Particle dynamics in polymer-metal nanocomposite thin films on nanometer-length scales

X-ray photon correlation spectroscopy was used in conjunction with resonance-enhanced grazing-incidence small-angle x-ray scattering to probe slow particle dynamics and kinetics in gold/polystyrene nanocomposite thin films. Such enhanced coherent scattering enables, for the first time, measurement of the particle dynamics at wave vectors up to approximately 1 nm(-1) (or a few nanometers spatially) in a disordered system, well in the regime where entanglement, confinement, and particle interaction dominate the dynamics and kinetics. Measurements of the intermediate structure factor f(q,t) indicate that the particle dynamics differ from Stokes-Einstein Brownian motion and are explained in terms of viscoelastic effects and interparticle interactions.     

Intrinsic bulk vortex dynamics revealed by time resolved small angle neutron scattering

Measurements of intrinsic vortex lattice (VL) dynamics in superconductors as for instance VL melting or Bragg glass transitions are typically performed by e.g. macroscopic transport or surface sensitive measurement techniques. Therefore, usually thin superconducting films are used for microscopic measurements of VL dynamics. A direct consequence of using thin films is the strong influence of surface effects and defects, sample quality and geometry. We succeeded to combine time resolved stroboscopic small angle neutron scattering (SANS) with an advanced, time varying magnetic field setup allowing to extend the time window for slow dynamical processes to the range of 10 ms up to several minutes. The new results demonstrate that it is possible to observe directly the intrinsic dynamics of the VL in a bulk niobium single crystal on a microscopic scale without limitations due to surface effects. Field and temperature dependent relaxation times of the VL from 100 to 500 ms could be observed for the first time, allowing to directly measure the VL of the tilt modulus. This new experimental technique provides the possibility to study also the dynamical magnetic properties of various strongly correlated electron systems.     

Normal form transforms separate slow and fast modes in stochastic dynamical systems

Modelling stochastic systems has many important applications. Normal form coordinate transforms are a powerful way to untangle interesting long term macroscale dynamics from insignificant detailed microscale dynamics. We explore such coordinate transforms of stochastic differential systems when the dynamics have both slow modes and quickly decaying modes. The thrust is to derive normal forms useful for macroscopic modelling of complex stochastic microscopic systems. Thus we not only must reduce the dimensionality of the dynamics, but also endeavour to separate all slow processes from all fast time processes, both deterministic and stochastic. Quadratic stochastic effects in the fast modes contribute to the drift of the important slow modes. Some examples demonstrate that the coordinate transform may be only locally valid or may be globally valid depending upon the dynamical system. The results will help us accurately model, interpret and simulate multiscale stochastic systems.     

质子化学位移各向异性的测量

测量质子化学位移各向异性（CSA）有助于表征分子结构与其动力学,但由于¹H-¹H同核偶极耦合相互作用很强及质子各向异性化学位移较小,测量质子化学位移各向异性仍具有巨大挑战,特别是对含有多种质子的生物大分子,如蛋白质.本文简要综述了测量质子化学位移各向异性的方法,包括同核去耦慢速魔角旋转方法、超快魔角旋转方法、对称重耦（RN_n^v）方法、xCSA方法以及量子化学计算方法.我们重点介绍了在高速魔角旋转条件下蛋白质氨基质子化学位移各向异性的测量及它们与氢键长度、蛋白质二级结构之间的关系.     

Enhanced sampling based on slow variables of trajectory mapping

Most current enhanced sampling(ES) algorithms attempt to bias a potential energy surface based on preset slow collective variables to improve simulation efficiency. However, due to difficulty in obtaining slow variables in complex molecular systems,approximate slow variables are usually applied in ES, which often fail to achieve the expected high efficiency and sufficient accuracy when reconstructing equilibrium properties. In this paper, we demonstrate that the trajectory mapping(TM) technique has the potential to provide the required slow variables for ES. We illustrate the application of a typical ES algorithm(metadynamics)based on the slow variables constructed from the TM in a hairpin peptide system. In this system, both the equilibrium properties and slow dynamics are accurately obtained within approximately two to three orders of magnitude shorter simulation time than in regular molecular dynamics simulation.     

Drift of slow variables in slow-fast Hamiltonian systems

We study the drift of slow variables in a slow-fast Hamiltonian system with several fast and slow degrees of freedom. Keeping the slow variables frozen, for any periodic trajectory of the fast subsystem we define an action. For a family of periodic orbits, the action is a scalar function of the slow variables and can be considered as a Hamiltonian function which generates some slow dynamics. These dynamics depend on the family of periodic orbits.Assuming that for the frozen slow variables the fast system has a pair of hyperbolic periodic orbits connected by two transversal heteroclinic trajectories, we prove that for any path composed of a finite sequence of slow trajectories generated by action Hamiltonians, there is a trajectory of the full system whose slow component shadows the path.     

On the use of slow manifolds in molecular and geophysical fluid dynamics

Constraints typically arise from the elimination of high frequency oscillations in mechanical systems. Examples are provided by bond constraints in molecular simulations and incompressibility constraints in fluid dynamics. A key issue is the accuracy of constrained dynamics with regard to the full dynamics. In this review we focus on the smooth solution components and discuss the concept of slow manifold and soft constraints in molecular and geophysical fluid dynamics. While the formal mathematical derivation of constraints is the same for both molecular and fluid dynamics, the predominant numerical techniques for dealing with constraints are different in the two fields. Semi-implicit time- stepping methods are often used in geophysical fluid dynamics while explicitly enforced constraints are more common in molecular dynamics.     

Method for monitoring slow dynamics recovery

Slow Dynamics is a specific material property, which for example is connected to the degree of damage. It is therefore of importance to be able to attain proper measurements of it. Usually it has been monitored by acoustic resonance methods which have very high sensitivity as such. However, because the acoustic wave is acting both as conditioner and as probe, the measurement is affecting the result which leads to a mixing of the fast nonlinear response to the excitation and the slow dynamics material recovery. In this article a method is introduced which, for the first time, removes the fast dynamics from the process and allows the behavior of the slow dynamics to be monitored by itself. The new method has the ability to measure at the shortest possible recovery times, and at very small conditioning strains. For the lowest strains the sound speed increases with strain, while at higher strains a linear decreasing dependence is observed. This is the first method and test that has been able to monitor the true material state recovery process.     

Evolutionary games with two timescales

We consider a two timescale model of learning by economic agents wherein active or ‘ontogenetic’ learning by individuals takes place on a fast scale and passive or ‘phylogenetic’ learning by society as a whole on a slow scale, each affecting the evolution of the other. The former is modelled by the Monte Carlo dynamics of physics, while the latter is modelled by the replicator dynamics of evolutionary biology. Various quanlitative aspects of the dynamics are studied in some simple cases, both analytically and numerically, and its role as a useful modelling device is emphasized. rights reserved.     

The Influence of Phosphate on the Determination of Calcium by Flame Methods of Analysis—A New Approach

The depression of the analytical signal of calcium by phosphate ion when using turbulent (H₂/0₂ and H₂/A/entrained air) and laminar (C₂/H₂/air) flames in flame spectro-ietry is studied. Measurements of flame emission of calcium as a function of calcium to phosphate and pyrophosphate molar ratios for various flame heights in both turbulent and laminar flames are made. The phosphate interference when using turbulent flames is shown to be a result of a slow vaporization of the calcium phosphate particles. The phosphate interference when using laminar flames with chamber type aspirators is only important at high calcium and phosphate concentrations. The cause of the interference is probably a result of either slow vaporization of the calcium phosphate particles or a slow rate of change of the orthophosphate to pyrophosphate during the decomposition step.     

Molecular rotations studied by nuclear resonant scattering

Nuclear resonant scattering techniques can be used to study both fast and slow dynamics of Mössbauer nuclei. The influence of rotational dynamics in molecular systems is studied applying three types of scattering techniques: (1) Synchrotron radiation perturbed angular correlation (SRPAC) yields direct and quantitative evidence for rotational dynamics in the μs-ns regime. (2) Nuclear inelastic scattering (NIS) monitors the relative influence of intra- and intermolecular forces via the vibrational density of states, which can be influenced by the onset of molecular rotation. (3) In nuclear forward scattering (NFS), information both on rotational and on translational dynamics can be extracted. Results using SRPAC and NIS on a plastic crystal and NFS on ferrocene confined in a molecular sieve are presented.     

Measurements of inner and outer streaming vortices in a standing waveguide using laser doppler velocimetry

Measurements of the axial streaming velocity are performed by means of laser doppler velocimetry in an experimental apparatus consisting of a waveguide having loudspeakers at each end for high intensity sound levels. Streaming is characterized by an appropriate Reynolds number Re(NL), the case Re(NL)<1 corresponding to the so-called slow streaming and the case Re(NL)>/=1 being referred to as fast streaming. The variation of axial streaming velocity with respect to the transverse coordinate is compared to the available slow streaming theory. Streaming fluid flow is measured both in the core region and in the near wall region. Streaming velocity in the center of the guide agrees reasonably well with the slow streaming theory for small Re(NL) but deviates significantly from such predictions for Re(NL)>20 and its evolution for further increasing Re(NL) is discussed. Then streaming behavior in the near wall region is particularly studied. For Re(NL)<70, two vortices are present across the guide section as predicted by slow streaming theory. Then it appears that, when the Reynolds number is increased, two other vortices become visible in the near wall region. Different stages for the generation and evolution of these inner streaming vortices are presented.     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

Slow Kinetics of viscoelastic properties of oil at low-frequency shear vibrations

The results of experiments on the observation of the slow evolution of viscoelasticity moduli of crude oil are presented. Measurements were carried out continuously for 72 h at frequencies of 0.5, 5, and 50 Hz at different temperatures. Based on the Boltzmann statistical approach to determination of the velocity of transition into an equilibrium state, a model differential equation describing slow kinetics of variations in the internal medium parameters is derived. As distinct from the accepted exponential time dependence, the slow kinetics is shown to yield a logarithmic time dependence of perturbation damping.     

Scattering theory of current-induced forces in mesoscopic systems

We develop a scattering theory of current-induced forces exerted by the conduction electrons of a general mesoscopic conductor on slow "mechanical" degrees of freedom. Our theory describes the current-induced forces both in and out of equilibrium in terms of the scattering matrix of the phase-coherent conductor. Under general nonequilibrium conditions, the resulting mechanical Langevin dynamics is subject to both nonconservative and velocity-dependent Lorentz-like forces, in addition to (possibly negative) friction. We illustrate our results with a two-mode model inspired by hydrogen molecules in a break junction which exhibits limit-cycle dynamics of the mechanical modes.     

Dynamical deformations of the electron density in an H2 molecule under strong,oscillating magnetic fields: an application of time-dependent quantum fluid density functional theory

Following the time-dependent quantum fluid density functional theory developed in our laboratory, the present quantum-mechanical, dynamical study of the H₂ molecule under strong, oscillating magnetic fields reveals a coexistence of both slow and fast dynamics, as seen earlier in the cases of hydrogen and helium atoms. Using the Deb–Chattaraj equation of motion we find that, contrary to the situation with static magnetic fields, the electron density now transiently expands. Consequently, the fate of the H–H bond under such strong TD magnetic fields has been addressed through detailed and accurate TD density profiles computed by direct numerical solution of the real-time evolution equation. A detailed interpretation of the slow dynamics has been made.     

Boundary-equilibrium bifurcations in piecewise-smooth slow-fast systems

In this paper we study the qualitative dynamics of piecewise-smooth slow-fast systems (singularly perturbed systems) which are everywhere continuous. We consider phase space topology of systems with one-dimensional slow dynamics and one-dimensional fast dynamics. The slow manifold of the reduced system is formed by a piecewise-continuous curve, and the differentiability is lost across the switching surface. In the full system the slow manifold is no longer continuous, and there is an O(?) discontinuity across the switching manifold, but the discontinuity cannot qualitatively alter system dynamics. Revealed phase space topology is used to unfold qualitative dynamics of planar slow-fast systems with an equilibrium point on the switching surface. In this case the local dynamics corresponds to so-called boundary-equilibrium bifurcations, and four qualitative phase portraits are uncovered. Our results are then used to investigate the dynamics of a box model of a thermohaline circulation, and the presence of a boundary-equilibrium bifurcation of a fold type is shown.     

Nematics with quenched disorder: how long will it take to heal?

Nematics with quenched disorder have been repeatedly predicted to form glass phases. Here we present turbidity experiments and computer simulations aimed at studying glass key features such as dynamics and history dependence in randomly perturbed nematics. Electric field-cooling alignment has been employed to prepare samples in suitably oriented starting states. Remarkable remnant order and slow dynamics are found both by experiment and simulations, indicating that random disorder can, by itself, induce a nematic glass state even without perturber restructuring.     

Phase stabilization of a large-mode-area ytterbium-doped fiber amplifier

Measurements are reported on the open and closed-loop phase stability of a large-mode-area ytterbium-doped fiber amplifier. Phase fluctuations are characterized by a high-frequency low-amplitude jitter superimposed on a slow power-dependent drift. The amplifier may be phase locked to a precision of lambda/20 by using a low-bandwidth feedback loop.