Irreversibility of the threshold field for dendritic flux avalanches in superconductors

Hysteretic effects are seen in the upper and lower threshold fields for the appearance of dendritic flux instabilities, first explained in Yurchenko et al. [Phys. Rev. B 76 (2007) 092504], in NbN-films. We have measured the threshold fields at increasing and decreasing applied fields at different temperatures and proposed a mechanism explaining how the hysteresis arises by analyzing the field profiles inside the sample.     

Giant transversal particle diffusion in a longitudinal magnetic ratchet

We study the transversal motion of paramagnetic particles on a uniaxial garnet film, exhibiting a longitudinal ratchet effect in the presence of an oscillating magnetic field. Without the field, the thermal diffusion coefficient obtained by video microscopy is D(0) ≈ 3 × 10(-4) μm2/s. With the field, the transversal diffusion exhibits a giant enhancement by almost four decades and a pronounced maximum as a function of the driving frequency. We explain the experimental findings with a theoretical interpretation in terms of random disorder effects within the magnetic film.     

Colloidal assembly on magnetically vibrated stripes

We investigate the collective organization of paramagnetic colloidal particles externally driven above the periodic stripes of a uniaxial ferrimagnetic garnet film. An external field modulation induces vibration of the stripe walls and produces random motion of the particles. Defects in the stripe pattern break the symmetry of the potential and favor particle nucleation into large clusters above a critical density. Mismatch between particle size and pattern wavelength generates assemblies with different morphological order. At even higher field strengths, repulsive dipolar interactions between the particles induce cluster melting. We propose a novel approach to generate and externally control a variety of colloidal assemblies.     

The study of geometric effects on the explosion front propagation in a horizontal channel with a layer of spherical beads

Experiments were performed in a horizontal channel partially filled with a layer of 12.7 mm ceramic-oxide beads filled with a nitrogen-diluted stoichiometric methane–oxygen mixture, i.e., CH₄ + 2(O₂ + 2/3N₂). Ionization probes and pressure transducers were used to track the explosion front velocity in the 1.22 m long, 76 mm wide and 152 mm high horizontal channel. Schlieren photography and smoked foil techniques are used to gain insight into the explosion front structure. The explosion propagation phenomenon was characterized by the combustion in the bead layer and the unobstructed gap above. It was determined that for a fixed gap height the bead layer thickness had very little effect on the explosion propagation phenomenon. However, for a fixed bead layer height the explosion propagation was strongly influenced by the gap height. The combustion products vented from the bead layer behind the flame propagating in the gap affects the structure of the shock-flame front in the gap and the maximum flame velocity achieved. The coupling between the vented products and the flame velocity in the gap was strongly influenced by the gap height. The gap height also affects the structure of the detonation wave propagating in the gap following DDT that always occurred in the gap. The DDT run-up distance was found to increase with increasing gap height and inversely with initial pressure.     

Mixed carboxylate-bridged dimanganese(II/III) compounds prepared by an O2-dependent oxidative cleavage of ketones

The structure of an O(2)-sensitive Mn(II)-Mn(II) complex with an unsymmetrical perchlorate bridge is reported. This complex promotes an O(2)-dependent oxidative cleavage of ketones giving mixed carboxylate complexes.     

Description of the photorefractive response in polymers

We employ a phenomenological approach to minimizing the assumptions underlying light-induced charge transport in polymers and expressing the steady-state photorefractive response of polymers through measurable medium characteristics. The relations obtained are shown to be useful for analysis of experimental data and theoretical predictions. The status of the phenomenological approach and its relationship with the known microscopic models are discussed.     

Critical enhancement of photorefractive beam coupling

We show that a hybridization of the optical and material nonlinearities takes place near the threshold of the subharmonic generation in photorefractive crystals. It results in a critical (with a singularity) enhancement of the rate of spatial amplification of light waves and leads to a variety of new optical critical phenomena.     

Forces between heavy nuclei in the independent-particle model

We use the linear response theory to study the force on a heavy nucleus moving near the symmetry axis of ²³⁸U, in an independent-particle model. We present the first detailed microscopic computation showing relaxation of the single-particle motion due to incoherence, and find a relaxation time of about $\text{0.3}\text{MeV}\text{≈ 2 × 10}{}^{\mathrm{?22}}\text{sec}$. We consider three stages of the reaction. While the nuclei approach each other, the induced force is very weak, and is computed as a function of the position, velocity and acceleration of the other nucleus; it involves excitons of 10 to 20 MeV. The computed force agrees well with phenomenological values. The second stage of the reaction is the formation of a neck, which we argue will begin already when the nuclear half-density surfaces are more than 4 fm from each other. For slow collective motion, thought to occur after the neck forms, we show how to represent the incoherent excitation of low-frequency excitons by a friction force, and present a method suitable for numerical computations of the friction coefficient.