Evidence of the harmonic Faraday instability in ultrasonic atomization experiments with a deep, inviscid fluid

A popular method for generating micron-sized aerosols is to submerge ultrasonic (ω~MHz) piezoelectric oscillators in a water bath. The submerged oscillator atomizes the fluid, creating droplets with radii proportional to the wavelength of the standing wave at the fluid surface. Classical theory for the Faraday instability predicts a parametric instability driving a capillary wave at the subharmonic (ω/2) frequency. For many applications it is desirable to reduce the size of the droplets; however, using higher frequency oscillators becomes impractical beyond a few MHz. Observations are presented that demonstrate that smaller droplets may also be created by increasing the driving amplitude of the oscillator, and that this effect becomes more pronounced for large driving frequencies. It is shown that these observations are consistent with a transition from droplets associated with subharmonic (ω/2) capillary waves to harmonic (ω) capillary waves induced by larger driving frequencies and amplitudes, as predicted by a stability analysis of the capillary waves.     

Estimation of the cross section of neutron scattering by spin waves in thin ferromagnetic layers

Inelastic neutron scattering by magnetic excitations in thin ferromagnetic films has not been observed so far owing to the small cross section of the interaction of neutrons with spin waves. To increase the probability of inelastic magnetic scattering, it has been proposed to implement three-layer structures in which the neutron wave functions exhibit resonant enhancement in a ferromagnetic layer. The cross section of neutron scattering by spin waves in the regime of the resonant enhancement of the neutron wave function has been estimated.     

Insensitive dependence of delay-induced oscillation death on complex networks

Oscillation death (also called amplitude death), a phenomenon of coupling induced stabilization of an unstable equilibrium, is studied for an arbitrary symmetric complex network with delay-coupled oscillators, and the critical conditions for its linear stability are explicitly obtained. All cases including one oscillator, a pair of oscillators, regular oscillator networks, and complex oscillator networks with delay feedback coupling, can be treated in a unified form. For an arbitrary symmetric network, we find that the corresponding smallest eigenvalue of the Laplacian λ(N) (0 >λ(N) ≥ -1) completely determines the death island, and as λ(N) is located within the insensitive parameter region for nearly all complex networks, the death island keeps nearly the largest and does not sensitively depend on the complex network structures. This insensitivity effect has been tested for many typical complex networks including Watts-Strogatz (WS) and Newman-Watts (NW) small world networks, general scale-free (SF) networks, Erdos-Renyi (ER) random networks, geographical networks, and networks with community structures and is expected to be helpful for our understanding of dynamics on complex networks.     

Callan-Rubakov effect for strings

Like monopoles, cosmic strings can catalyze proton decay. The cross section for this process is given by the geometrical cross section. Unlike in the case of monopole-catalyzed proton decay, there is no amplification of the fermion wave function near the core of the defect and hence there is no amplification of the cross section. The scattering cross section of bosons off of cosmic strings is larger than the geometrical cross section, as a consequence of the statistics.     

An oscillator using high order Lamb wave modes

An oscillator using high order Lamb wave modes is presented. Due to the multimode property of Lamb waves, high frequency oscillators using high order mode Lamb waves can be fabricated. To select the operating mode of the Lamb wave oscillator, a high order mode selector is inserted into the feedback loop. An oscillator using the 13th antisymmetric mode (a₁₃) in Lamb wave is achieved in experiments and the oscillating frequency is 5.30 times higher than that of the a₀ mode excited by the interdigital transducer.     

Calculation of external circuit impedances of mm-wave oscillator cavities by a rigorous field analysis method

This paper presents a generalized field analysis method for calculating the external circuit impedances looking outside from the active devices of some millimeter wave oscillators. For a simple oscillator model with a radial disc and a diode existing in a rectangular waveguide, the numerical results are in agreement with the experimental data. Impedance characteristics for several different oscillator configurations are given, and some useful conclusions for oscillator design are derived.     

Spiral wave chimeras in populations of oscillators coupled to a slowly varying diffusive environment

Chimera states are firstly discovered in nonlocally coupled oscillator systems. Such a nonlocal coupling arises typically as oscillators are coupled via an external environment whose characteristic time scale τ is so small (i.e., τ → 0) that it could be eliminated adiabatically. Nevertheless, whether the chimera states still exist in the opposite situation (i.e., τ ≫ 1) is unknown. Here, by coupling large populations of Stuart−Landau oscillators to a diffusive environment, we demonstrate that spiral wave chimeras do exist in this oscillator-environment coupling system even when τ is very large. Various transitions such as from spiral wave chimeras to spiral waves or unstable spiral wave chimeras as functions of the system parameters are explored. A physical picture for explaining the formation of spiral wave chimeras is also provided. The existence of spiral wave chimeras is further confirmed in ensembles of FitzHugh−Nagumo oscillators with the similar oscillator-environment coupling mechanism. Our results provide an affirmative answer to the observation of spiral wave chimeras in populations of oscillators mediated via a slowly changing environment and give important hints to generate chimera patterns in both laboratory and realistic chemical or biological systems.     

Integral cross sections for electron impact excitations of argon and carbon dioxide

Electron-impact excitation integral cross sections play an important role in understanding the energy transfer processes in many applied physics. Practical applications require integral cross sections in a wide collision energy range from the excitation threshold to several keV. The recently developed BE-scaling method is able to meet the demands of integral cross sections for dipole-allowed transitions while the prerequisite relies on the accurate generalized oscillator strengths. Fast electron and x-ray scatterings are the conventional experimental techniques to approach the generalized oscillator strengths, and the joint study by both methods can provide credible cross-checks. The validated generalized oscillator strengths can then be used to extrapolate optical oscillator strengths by fitting the data with the Lassettre formula. The fitted curve also enables the integration of generalized oscillator strengths over the whole momentum transfer region to obtain the BE-scaled integral excitation cross sections. Here, experimental measurements by both fast electron and x-ray scattering of argon and carbon dioxide are reviewed. The integral cross sections for some low-lying states are derived from the cross-checked generalized oscillator strengths for the first time. The integral cross sections presented in this paper are openly available at https://doi.org/10.11922/sciencedb.01466.     

广义振子强度密度

根据Born近似,可以计算高能电子碰撞激发的总截面和微分截面。微分截面是和所谓的广义振子强度成正比的。电子碰撞过程可将靶原子或离子激发至无数的束缚态、自电离态和对应的连续态,多通道量子数亏损理论能够统一地处理这些激发态。因此可以定义每单位激发能内的广义振子强度为广义振子强度密度。本文以Li原子为例,总结其广义振子强度密度随激发能量和动量转移而变化的规律,并和最近精确的实验结果进行了比较,明确了Born近似的适用范围。 关键词：     

Hypochromism and optical rotation in helical polymers

We show that the decreased light absorption and the anomalous optical rotatory dispersion in helical polynucleotides and polypeptides may be interpreted purely as a local field effect. The electric field of the incident light wave is screened off from each residue by the induced electric dipoles in the others. Quantum-mechanical calculations based on time-dependent Hartree theory and this local field picture correspond precisely with the formulae derived in Tinoco's, Rhodes's, and Moffitt's exciton theories, provided that the Coulomb interactions are small. The degenerate exciton waves in our theory correspond to normal modes of a set of coupled oscillators, and the rotational strengths and oscillator strengths are conserved. There is no conflict between Tinoco's theory of hypochromism and the ones proposed by Bolton and Weiss and by Nesbet. One new conclusion is that the energy shifts accompanying hypochromism should not vary much when the exciton coupling changes from the strong to the weak coupling limits.     

Photoacoustic cavitation in spherical and cylindrical absorbers

Photomechanical damage in absorbing regions or particles surrounded by a non-absorbing medium is investigated experimentally and theoretically. The damage mechanism is based on the generation of thermoelastic pressure by absorption of pulsed laser radiation under conditions of stress confinement. Principles of photoacoustic sound generation predict that the acoustic wave generated in a finite-size absorbing region must contain both compressive and tensile stresses. Time-resolved imaging experiments were performed to examine whether the tensile stress causes cavitation in absorbers of spherical or cylindrical shape. The samples were absorbing water droplets and gelatin cylinders suspended in oil. They were irradiated with 6-ns-long pulses from an optical parametric oscillator. Photoacoustic cavitation was observed near the center of the absorbers, even if the estimated temperature caused by absorption of the laser pulse did not exceed the boiling point. The experimental findings are supported by theoretical simulations that reveal strong tensile stress in the interior of the absorbers, near the center of symmetry. Tensile stress amplitudes depend on the shape of the absorber, the laser pulse duration, and the ratio of absorber size to optical absorption length. The photoacoustic damage mechanism has implications for the interaction of ns and sub-nslaser pulses with pigmented structures in biological tissue. Received: 9 October 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999     

Phonon absorbing boundary conditions for molecular dynamics

With the goal of minimizing the domain size for molecular dynamics (MD) simulations, we develop a new class of absorbing boundary conditions (ABCs) that mimic the phonon absorption properties of an unbounded exterior. The proposed MD-ABCs are extensions of perfectly matched discrete layers (PMDLs), originally developed as an absorbing boundary condition for continuous wave propagation problems. Called MD-PMDL, this extension carefully targets the absorption of phonons, the high frequency waves, whose propagation properties are completely different from continuous waves. This paper presents the derivation of MD-PMDL for general lattice systems, followed by explicit application to one-dimensional and two-dimensional square lattice systems. The accuracy of MD-PMDL for phonon absorption is proven by analyzing reflection coefficients, and demonstrated through numerical experiments. Unlike existing MD-ABCs, MD-PMDL is local in both space and time and thus more efficient. Based on their favorable properties, it is concluded that MD-PMDL could provide a more effective alternative to existing MD-ABCs.     

Propagation of electromagnetic waves in a waveguide with a modulated anisotropic insert

The propagation of transverse electric (TE) and transverse magnetic (TM) waves in a waveguide of arbitrary cross section with an anisotropic periodically space-and time-modulated insert is considered. Under the assumption that the modulation indices are small, analytical expressions for the TE and TM fields in the domain of weak interaction between the signal wave and modulating wave are derived. The frequency near which strong interaction between these waves occurs is found, and specific features of the fields in this domain are revealed.     

等电子系列离子的广义振子强度密度之标度关系

利用Born近似可以计算高能电子与原子或离子碰撞激发的微分截面和总截面。微分截面正比于广义振子强度。电子碰撞过程可将靶原子或离子激发到无数个束缚态以及相应的连续态。量子亏损理论能够统一处理这些激发态;因此可定义广义振子强度密度——即每单位激发能内的广义振子强度。我们计算了Li的等电子系列Li,Be⁺,B⁺⁺,C³⁺,Ne⁷⁺,Na⁸⁺,K¹⁶⁺等从基态到S,P,D,F通道的激发的广义振子强度密度,总结了类Li等电子系列离子的广义振子强度密度的标度关系。 关键词：     

非相干光宽带腔增强吸收光谱技术应用于SO_2弱吸收的测量

非相干光宽带腔增强吸收光谱作为高灵敏检测技术,已成功应用于多种大气痕量气体浓度的测量。根据腔增强吸收光谱技术测量原理可知,若已知测量气体准确浓度,镜片反射率随波长的变化曲线、有效吸收长度、光学腔内有无测量气体吸收前后的光辐射变化,可测量出待测气体的吸收截面。SO_2由于a~3 B_1—X~1 A_1自旋禁阻跃迁,在345~420nm波段吸收截面较低(~10~(-22) cm2/molecule),其测量有一定难度,而准确的弱吸收截面对于卫星反演大气痕量气体浓度以及大气研究等方面均有重要意义。采用365nm LED光源的宽带腔增强吸收光谱实验装置测量357~385nm波段范围SO_2的弱吸收,获得该波段SO_2弱吸收截面,并与已公开发表的SO_2吸收截面进行对比,相关系数r为0.997 3,验证了非相干光宽带腔增强吸收光谱技术准确测量气体弱吸收截面的适用性。     

Generalized deformed para-Bose oscillator and its coherent states

Generalized deformed commutation relations for a single-mode para-Bose oscillator are constructed. The connection of generalized deformed para-Bose oscillators with para-Bose oscillators is determined. From these, the energy spectrum of generalized deformed para-Bose oscillators and the coherent states of the annihilation operators are discussed. On leave from Department of Physics, Hanoi National University, 90 Nguyen Trai, Hanoi, Vietnam.     

Ultrasonic waves propagation in absorbing thin plates application to paper characterization

Guided wave theory is applied to a thin orthotropic and absorbing plate for low frequency propagation of ultrasonic waves. The aim of this paper is to give some physical interpretations of the non-destructive characterization of paper materials, which are cellulosic fibrous networks. It is shown that the propagation problem reduces to two normal modes of propagation in the plane of the plate. Each of them depends on four complex and independent stiffnesses that are combinations of elementary complex stiffnesses of the media. The imaginary part of these stiffnesses corresponds to a possible mechanism of energy dissipation during the wave propagation for this kind of material. The reverse problem, which gives four complex values, is then numerically solved using a small attenuation assumption. The specially designed experimental set-up has led to the first measurements of tracing paper damping factors. The phase velocity measurements of the plate waves agree with the results already found by several paper researchers. As a particular and new result, the shear wave velocities are found to present a quasi-isotropic repartition in the plane of the paper sheet. It was found that the absorbing phenomenon can occur for each propagation mode in such a material. The attenuation values are small, except for one of them that corresponds to a coupling term in the propagation model. The anisotropy of their repartition is also shown in the case of quasi-longitudinal waves.     

Meson-exchange currents in thermal n-3He radiative capture

The cross section for radiative capture of thermal neutrons by ³He arising from 1π, 1ρ and 1ω meson-exchange processes is calculated using oscillator and exponential wave functions for ³He (including a D-wave component) and oscillator wave functions for ⁴He (also including a D-wave component). A correlation function with a range of 0.25 fm is incorporated in the wave function to describe the short-range behaviour. Results are sensitive to the assumed range of the correlation function and to the triplet scattering length in the neutron scattering wave function, which are not known accurately. For a reasonable set of parameter choices, a combination of one-body impulse approximation (between D-states) and two-body meson-exchange current processes can explain the entire experimental cross section.     

An experimental determination of the cross section of the swings band system of C3

The spectral absorption cross section of the Swings band of C₃ was determined from measurements behind incident shock waves that heated a gas mixture of argon and acetylene. These measurements spanned the spectral region between 300 and 540 nm, and were obtained at temperatures between 3200 and 4000°K. An electronic oscillator strength of 0.033 was deduced from the measurements.     

Stability analysis of plane wave solutions of the discrete ginzburg-landau equation

The discrete Ginzburg-Landau model for a family of oscillators linearly coupled with their first neighbors is studied. The full linear stability analysis of the nonlinear plane wave solutions is performed by considering both the wave number (k) of the basic states and the wave number (q) of the perturbations as free parameters. In particular, it is shown that nonlinear plane waves can be destabilized not only by long (q-->0) or short (q=pi) wave perturbations, but also by intermediate wave numbers (0相似文献