Wave Vector Kz Research Articles

TRIVELPIECE-GOULD MODES OF WAVEGUIDE PARTIALLY FILLED WITH NON-NEUTRAL PLASMA. PART 3

The possibility of explaining the nature of low-frequency oscillations observed in devices with non-neutral plasma by the instability of the relative azimuth motion of non-neutral plasma components and by anisotropy of the ion distribution function is discussed. The resonance condition for ions with a diocotron mode with a finite value of the longitudinal wave vector kz is studied. Numerical estimations are made for the plasma parameters and unstable oscillations, that are characteristic for experiments. Frequencies, growth rates, and other characteristics and features of the expected electron-ion instability are estimated. The conclusion is made that the nature of low-frequency oscillations observed in devices with non-neutral plasma can be explained by this instability.

Chirality-Induced Spin-Orbit Coupling, Spin Transport, and Natural Optical Activity in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) with chiral organic ligands exhibit highly spin-dependent transport and strong natural optical activity (NOA). Here we show that these remarkable features can be traced to a chirality-induced spin-orbit coupling (SOC), Hso = ατkzσz, which connects the carrier's spin (σz), its wave vector (kz), and the material's helicity (τ) along the screw direction with strength α controlled by the geometry of the organic ligands. This SOC leads to a macroscopic spin polarization in the presence of an electrical current and is responsible for the observed spin-selective transport. NOA originates from a coupling between the exciton's center-of-mass wave vector Kz and its circular polarization jzex, Hso' = α'τKzjzex, contributed jointly from the electron's and the hole's SOCs in an exciton. Our model provides a roadmap to achieve a strong and tunable chirality in HOIPs for novel applications utilizing carrier spin and photon polarization.

BO: A unified tool for plasma waves and instabilities analysis

A unified numerically solvable framework for dispersion relations with an arbitrary number of species drifting at arbitrary directions and with Krook collision is derived in linear uniform/homogenous kinetic plasma model, which greatly extends the standard one given by Stix (1992). The purpose of this work is to provide a kinetic plasma dispersion relation tool not only in the physical model but also the numerical approach as general/powerful as possible. As a general application, we give the dispersion relations which assume the equilibrium distribution function to be bi-Maxwellian including parallel drift, two directions of perpendicular drift (i.e., drift across magnetic field), ring beam and loss-cone. Both electromagnetic and electrostatic versions are provided, with also the Darwin (a.k.a., magnetoinductive or magnetostatic) version. The species can be treated either magnetized or unmagnetized. Later, the equations are transformed to the matrix form that can be solved by using the powerful matrix algorithm (Xie and Xiao, 2016), which is the first approach can give all the important solutions of a linear kinetic plasma system without requiring initial guess for root finding. To the best of our knowledge, the present model is the most comprehensive model in literature for the distribution function constructed from Maxwellian distributions, which thus can be applied widely for studying waves and instabilities in space, astrophysics, fusion and laser plasmas. We limit the present work to non-relativistic case. Program summaryProgram Title: BOProgram Files doi:http://dx.doi.org/10.17632/cvbrftzfy5.1Licensing provisions: BSD 3-clauseProgramming language: MatlabNature of problem: The linear fluid and kinetic waves and instabilities in plasma can be described by dispersion relations. The challenges are to provide a dispersion relation as general as possible and to obtain all the solutions of it, which is the goal of BO tool. The kinetic version of BO tool provides a unified numerically solvable framework for kinetic dispersion relations, which greatly extends the standard one (Stix, 1992), with an arbitrary number of species. It contains many new features, including anisotropic temperature/loss cone/drift in arbitrary direction/ring beam/collision, unmagnetized/magnetized, electrostatic/electromagnetic/Darwin, can support parallel wave vector kz≤0, etc.Solution method: Approximating the plasma dispersion function in the kinetic dispersion relation with J-pole Padé expansion. Transforming the dispersion relation to an equivalent matrix eigenvalue problem and find all the solutions using standard matrix eigenvalue library function.Additional comments including restrictions and unusual features: Kinetic relativistic effects are not included in the present version yet.

Analysis of mode radiation characteristics in a non-magnetized cold plasma column

The electromagnetic surface waves which propagate along a non-magnetized cold plasma column have a great value in the application of plasma antenna. In this paper, the dispersion properties, the transmission power distributions, and the radiation patterns for these electromagnetic surface waves which have lower frequencies than the electron plasma frequency are analyzed numerically. Based on Helmholtz equation, the specific expression of dispersion equation is derivedby the field matching method, then the exact values of complex axial wave vector kz under different wave frequencies are obtained by solving the transcendental dispersion relation. Using the specific value of kz obtained above, the exact expressions of transmission power profile in the plasma column and field profiles in the three regions, i.e., plasma, dielectric, and free space are derived, respectively. Finally, based on the complex form of electric conductivity that is derived from the Boltzmann-Vlasov equation with Krook term and the complex axial wave vector kz obtained above, the influence of the parameter pea/c on phase property, and the dependence of radiation pattern and transmission power profile on wave frequency of the non-magnetized cold plasma column in a cylindrical dielectric tube system are analyzed. The results show that the electron plasma frequency has a significant influence on the phase property, which is evidently confirmed by the fact that the propagation velocities of the three modes m=0, m=1 and m=2 are all near to the light speed when the value of parameter pea/c gradually increases. Meanwhile, through the investigation of the radiation patterns for the three modes, an important conclusion is that the radiation pattern has evident dependence on wave frequency. While the radiation direction of the main lobe is in the axial direction for the m=1 mode, the m1 modes each have an angle between the radiation direction of the main lobe and the axial direction, this crucial conclusion is in good agreement with the theoretical calculation results obtained from other researcher. Further, we find that with the increase of wave frequency, the angle between the main lobe radiation direction and the axial direction turns smaller for each of m=0 and m=2 modes, and the width of main lobe gradually narrows for each of all modes, and the amplitude of the first side lobe becomes notable for each of m=0 and m=2 modes and ignorable for the m=1 mode. Also, the transmission power increases as the wave frequency increases for each of all modes. These theoretical calculation results provide a detailed theoretical reference for the designing of plasma stealth and high-precision requirements of plasma antenna design, and giving a comprehensive optimization guidance for the modulation of plasma antenna.

Crystalline and spin chiralities in multiferroics with langasite-type structure and Fe1–x Co x Si crystals

It is shown that, when magnetic ordering occurs in layered iron-containing langasites (sp. gr. P321), one of the reasons for spin chiralities of different signs is the presence of structural chirality (the existence of inversion twins), which, in turn, is due to the nonsymmetricity of these crystals. Spin helicoids arise in these multiferroics at split sites of Fe3+ ions below the Neel point. The direction of electric polarization vectors coincides with the direction of the magnetic helicoid axes because of the piezoelectric properties of these materials. Due to the magnetostriction effects, structural chirality wave vector kz exceeds the magnetic helicoid wave vector by a factor of 2: kz = 2qz. The temperatures of transitions to the chiral structural and chiral magnetic states may differ. In particular, if the structural transition initial temperature exceeds the magnetic transition temperature (ТU> ТМ), structural displacements may arise in the absence of magnetism at ТМ < Т < ТU. In noncentrosymmetric Fe1–xCoxSi crystals (sp. gr. P213), which are not multiferroics, magnetic chirality is due to the Dzyaloshinski–Moriya interaction. The dependence of the moduli of incommensurate wave number of the corresponding helicoid on the atomic composition of the crystals under consideration is nonmonotonic.

Shear Thickness Modes in a Presence of Magnetoelastic Waves Parallel to a Surface Guided by Ferromagnetic Film

Thickness modes of magnetoelastic waves (MEW) guided by ferromagnetic film have been investigated. Propagation of two mutually perpendicular MEW one of which is parallel to a normal to the surface has been considered. It has been shown that resonant frequencies of standing MEW depend on the wave number of a wave propagating along a surface. Plots illustrating dependence of thickness shear modes upon wave vector kz of MEW traveling in a surface plane have been presented.

Excitons in motion: universal dependence of the magnetic moment on kinetic energy

AbstractWe have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence on the translational wavevector Kz. The behaviour of the Zee‐man splittings corresponds to enhancement of the magnetic moments of the excitons. This enhancement is particularly marked when their translational kinetic energy becomes comparable with the exciton Rydberg and can be described by what appears to be a universal function of Kz. A model for the behaviour is outlined which involves motionally‐induced mixing between the 1S hydrogenic exciton ground state and excited nP states. The observations imply that there are significant changes in the structure of the exciton as its translational kinetic energy increases. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Resolving the wave vector and the refractive index from the coefficient of reflectance

We resolve the existing controversy concerning the selection of the sign of the normal-to-the-interface component of the wave vector k(z) of an electromagnetic wave in an active (gain) medium. Our method exploits the fact that no ambiguity exists in the case of a film of the active medium, since its coefficient of reflectance is invariant under the inversion of the sign of k(z). Then we show that the limit of the infinite film thickness determines a unique and physically consistent choice of the wave vector and the refractive index. Important practical implications of the theory are identified and discussed.

Phonon-pumped terahertz gain in n-type GaAs/AlGaAs superlattices

Local population inversion and far-IR gain are proposed and theoretically analyzed for an unbiased n-doped GaAs/Al0.15Ga0.85As superlattice pumped solely by phonons. The lasing transition occurs at the Brillouin zone boundary of the superlattice wave vector kz between the two conduction minibands CB1 and CB2 of the opposite curvature in kz space. The proposed waveguided structure is contacted above and below by heat sinks at 300 K and 77 K, respectively. Atop the superlattice, a heat buffer layer confines longitudinal optical phonons for enhanced optical-phonon pumping of CB1 electrons. A gain of 345 cm−1 at 4.5 THz is predicted for a doping density of 2.8×1016 cm−3.

Plasmon coupling and finite size effects in metallic multilayers

This work uses electron energy loss spectroscopy (EELS) to study the excitation of interface and bulk plasmons in artificial, nonmagnetic metallic multilayers. We obtain EELS spectra for W/Si and Ta/Si multilayers with layer periodicity d using an off‐axis acquisition technique and compare the results with dispersion relations and energy loss probability distributions predicted from dielectric theory. Our energy loss spectra, taken for a fixed out‐of‐plane wavevector kz = π/d and low in‐plane wavevectors ky &lt; 1/d, contain features which are identified as coupled interface modes by solving the appropriate superlattice dispersion relation. These interface modes are accompanied by bulk plasmons of the individual media. We find that dielectric theory greatly overemphasizes these bulk modes. This behaviour is interpreted phenomenologically as a finite size effect and is modelled by introducing a cut‐off for the long‐wavelength bulk modes.

Statics and dynamics of flux line lattices of high-Tc superconductors

The authors investigate the energy, elastic modulus and normal modes of oscillation of flux line lattices. To deal with the long range nature of the flux line potential, the Ewald sum technique was used. The dependence of the normal modes as a function of the transverse wavevector in the 2D hexagonal Brillouin zone and the z wavevector kz is discussed in detail. They found two normal modes corresponding to the shear and the compression of the lattice. The frequencies of these two are usually very different from each other. Because of the long range nature of the potential, the compressive mode behaves like a plasma oscillation. At zero momentum the excitations are gapless, because the potentially is exponential in character at large distances. At a finite z wavevector they found that the transverse frequencies drop off rapidly as the transverse wavevector is increased. The elastic moduli are compared with those computed using a continuum approximation by Sudbo and co-workers. Quantitative differences are found in the low field, small k region as well as the high field, large k region.

Axial and circular birefringence due to static electric and magnetic field combinations

Axial and circular birefringence are derived for a combination of static electric field strength (ES) and static magnetic flux density (BS). For ES parallel to BS in the Z axis of the laboratory frame (X, Y, Z) the refractive index measured in a chiral ensemble with unpolarized light with wave vector KZ, propagating in Z, is different for KZ parallel and antiparallel to ESZ for fixed BSZ or vice versa. The effect is c times greater in magnitude than axial birefringence due to BSZ alone and is proportional to the product BS⋅ES and to the ensemble average 〈A′1XXZZ〉, where A1 is an odd parity tensor mediating the effect of BS⋅ES on the molecular polarizability. Circular birefringence due to ES⋅BS also occurs in chiral ensembles only, but is c times smaller than axial birefringence. It is proportional to the ensemble average 〈A′2XXZZ〉, where A2 is an even parity tensor mediating the effect of ES⋅BS on the molecular Rosenfeld tensor. There are no equivalent effects due to ES alone, ES⋅ES or BS⋅BS, because they violate reversality. The ES⋅BS effects conserve reversality both in achiral and chiral ensembles, and provide information on new molecular property tensors. Their frequency dependencies provide novel molecular spectroscopies, and experimental configurations are suggested.