been mentioned in the literature [2-6]. I t has been shown that rubbers (natural and synthetic) give very narrow NMI~ lines at room temperature. This is explained by the high degree of quasi-free motion of the segments of the polymer molecules. The lines broaden with reduction in temperature, the movement of the segments being retarded. The data on the effect of fillers on the N ~ R signal of rubbers are less clear. According to reference [6] various fillers in butyl rubber showed no effect over the temperature interval of --50 ~ to + 7 0 ~ , but data on the spin-spin relaxation times in compositions based on various rubbers and carbon blacks, obtained b y means of a spin-echo impulse apparatus, have clearly shown differences in these times for different carbon blacks [1].

The electronic states around a dislocation

The propagation of an electron through a crystal is conceived as involving two qualitatively different mechanisms, a quasifree motion as in empty space and a jumping from one localized state attached to an atom to another. A Hamiltonian containing two terms corresponding to these mechanisms is examined, and its behavior in the presence of a dislocation is studied. In particular, the case of a screw dislocation is investigated, and all the eigenfunctions are found under the assumption that there is no screening. The electron density in the neighborhood of the screw dislocation is computed. It is shown that there is a net repulsion of the electrons from the dislocation axis which results in a well denned positive effective charge per unit length, even though there is no volume change.

Transport Coefficients in Moderately Dense Gases

The first virial corrections for shear viscosity η and thermal conductivity λ are calculated, starting from the BOLTZMANN-LANDAU equation of a quantum gas in binary collision approximation. The first order CHAPMAN-ENSKOG solution is used. With a very simplified collisional operator numerical values are got and compared with experiments. The result is that the kinetic equation including the interaction effects in quasi-free motion is able to explain the empirical order of magnitude of dη/dn.