Hybridization of optical phonons with double quantum-wire plasmons including drift instabilities

We examine the collective mode spectrum for a double quantum-wire system embedded in semi-infinite polar medium, taking account of the role of optical phonons in interaction with the double quantum-wire plasmons and bulk/surface plasmons. The collective mode frequencies are exhibited as functions of q_x (wavenumber parallel to the wires) and as functions of z₀ (distance of first wire from the interface). We also find that the region of drift instability of the collective modes is split into two disjoint regions of instability by the optical phonons, with a region of stability between them.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

Origin of laser-output noise of cataphoretic HeSe+ lasers

We investigated noise and fluctuations of the output power of cataphoretic HeSe⁺ lasers in positive column plasma. Direct coupling of laser output power noise and fluctuations of the local population inversion was found. An investigation of the positive column plasma showed moving striations being responsible for the gain fluctuations. Whereas the local plasma properties are dominated by high frequency striations in the 100 kHz range, integrated quantities such as laser gain per pass are most strongly influenced by low-frequency waves with a continuous noise spectrum below 200 kHz. External modulation of the discharge voltage or current at a frequency near thehf striations reduces laser noise and increases laser output power.     

Critical behavior at dirty surfaces

The influence of quenched surface disorder — i.e. quenched disorder that is located at the bounding surface of a macroscopic system — on the surface critical behavior of such systems is investigated. To this end a class of semi-infinite continuum models of then-vector type with random surface interactions is studied. Both the case of surface transitions at a bulk critical point as well as that of surface transitions at a bulk tricritical point is considered. General irrelevance/relevance criteria of the Harris type are derived for both short-range and long-range correlated random surface interactions. These are used to assess the stability of the pure system critical behavior and to point out when random surface field or enhancement disorder is expected to be relevant.     

Surface defect solitons at interfaces between dual-frequency and simple lattices

We reveal theoretically the existence and stability of surface defect solitons (SDSs) at interfaces between dual-frequency and simple lattices with focusing saturable nonlinearity. Solitons with some unique properties exist in such composite structures with the change of defect intensity. For zero defect or positive defect, the surface solitons exist at the semi-infinite gap and cannot exist in the first gap, and solitons are stable at lower power but unstable at high power. For the case of negative defect, the surface solitons exist not only in the semi-infinite gap, but also in the first gap. With increasing the defect depth, the stable region of surface solitons becomes narrower in the semi-infinite gap, these solitons are stable within a moderate power region in the first gap within unstable solitons in the entire semi-infinite gap.     

中心对称光折变晶体中Kagome光子晶格内带隙孤子的研究

研究了中心对称光折变晶体中Kagome光子晶格内带隙孤子的存在及其稳定性。结果表明:带隙孤子只存在于半无限带隙内,中功率的带隙孤子是稳定的,高功率和低功率的带隙孤子是不稳定的。在高功率和中功率区域内,带隙孤子的功率随传播常数的增加而减小。在低功率区域内,带隙孤子的功率随传播常数的增加而变大。     

Dynamical behaviour of transverse plasmons in pair plasmas

This paper analytically investigates the nonlinear behaviour of transverse plasmons in pair plasmas on the basis of the nonlinear governing equations obtained from Vlasov--Maxwell equations. It shows that high frequency transverse plasmons are modulationally unstable with respect to the uniform state of the pair plasma. Such an instability would cause wave field collapse into a localized region. During the collapse process, ponderomotive expulsion is greatly enhanced for the increase of wave field strength, leading to the formation of localized density cavitons which are significant for the future experimental research in the interaction between high frequency electromagnetic waves and pair plasmas.     

Non-ohmic conduction and electrical switching under pressure of the charge transfer complexo-tolidine-iodine

The current voltage characteristics ofo-tolidine-iodine, with stoichiometry 1:1 grown from benzene, have been studied under high pressures upto 6 GPa atT=300 K andT=77 K. The characteristics show a pronounced deviation from ohmicity beyond a certain current for all pressures studied. At room temperature, beyond a threshold field the system switches from a low conductingOFF state to a high conductingON state with σ_ON/σ_OFF ∼ 10³. TheOFF state can be restored by the application of an a.c. pulse of low frequency. The temperature dependence of the two states studied indicates that theOFF state is semiconducting while theON state, beyond a certain applied pressure is metallic. The characteristics atT=77 K do not show any switching.     

侧向铁磁/铁磁超晶格的推迟模式

用等效介质理论计算了半无限侧向铁磁/铁磁超晶格的推迟模式.且以Co/Ni体系超晶格为例具体计算了该超晶格的表面模式和体模式,展示出一些与磁性/非磁性超晶格不同的有趣性质.侧向磁性/磁性超晶格具有较复杂的推迟模式,这是一种具有高度一般性的体系,在改变构成超晶格的两种铁磁层的厚度的比值、外场时,可以调节两支表面模式的频率以及体模式的频带,这种调节作用是与两种铁磁层的饱和磁化值有关的.当饱和磁化值相差较大时,调制效果是很明显的.当第二种铁磁介质饱和磁化值趋于零时,该体系演变成熟知的磁性/非磁性超晶格.当取麦克斯 关键词： 铁磁/铁磁超晶格 推迟模式 等效介质理论 自旋波谱     

Dust‐acoustic envelope solitons in super‐thermal plasmas

The modulational instability (MI) of the dust‐acoustic waves (DAWs) in an electron‐positron‐ion‐dust plasma (containing super‐thermal electrons, positrons, and ions along with negatively charged adiabatic dust grains) is investigated by the analysis of the non‐linear Schrödinger equation (NLSE). To derive the NLSE, the reductive perturbation method was employed. Two different parametric regions for stable and unstable DAWs are observed. The presence of super‐thermal electrons, positrons, and ions significantly modifies both the stable and unstable regions. The critical wave number k_c (at which MI sets in) depends on the super‐thermal electron, positron, and ion, and adiabatic dust concentrations.     

Dynamics of ion-acoustic rogue waves in electron-positron-ion magneto-plasmas

A more general and realistic four-component magnetized plasma medium consisting of opposite polarity ions and nonthermal distributed positrons and electrons is considered to investigate the stable/unstable frequency regimes of modulated ion-acoustic waves (IAWs) in the D-F regions of Earth's ionosphere. A (3 + 1) -dimensional nonlinear Schrödinger equation, which leads to the modulation instability (MI) of IAWs, is derived. The parametric regimes for the existence of the MI, first- and second-order rogue waves, and also their basic features (viz., amplitude, width, and speed) are found to be significantly modified by the effect of physical plasma parameters and external magnetic field. It is found that the nonlinearity of the different types of electronegative plasma system depends on the positive to negative ion mass ratio. It is also shown that the presence of nonthermal distributed electrons and positrons modifies the nature of the MI of the modulated IAWs. The implication of our results for the laboratory plasma [e.g., (Ar⁺, F⁻ ) electronegative plasma] and space plasma [e.g., (H⁺, H⁻ ), () electronegative plasma in D-F regions of Earth's ionosphere] are briefly discussed.     

Theory of two-phonon modes in layered charge-density-wave systems

A theoretical model with electron-phonon and anharmonic interactions is proposed to explain the two-phonon mode observed in the Raman spectra of layered transition metal dichalcogenides, which exhibit charge density wave (cdw) phase transition. The phonon self-energy, which involves the electron response function and the two-phonon Green’s function, is calculated using the double-time Green’s function formalism. It is shown that in these low-dimensional systems there exists an anharmonicity-mediated two-phonon mode in the phonon spectral function both in the normal and in thecdw phases. In the normal phase since the phonon Raman scattering proceeds through a single optic phonon the calculations are carried out for zero wave vector and hence the contribution of the electron response function to the self-energy vanishes. On the other hand explicit evaluation of the two-phonon Green’s function shows that the frequency of the two-phonon mode is twice that of the Kohn anomaly phonon and decreases with decreasing temperature. The strength of two-phonon peak is found to be comparable to that of the original optic phonon. In thecdw phase the phonon which enters into the Raman scattering is taken to be the one with thecdw wave vectorQ, which when zone-folded becomes equivalent to zero wave vector. The evaluation of the electron response function in this phase generates a phonon corresponding to thecdw-amplitude mode. The two-phonon Green’s function is assumed to be of similar form as in the normal phase. The spectral function evaluated at zero temperature shows a weak two-phonon peak besides the prominentcdw-amplitude mode. Numerical results are presented for the system 2H-NbSe₂ and are found to be in qualitative agreement with the experimental data.     

The convergence of the slaving principle in a simplified model

We prove the convergence of the slaving principle in a model consisting of two nonlinear equations with two variables,s andu, which represent in the linear regime the stable mode and unstable mode, respectively.We show explicitly how the stable modes becomes increasingly dependent on the unstable modeu and approaches a definite power series ofu regardless of the initial condition fors. This power series is called slaving function and is shown to be absolutely and uniformly convergent on a closed disc, which contains the point describing the asymptotic behavior of the system. For some finite time, we show that the approximation involved in the substitution of the slaving function for the original stable modes decreases exponentially with time.     

Surface impedance and boundary scattering in metals for the conditions of the anomalous skin depth

The non-local conductivity for a metal satisfying the conditions of the anomalous skin depth is characterized by a logarithmic divergence. It is postulated that this divergence can be directly related to the surface impedance of a semi-infinite metal in the extreme anomalous limit. Methods are presented for calculating theGreen's function for the conduction electron distribution function and the non-local conductivity. These methods are applied to a model where the electrons cannot drift along the magnetic field. Approximate expressions are obtained for the dependence of the divergence upon the value of the static magnetic field. Experiments are in progress to attempt to relate a model of surface scattering to the behavior of the surface impedance for small magnetic fields. Work supported principally by the Joint Services Electronics Program [Contract DA-28-043-AMC-02536(E)].     

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF₂/ZnF₂)/FeF₂.     

Effect of magnetic field on nonlinear interactions of electromagnetic and surface waves in a plasma layer

Investigation is made for nonlinear interaction between incident radiation and a surface wave in a magnetized plasma layer. Both interacting waves are ofP polarization. We get the generated currents and fields at combination frequencies analytically. Unlike theS-polarized interacting waves, the magnetic field affects the fundamental waves and leads to an amplification of generated waves when their frequencies approach the cyclotron frequency.     

Numerical flow visualization of stall suppression of a symmetrical aerofoil by leading-edge moving durface

This paper applies the numerical simulation techniques based on the generalized conservation of circulation (GCC) method to investigate the effects of momentum injection by a leading-edge moving surface on flow past a two-dimensional aerofoil at a Reynolds number of 1000. The stream function and vorticity contours obtained together with the animated flow visualization show that the stall flow region is highly unsteady and consist mainly of large vortices being shed alternately. They are confined to a narrow region near the upper surface of aerofoil asC _u (the ratio of the speed of the moving surface to the free stream velocity) is raised. The proximity of vortices to the upper surface of aerofoil at highC _u is caused by the ability of free stream to negotiate around the leading edge since the leading-edge moving surface suppresses the growth of boundary layer by reducing the relative between the inviscid flow and the wall. As well-formed large scale vortices are associated with low pressure regions, their proximity to the aerofoil leads to increase in lift as speed ratio increases     

Infrared studies on ion irradiated quartz

Damage produced inα- and fused quartz bombarded with low energy (∼ 100 keV) D⁺, He⁺ and Ar⁺ ions, has been studied by observing the changes in their spectrum. Besides bulk reflectivity, the attenuated total reflection spectrum has also been studied, the latter with a view to obtaining the surface polariton frequencies. It is observed that for the same fluence, the changes following D⁺ irradiation are much higher compared to that for Ar⁺ irradiation. The variation of the surface polariton frequency inα-quartz with the damage energy deposited has the same trend as observed earlier for refractive index. Some annealing studies have also been performed in argon-irradiated samples. These studies indicate that whereas in fused quartz the damaged layer recovers completely, inα-quartz there is a residual amorphization even after annealing. A two-layer model is proposed which gives a reasonable simulation of the observedir properties.     

Jeans-Buneman instability in a dusty plasma

A self consistent formulation of the Jeans instability of a dusty plasma with proper inclusion of charge dynamics is described. It is shown that charge fluctuations significantly affect the Jeans as well as the Buneman mode. For plasma particles (electrons and ions) in local thermal equilibrium, the Jeans lengthλ _J is given byλ _J≈λ _g F(R, ε, β/η), whereλ _g is the Debye length of the charged grains,R is the square of the ratio of the Jeans to the plasma frequency of the grains,ε is the square of the ratio of the Debye length of the grains and the plasma particles andβ/η is the ratio of the attachment to the decay frequency of the electronic charges to the grain surface. The functional form ofF is given in the text. Numerical investigation of the Jeans-Buneman mode for a two and three component plasma shows that the Jeans mode dominates atkλ _D≪1 (wherek is the wave number andλ _D is the Debye length of plasma particles), whereas atkλ _D≫1 only the Buneman mode operates. Charge fluctuations reduce the area of overlap of the two modes. Furthermore, in the absence of gravity, there exists a new, charge fluctuation induced unstable mode in a streaming dusty plasma. Astrophysical applications of the results are discussed.     

Propagation of a moving point source in a stratified medium with fluid flow

Summary The problem of an acoustic field generated by a point source moving with arbitrary velocity in a stratified medium in still air has been addressed by Limet al. (Lim P. H. andOzard J. M.,J. Acoust. Soc. Am.,95 (1994) 131). The aim of this paper is to investigate the same problem in the presence of a moving fluid.