Two-zone elastic-plastic single shock waves in solids

By decoupling time and length scales in moving window molecular dynamics shock-wave simulations, a new regime of shock-wave propagation is uncovered characterized by a two-zone elastic-plastic shock-wave structure consisting of a leading elastic front followed by a plastic front, both moving with the same average speed and having a fixed net thickness that can extend to microns. The material in the elastic zone is in a metastable state that supports a pressure that can substantially exceed the critical pressure characteristic of the onset of the well-known split-elastic-plastic, two-wave propagation. The two-zone elastic-plastic wave is a general phenomenon observed in simulations of a broad class of crystalline materials and is within the reach of current experimental techniques.     

Incoherently coupled photorefractive bright-bright soliton pairs in one and two dimensions

We investigate incoherently coupled bright-bright soliton pairs in photorefractive crystals under steady-state condition in both one and two transverse dimensions. The novel numerical scheme according to the Crank-Nicholson method generalized in three dimensions accompanied by the central difference method was used to solve the coupled wave and potential equations in order to find soliton solution and pairs distribution. Simulation of propagation is performed numerically and show that the presence of both components is required for stable propagation. This simulation approach demonstrates beams evolution after decoupling clearly.     

Bright-dark incoherently coupled soliton pairs composed of spatially incoherent solitons

It is shown that bright-dark incoherently coupled soliton pairs can exist in photorefractive (PR) crystals under steady-state conditions, each soliton constituent of which is spatially incoherent. The characteristics of bright-dark incoherently coupled soliton pairs are studied by the coherent density approach and the intensity expressions of soliton pairs are obtained. The propagation properties of coherent components of each constituent in a soliton pair are also discussed in detail.     

闭路光伏光折变聚合物中非相干耦合亮-暗空间孤子对

为了得到闭路光折变有机聚合物中存在非相干耦合亮-暗光伏空间孤子对,采用数值模拟的方法,对稳态情况下两束互不相干的光束,在闭路光伏光折变有机聚合物中的传播进行了研究。结果表明,具有相同偏振和相同波长的两束互不相干的光束,可在光伏光折变有机聚合物中形成非相干耦合亮-暗光伏孤子对。在给定适当参量或扰动不太大的情况下,孤子对中的亮孤子和暗孤子都能在光伏光折变有机聚合物中稳定传播。研究结果可为光折变有机聚合物空间孤子理论的发展提供理论依据。     

Equations of state and shock-induced transformations in solid I-solid II-liquid bismuth

An equation of state is determined for each phase in the solid I-solid II-liquid region of bismuth. Equality of the Gibbs free energy for each pair of phases determines the phase boundaries and triple point in the temperature-pressure plane. From the complete equation of state, the shock-wave response of bismuth preheated to 493 °K. is calculated and comparison is made with experimental data obtained by means of quartz transducer instrumentation. These results suggest that bismuth undergoes a certain degree of melting on the microsecond time scale of the shock-wave experiment. This is contrary to previous results and conclusions drawn from data on the shock-wave propagation in bismuth. These seemingly contradictory results may be due to microstructural differences in the samples or to differences in measuring techniques.     

Velocity of propagation in the shock-crystallization of sputtered amorphous germanium

Shock-crystallization in sputtered amorphous germanium thin films has been studied by a high speed movie technique. One hundred cm/sec has been obtained as the velocity of propagation of the crystallization wave. This value is much smaller than that of the sound velocity and rules out the contribution of the acoustic shock-wave to this phenomenon.     

Observations on sound propagation in rapidly rotating Bose-Einstein condensates

Repulsive laser potential pulses applied to vortex lattices of rapidly rotating Bose-Einstein condensates create propagating density waves which we have observed experimentally and modeled computationally to high accuracy. We have observed a rich variety of dynamical phenomena ranging from interference effects and shock-wave formation to anisotropic sound propagation.     

Structural transformations in single-crystal iron during shock-wave compression and tension: Molecular dynamics simulation

The molecular dynamics method is used to simulate shock-wave propagation in the [100] direction of a single-crystal bcc iron target in order to study structural transformations in compression and rarefaction waves and the mechanisms of spall fracture. The specific features of structural transformations near the lateral target surface have been analyzed.     

Optimally stabilized beam pairs

In this paper, evolution of two co-axially co-propagating beams with the propagation distance in a Kerr type nonlinear medium has been analyzed for all possible physical parameters and situations. For the first time, the paper analyzes useful “optimally stabilized” (minimum width oscillations of beams with distance of propagation) pairing of bright-bright, bright-dark, dark-dark beams of same/different wavelengths and of same/different widths using coupled nonlinear Schrödinger equation (NLSE) that has been solved using split-step Fourier method. Soliton pairs of two unequal beam widths are known to be impossible, however, it is shown that “optimally stabilized” pairing is possible in such cases. Existence surfaces for optimally stabilized pairing have also been revealed for different interaction coefficient of the coupled beams.     

Wave velocities in shock-compressed cubic and hexagonal single crystals above the elastic limit

When solids are subjected to high-pressure shock-wave loading, multiple stress waves propagate with velocities dependent upon the elastic and inelastic compressibilities of the solid. The present paper shows that the inelastic or plastic waves in cubic and hexagonal single crystals do not necessarily propagate with the bulk sound speed as they do in isotropic elastic-plastic solids. This result is a consequence of anisotropy in the plastic deformation which depends on the slip plane orientation in the crystal and has important consequences with regard to the determination of compressibilities from shock-wave data. In particular, for wave propagation in the <110> directions of cubic crystals the departure from the bulk velocity can be significant (5–25 per cent). For wave propagation normal to the c-axis in hexagonal crystals, the plastic wave velocity also differs from the bulk sound speed (10–25 per cent). Plastic wave velocities are tabulated for a number of cubic crystals on the basis of the various slip systems common to these materials. The calculated velocities are then compared with experimental data on shock-loaded single-crystal aluminum and sodium chloride.     

SR-beam investigations of ultradispersed silver obtained by shock-wave synthesis under cryogenic conditions

Results from studying ultradispersed silver samples obtained using the shock-wave approach, both in liquid nitrogen and at normal temperatures, are described. Some possible initial substances for such synthesis techniques are considered and a circuit for an immunosensor based on coupled charge devices is proposed.     

One-dimensional optical dark screening photovoltaic-photorefractive solitons, soliton pairs and incoherent interaction between them in BaTiO3 crystal

We studied the theory of the optical solitons and introduced the nonlinear equations governed by the screening photovoltaic-photorefractive dark solitons. By the use of the numerical methods, the intensity profile, refractive index change of the media and also stability under propagation are investigated. Also, we investigated incoherent coupled dark-dark soliton pairs and incoherent interaction between them.     

One-dimensional optical bright screening photovoltaic photorefractive solitons, soliton pairs and incoherent interaction between them in BaTiO3 crystal

By the use of theory of optical solitons, nonlinear equations governed by screening photorefractive photovoltaic solitons are introduced. By means of numerical methods, intensity profile, refractive index change of the media and also stability under propagation are investigated. Self-bending of solitons under first-order diffusion is also studied. Besides, incoherent coupled bright-bright soliton pairs and incoherent interaction are studied.     

Coupled nonlinear Schrödinger equations with cubic-quintic nonlinearity: Integrability and soliton interaction in non-Kerr media

We propose an integrable system of coupled nonlinear Schr?dinger equations with cubic-quintic terms describing the effects of quintic nonlinearity on the ultrashort optical soliton pulse propagation in non-Kerr media. Lax pairs, conserved quantities and exact soliton solutions for the proposed integrable model are given. The explicit form of two solitons are used to study soliton interaction showing many intriguing features including inelastic (shape changing or intensity redistribution) scattering. Another system of coupled equations with fifth-degree nonlinearity is derived, which represents vector generalization of the known chiral-soliton bearing system.     

Shock-wave generation during dry laser cleaning of particles

Generation of shock waves during a dry laser-cleaning process was examined with the probe-beam-deflection technique. Our experimental set-up allows measurement of a density gradient and a time of flight of the generated optoacoustic waves. With analysis of the measured signals we observed supersonic propagation velocities, density profiles matching the shock-wave profile, and reasonable agreement between the shock-wave theory and measured data. Furthermore, we inspected the mechanisms for the generation of these optoacoustic waves. For the clean surface a rapid thermoelastic surface expansion is the main generation mechanism, creating only weak shock waves; however, during the dry laser cleaning the ejected contaminants generate relatively strong shock waves. This difference is easily observed with our experimental set-up, thus enabling the on-line monitoring of the laser-cleaning process. PACS 79.20.Ds; 81.65.Cf; 43.25.+y     

Incoherently coupled Hermite-Gaussian breather and soliton pairs in strongly nonlocal nonlinear media

We theoretically investigate the propagation of incoherently coupled Hermite-Gaussian breather and soliton pairs in strongly nonlocal nonlinear media. It is found that multipole-mode soliton pairs with arbitrary different orders of Hermite-Gaussian shape can exist when the total power of two beams equals the critical power and the ratio of the beam widths for the Gaussian part is inversely proportional to the square root of the ratio of the wave numbers. When the total power does not equal the critical power, the Hermite-Gaussian breather pair exists and their beam widths evolve analogously. For general cases where the ratio of the beam widths is arbitrary, soliton-breather pairs or breather-breather pairs can be formed and their beam widths evolve synchronously in-phase or out-of-phase. Numerical simulations directly based on the nonlocal nonlinear Schrödinger equation are conducted for comparison with our theoretical predictions. The numerical stability analysis shows the higher-order Hermite-Gaussian solitons can not be stable for small nonlocality or for some media like liquid crystals.     

Energy-based coupling of smooth particle hydrodynamics and molecular dynamics with thermal fluctuations

We propose a thermodynamically consistent and energy conserving coupling scheme between the atomistic and the continuum domain. The coupling scheme links the two domains using the DPDE (Dissipative Particle Dynamics at constant Energy) thermostat and is designed to handle strong temperature gradients across the atomistic/continuum domain interface. The fundamentally different definitions of temperature in the continuum and atomistic domain – internal energy and heat capacity versus particle velocity – are accounted for in a straightforward and conceptually intuitive way by the DPDE thermostat. We verify the here proposed scheme using a fluid, which is simultaneously represented as a continuum using Smooth Particle Hydrodynamics, and as an atomistically resolved liquid using Molecular Dynamics. In the case of equilibrium contact between both domains, we show that the correct microscopic equilibrium properties of the atomistic fluid are obtained. As an example of a strong non-equilibrium situation, we consider the propagation of a steady shock-wave from the continuum domain into the atomistic domain, and show that the coupling scheme conserves both energy and shock-wave dynamics.     

Simulation of shock-wave propagation in the argon plasma of a positive column discharge

The computer simulation of shock-wave propagation in the argon plasma of positive column discharge was performed. A one-dimensional model of the gas-discharge plasma is used, which comprises the continuity equations for the electron and ion plasma components and the equation of electrostatics with allowance for initial and boundary conditions. The distribution of plasma parameters in the shock wave was obtained; the effect of its intensity was evaluated. The simulation results were compared with experimental data.