On the collapse of wave packets in a medium with normal group velocity dispersion

A solution to the problem of realizing the collapse of three-dimensional wave packets in nonlinear media with normal group velocity dispersion is proposed. Wave packets with pronounced hyperbolic topology are shown to collapse; i.e., the field increases infinitely near the system axis. In particular, wave collapse of the tubular axisymmetric packets occurs through the concentration of the compressed ring field distribution at the axis. The collapse is shown to stabilize due to the saturation of nonlinearity or nonlinear dissipation, which restrict the field increase and lead to the packet splitting in the transverse direction.     

Stability criteria of Rayleigh ‐ Taylor modes in magnetized plasmas

The Rayleigh ‐ Taylor (RT) instability is investigated analytically in an inhomogeneous plasma in an external magnetic field. For the case of two distinct fluid layers separated by a sharp boundary and for a fluid of a continuously varying density, RT dispersion relations have been obtained and analyzed. Stability criteria of the excited modes are disscussed with respect to the mode propagation relative to the applied magnetic field. The magnetic field is found to act as a stabilizer up to a threshold value that can be determined from the dispersion relation.     

The influence of the orientation and strength of a magnetic field on the coherent population trapping in the Λ system

The influence of the orientation and strength of a magnetic field on the dynamics and dispersion of the populations of the multilevel Λ system upon spontaneous decay into thermostat levels is considered. The radiation field consists of two components and is specified by the vector-potential in the electric dipole approximation. From the solution of the Schödinger equation for a system consisting of an atom in a magnetic field + radiation field, the probability of populating a common level for the generalized Λ system is determined in the resonance approximation. The calculation of the dynamics and dispersion of the populations demonstrates their dependence on the orientation of the magnetic field vector with respect to the light field polarization vector and on the relationship between the magnetic field strength and radiation field intensities. The coherent population trapping occurs only in the case when Rabi frequencies either exceed or are comparable to the Zeeman splitting of magnetic sublevels. By varying the orientation of the magnetic field, it is possible to change the dynamics and dispersion of the populations, thus affecting the coherent population trapping.     

Hole with diluted density in the presence of self-generated magnetic fields

The collapse behavior of the fields and hole with diluted density described by the nonlinear coupling equations in laser-produced plasma are investigated in the condition of non-static limit. The results show that the nonlinear plasma currents give rise to intermittent magnetic fields, leading to collapsing hole with diluted density. The self-generated magnetic field is very important for the formation of hole with diluted density. The resulting density perturbation rate is very similar to that observed experimentally.     

Stability of a self-gravitating homogeneous resistive plasma

In this paper, we analyze the stability of a homogeneous self-gravitating plasma, having a non-zero resistivity. This study provides a generalization of the Jeans paradigm for determining the critical scale above which gravitational collapse is allowed.We start by discussing the stability of an ideal self-gravitating plasma embedded in a constant magnetic field. We outline the existence of an anisotropic feature of the gravitational collapse. In fact, while in the plane orthogonal to the magnetic field the Jeans length is enhanced by the contribution of the magnetic pressure, outside this plane perturbations are governed by the usual Jeans criterion. The anisotropic collapse of a density contrast is sketched in detail, suggesting that the linear evolution provides anisotropic initial conditions for the non-linear stage, where this effect could be strongly enforced.The same problem is then faced in the presence of non-zero resistivity and the conditions for the gravitational collapse are correspondingly extended. The relevant feature emerging in this resistive scenario is the cancelation of the collapse anisotropy in weakly conducting plasmas. In this case, the instability of a self-gravitating resistive plasma is characterized by the standard isotropic Jeans length in any directions. The limit of very small resistivity coefficient is finally addressed, elucidating how reminiscence of the collapse anisotropy can be found in the different values of the perturbation frequency inside and outside the plane orthogonal to the magnetic field.     

Stability of stratified fluid in porous medium in the presence of suspended particles and variable magnetic field

General equations governing the stability of stratified fluid in a stratified porous medium in the presence of suspended particles and variable horizontal magnetic field, separately, have been derived. Assuming stratifications in density, viscosity, suspended particles number density, medium porosity, medium permeability and a magnetic field of exponential form the dispersion relations have been obtained. Systems have been found to be stable for stable stratifications and unstable for unstable stratifications. A system which was unstable in the absence of magnetic field can be completely stabilized by a magnetic field for a certain wave-number range. The behaviour of growth rates with respect to fluid viscosity, medium permeability, suspended particles number density and magnetic field has been examined analytically.     

Half-metallicity induced by out-of-plane electric field on phosphorene nanoribbons

Exploring the half-metallic nanostructures with large band gap and high carrier mobility is a crucial solution for developing high-performance spintronic devices. The electric and magnetic properties of monolayer zigzag black-phosphorene nanoribbons (ZBPNRs) with various widths are analyzed by means of the first-principles calculations. Our results show that the magnetic ground state is dependent on the width of the nanoribbons. The ground state of narrow nanoribbons smaller than 8ZBPNRs prefers ferromagnetic order in the same edge but antiferromagnetic order between two opposite edges. In addition, we also calculate the electronic band dispersion, density of states and charge density difference of 8ZBPNRs under the action of out-of-plane electric field. More interesting, the addition of out-of-plane field can modulate antiferromagnetic semiconductor to the half metal by splitting the antiferromagnetic degeneracy. Our results propose a new approach to realize half-metal in phosphorene, which overcomes the drawbacks of graphene/silicene with negligible band gap as well as the transitional metal sulfide (TMS) with low carrier mobility.     

Phase diagram of magnetic polymers

We consider polymers made of magnetic monomers (Ising or Heisenberg-like) in a good solvent. These polymers are modeled as self-avoiding walks on a cubic lattice, and the ferromagnetic interaction between the spins carried by the monomers is short-ranged in space. At low temperature, these polymers undergo a magnetic induced first order collapse transition, that we study at the mean field level. Contrasting with an ordinary point, there is a strong jump in the polymer density, as well as in its magnetization. In the presence of a magnetic field, the collapse temperature increases, while the discontinuities decrease. Beyond a multicritical point, the transition becomes second order and -like. Monte Carlo simulations for the Ising case are in qualitative agreement with these results. Received 11 February 1999     

Nonsymmetrical Electromagnetic Waves in Partially Plasma — Filled Waveguide

The dispersion of nonsymmetrical electromagnetic waves in waveguide, partially filled with plasma, is studied numerically and experimentally. An external d.c. coaxial magnetic field is applied to the waveguide. The dispersion equation of these waves is obtained, and it is solved numerically for the experimental conditions. Results are obtained, which show coupling of the waveguide HE_l1-modes with the family of the high-frequency plasma HE_ln-modes. The numerical dispersion curves are experimentally examined, and the influence of the magnetic field and the plasma density is studied.     

Chiral optical Tamm states at the boundary of the medium with helical symmetry of the dielectric tensor

We report the observation of photovoltage oscillations in back-gated two-dimensional electron systems when tuning the density under incident microwaves and in the absence of a magnetic field. The oscillations are periodic in the inverse of the square root of the density. They originate from the interference of screened bulk plasmons with a linear dispersion. This phenomenon can be exploited to devise a spectrometer-on-a-chip for millimeter waves. The influence of a perpendicular magnetic field is investigated and reveals a transformation of screened bulk plasmons waves into screened edge magnetoplasmons.     

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由色散方程和流体方程推导得到了横等离激元和对等离子体相互作用的非线性控制方程,利用场论的方法构建非线性控制方程的拉格朗日密度函数并求得相互作用过程中守恒的等离激元数和总能量。通过标度变换发现横等离激元由于调制不稳定性将坍塌形成小尺度的局域结构。在坍塌过程中横等离激元产生的有质动力随场强的增强而增强。由于有质动力对粒子的排斥,对等离子体中将产生小尺度的密度空穴。     

Two-dimensional magnetoplasmon in the silicon inversion layer

The influence of a magnetic field on the behavior of the two-dimensional plasmon is studied in the n-inversion layer of p-type silicon at fixed wavevector as a function of electron density and magnetic field. Most of the behavior is well described by a dispersion relation and lineshape based on a classical theory of the electron gas. However, deviations from the theory are observed which may indicate an interaction with other modes of the system.     

Rayleigh-taylor instability of viscoelastic fluids with suspended particles in porous medium in hydromagnetics

The instability of the plane interface between two viscoelastic (Oldroydian) superposed conducting fluids permeated with suspended particles in porous medium is studied when the whole system is immersed in a uniform magnetic field. The dispersion relation for the Oldroydian viscoelastic fluid is obtained which also yields dispersion relations for Maxwellian and Newtonian fluids in special cases, in the presence of suspended particles in porous medium in hydromagnelics. The system is found to be stable for potentially stable case. The presence of magnetic field stabilizes certain wave number band whereas the system was unstable for all wave numbers in the absence of magnetic field, for the potentially unstable configuration. The growth rates increase (for certain wave numbers) and decrease (for other wave numbers) with the increase in stress relaxation time, strain retardation time, suspended particles number density and medium permeability.     

Dependence of resonance condition on pressure in an RF resonancemethod

The plasma density is shown as functions of pressure and magnetic flux density in an RF resonance method using the XPDP1 simulation code. The RF resonance method has the unique feature that a strong electric field in bulk controls the plasma density. Owing to the balance between the electric field decrease and the collision rate increase, the plasma density in the RF resonance method has a peak with respect to pressure. The plasma density with respect to the magnetic flux density depends on the condition of the RF resonance method, and the dependence is strong at low pressure because of the strong resonance. Sheath thickness is the most important parameter that determines the strength of the resonance induced. It is shown that the sheath thickness s is related to the plasma density n as a function of ns, obtained from a dispersion relation at constant external parameters. The magnetic flux density which induces the strong resonance is determined from sheath thickness. The plasma density in the RF resonance method can be predicted from discharge parameters using the relation between plasma density and sheath thickness     

Perfectly conducting ferrofluid in general relativity

The growth of magnetic energy density is considered in a collapsing ferrofluid with infinite electrical conductivity whose magnetic permeability is not a constant. It is shown that the variation of the magnetic permeability affects the growth of magnetic energy. The case of isotropic collapse and a specific case of anisotropic collapse are examined.     

Magnetoacoustic solitons in pair-ion fullerene plasma

ABSTRACT

The propagation of magnetoacoustic (fast magnetohydrodynamic) waves in pair-ion (PI) fullerene plasma is studied in the linear and nonlinear regimes. The pair-ion (PI) fullerene plasma is theorized as homogeneous, magnetized, warm and collisionless. Employing multi-fluid magnetohydrodynamic model, the dispersion relation is obtained and wave dispersion effects which appear through ion inertial length are discussed. Using reductive perturbation technique (RPT), the Korteweg–de Vries (KdV) equation is derived and its solution for small but finite amplitude magnetoacoustic solitons propagating in the direction perpendicular to the external magnetic field is presented. The compressive magnetoacoustic soliton (i.e. positive potential pulse) propagating with super Alfvénic speed is obtained in magnetized PI fullerene plasma. The variations in the amplitude and width of the magnetoacoustic soliton structures are also illustrated by using numerical values of the plasma parameters such as ions' density, temperature difference between fullerene ions and magnetic field intensity, which have been taken from the PI plasma experiments already published in the literature.     

等离子体填充波纹波导中低频模式特性分析

采用等离子体流体模型和线性场理论，导出了在强引导磁场下，一无限薄环形等离子体加载 波纹波导中电磁波传播的色散关系.数值计算并分析了在不同的等离子体填充密度下，低频 等离子体模式的色散特性和行波管的小信号增益.研究发现，低频等离子体模式可与相对论 电子束发生同步互作用使高频信号放大.同时，在无限薄环形等离子体填充条件下，波纹波 导中的低频等离子体模式严格满足Floquet定理所要求的周期性，其上截止频率不再受到等 离子体频率的限制，当密度较大时，等离子体模式还可与TM模式发生耦合. 关键词： 等离子体 波纹波导 色散关系 增益     

Effect of Static Magnetic Field on Photocatalytic Degradation of Methylene Blue over ZnO and TiO2 Powders

Magnetic field effects (MFEs) on photocatalytic degradation of methylene blue (MB) solution over ZnO and TiO₂ powders are investigated under static magnetic field up to 0.7 T with light irradiation by ultraviolet (UV) light emitting diode. The UV–visible-near-infrared spectrometer is used to monitor the MB concentrations. The positive MFE is observed for ZnO, while the negative MFE for TiO₂, and both MFEs are increased with the increase in the magnetic field applied. By increasing the settling time (the time interval between the preparation of MB solution and the powder dispersion into the MB solution), the photodegradation abilities under MFEs are decreased for both the catalysts. The cause of MFE is discussed in terms of dissolved oxygen in the MB solution and magnetic adsorption of the constituent molecules.     

Propagation of waves in a multicomponent plasma having charged dust particles

Propagation of both low and high frequency waves in a plasma consisting of electrons, ions, positrons and charged dust particles have been theoretically studied. The characteristics of dust acoustic wave propagating through the plasma has been analysed and the dispersion relation deduced is a generalization of that obtained by previous authors. It is found that nonlinear localization of high frequency electromagnetic field in such a plasma generates magnetic field. This magnetic field is seen to depend on the temperatures of electrons and positrons and also on their equilibrium density ratio. It is suggested that the present model would be applicable to find the magnetic field generation in space plasma.     

Potential Surface Waves in a Warm Nonisothermal Plasma Bounded by Metal

The dispersion properties of potential surface waves (SW), propagating at a warm nonisothermal plasma-metal boundary across the external magnetic field are studied in this paper. The existence of potential SW is shown to be possible in frequency ranges, where electromagnetic SW do not exist. The influence of transverse plasma density inhomogeneity on dispersion properties of the SW considered is studied as well.