Effect of transverse static magnetic field on stimulated brillouin scattering of electromagnetic wave

Stimulated Brillouin scattering of a plane polarised electromagnetic wave propagating perpendicular to a static magnetic field has been investigated analytically in ann-type piezoelectric semiconductor-plasma. Using coupled mode theory the dispersion relation is obtained and the threshold value of the amplitude of electromagnetic wave for the onset of instability is studied for both the forward and back-scattered modes. The role of the magnetostatic field on the threshold conditions for the unstable mode has been discussed.     

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.     

Beat excitation of terahertz radiation in a semiconductor slab in a magnetic field

Two infrared lasers of frequencies ω₁ and ω₂ propagating in the TM/TE mode along $\stackrel{?}{z}$ direction in a rippled density semiconductor waveguide are shown to resonantly excite terahertz radiation at the beat frequency when ripple wave number is suitably chosen to satisfy the phase matching. The wave vector of the density ripple is along the direction of laser propagation while a static magnetic field is applied transverse to it. The lasers exert a ponderomotive force on the electrons at the beat frequency. This force, in the presence of density ripple and transverse magnetic field, produces a nonlinear current at the terahertz frequency. The magnetic field enhances the amplitude of the terahertz wave. However terahertz yield is significantly higher in the TM mode laser beating than in the TE mode laser beating.     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

Effect of Temperature Anisotropy on Various Modes and Instabilities for a Magnetized Non-relativistic Bi-Maxwellian Plasma

Using kinetic theory for homogeneous collisionless magnetized plasmas, we present an extended review of the plasma waves and instabilities and discuss the anisotropic response of generalized relativistic dielectric tensor and Onsager symmetry properties for arbitrary distribution functions. In general, we observe that for such plasmas only those modes whose magnetic-field perturbations are perpendicular to the ambient magnetic field, i.e., B ₁ $\bot $ B ₀, are effected by the anisotropy. However, in oblique propagation all modes do show such anisotropic effects. Considering the non-relativistic bi-Maxwellian distribution and studying the relevant components of the general dielectric tensor under appropriate conditions, we derive the dispersion relations for various modes and instabilities. We show that only the electromagnetic R- and L- waves, those derived from them (i.e., the whistler mode, pure Alfvén mode, firehose instability, and whistler instability), and the O-mode are affected by thermal anisotropies, since they satisfy the required condition $\mathbf{B}_{1}\bot \mathbf{B}_{0}$ . By contrast, the perpendicularly propagating X-mode and the modes derived from it (the pure transverse X-mode and Bernstein mode) show no such effect. In general, we note that the thermal anisotropy modifies the parallel propagating modes via the parallel acoustic effect, while it modifies the perpendicular propagating modes via the Larmor-radius effect. In oblique propagation for kinetic Alfvén waves, the thermal anisotropy affects the kinetic regime more than it affects the inertial regime. The generalized fast mode exhibits two distinct acoustic effects, one in the direction parallel to the ambient magnetic field and the other in the direction perpendicular to it. In the fast-mode instability, the magneto-sonic wave causes suppression of the firehose instability. We discuss all these propagation characteristics and present graphic illustrations. The threshold conditions for different instabilities are also obtained.     

Study of dust ion-acoustic wave propagation in the Lorentzian magnetized dusty plasma

Dust ion-acoustic waves propagation in the magnetized dusty plasma including ions, electrons and dust particulates are studied by using kinetic equation. For unbounded and collisionless plasma and in the presence of uniform external magnetic field B₀, electrons and ions with Lorentzian distribution function and dust particles with Maxwellian one are considered. Calculating dielectric tensor through the Vlasov equation solution, in the parallel propagation, dispersion relation is derived and suprathermal particle effects on the Landau damping is studied. It is shown that the Landau damping effect vanishes for parallel propagation.     

Green’s function approach to the low temperature properties of Cs<Subscript>2</Subscript>CuCl<Subscript>4</Subscript>: anisotropy effects

We have studied the effect of both axial and transverse anisotropy on the critical field and thermodynamic properties of the field induced three dimensional antiferromagnetic Heisenberg model on the frustrated hexagonal lattice for Cs₂CuCl₄ compound. The spin model is mapped to a bosonic one with the hard core repulsion constraint and the Green’s function approach has been implemented to get the low energy spectrum and the corresponding thermodynamic properties. To find the critical field (B _c ) we have looked for the Bose-Einstein condensation of quasi-particles (magnons) which takes place when the magnon spectrum vanishes at the ordering spiral wave vector. We have also obtained the dispersion of magnon spectrum in the critical magnetic field for each anisotropy parameter to find the spiral wave vector where the spectrum gets its minimum. The magnon energies show a linear dispersion relation close to the quantum critical point. The effect of hard core boson interaction on the single particle excitation energies leads to a temperature dependence of the magnon spectrum versus magnetic field. We have also studied the behavior of specific heat and static structure factor versus temperature and magnetic field.     

Ultrasound and vortex oscillations in high-temperature superconductors

An HTSC powder sample with grain (particle) diameter of 20–50 μm placed in a dc magnetic field B ₀ and cooled to a temperature below the superconducting transition temperature was exposed to the radiofrequency (rf) pulsed magnetic field B _∼ (B _∼ ⊥ B ₀) at a carrier frequency of 30.7 MHz. Stable echo signals were recorded which followed different rf-pulse trains. This phenomenon has the following mechanism. The rf magnetic field stimulates fluxoid oscillations on the HTSC grain surface, which are transformed into lattice oscillations through the pinning centers and induce a propagating sound wave. The second-order nonlinearity with respect to the gradient of the crystal lattice deviation from the equilibrium position taken into account in the sound wave equation yields the dependence of the crystal lattice natural frequency on the amplitude and length of the pulses which excite these oscillations. This dependence is responsible for the emergence of echo signals.     

Collective modes in quasi-two-dimensional conductors under strong spatial dispersion

Propagation of electromagnetic and spin waves in layered conductors with a quasi-two-dimensional dispersion law of charge carriers is investigated theoretically in the presence of an external magnetic field with induction B₀. In layered conductors, the drift velocity v_D of electrons along B₀ is an oscillatory function of the angle between the magnetic field direction and the normal to the layers. For certain orientations of the magnetic field with respect to the layers of the conductor, v_D is close to zero. In these directions, there is no collision-free absorption, and weakly damped waves may propagate even under strong spatial dispersion. In the short-wave-length limit, there may exist collective modes with frequencies in the neighborhood of resonances for arbitrary orientation of the wavevector k relative to B₀. Similar types of excitations in quasi-isotropic metals are possible only when k is perpendicular to the direction of the external magnetic field.     

Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.     

Spin waves in quasiequilibrium spin systems

Using the Landau Fermi liquid theory we discovered a new propagating transverse spin wave in a paramagnetic system which is driven slightly out of equilibrium without applying an external magnetic field. We find a gapless mode which describes the uniform precession of the magnetization in the absence of a magnetic field. We also find a gapped mode associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to a T3/2 contribution to the specific heat. These modes significantly contribute to the dynamic structure function.     

A Mathematical Representation of Quantizing the Motion of Relativistic Electrons in a Magnetic Field

It is shown that the quadratic component of the kinetic energy of continuous longitudinal motion of relativistic electrons in the external magnetic field is varied continuously between 0 and 2(2m _e c ²_B H) within each Landau energy level, undergoing an abrupt change at the boundaries of the levels. This results in the fact that in the quantum limit of a superstrong magnetic field where all electrons are at the zero Landau level, the maximum quadratic component of the kinetic energy of free longitudinal electron motion along the direction of the magnetic field is twice as high as the maximum quadratic component of the kinetic energy of its bound transverse motion.     

Experimental investigation of ultrasonic velocity anisotropy in magnetic fluids: Influence of grain–grain interaction

Magnetic field-induced dispersion of ultrasonic velocity in a Mn_0.7Zn_0.3Fe₂O₄ fluid (applied magnetic field is perpendicular to the ultrasonic propagation vector) is determined by employing continuous wave method. The magnitude of dispersion initially decreases with increasing field, then increases and reaches a plateau at higher fields. Results indicate that the velocity anisotropy is dominated by grain–grain interactions rather than grain–field interaction. At the critical temperature, the grain–grain interaction becomes weak as the transverse component of the particle/cluster moment is larger than the longitudinal one and the system reaches saturation even at low field. These observed variations in the field-induced anisotropy are analysed by incorporating the moment distribution of particles in Tarapov’s theory (J. Magn. Magn. Mater. 39, 51 (1983)).     

Energy spectra and quantum crystallization in two-electron quantum dots in a magnetic field

Quantum crystallization of electrons in a quantum dot (QD) subjected to an external magnetic field is considered. Two-electron QDs with two-dimensional (2D) parabolic confining potential in an external transverse magnetic field are calculated. The Hamiltonian is numerically diagonalized in the basis of one-particle functions to find the energy spectra and wave functions for the relative motion of electrons with inclusion of electron-electron interaction for a broad range of the confining-potential steepness (α) and external magnetic fields (B). The region of the external parameters (α, B) within which a gradual transition to quantum crystalline order occurs is numerically determined. In contrast to a 2D unbounded system, a magnetic field acts nonmonotonically on “crystallization” in a quantum dot with several electrons because of a competition between two effects taking place with increasing B, namely, decreasing spread of the electron wave functions and increasing effective steepness of the confining potential, which reduces the average separation between electrons. Fiz. Tverd. Tela (St. Petersburg) 40, 1753–1759 (September 1998)     

Magnetogravitational Stability of Resistive Plasma through Porous Medium with Thermal Conduction and FLR Corrections

The problem of stability of self gravitating magnetized plasma in porous medium is studied incorporating electrical resistivity, thermal conduction and FLR corrections. Normal mode analysis is applied to derive the dispersion relation. Wave propagation is discussed for parallel and perpendicular directions to the magnetic field. Applying Routh Hurwitz Criterion the stability of the medium is discussed and it is found that Jeans' criterion determines the stability of the medium. Magnetic field, porosity and resistivity of the medium have no effect on Jeans' Criterion in longitudinal direction. For perpendicular direction, in case of resistive medium Jeans' expression remains unaffected by magnetic field but for perfectly conducting medium magnetic field modifies the Jeans' expression to show the stabilizing effect. Thermal conducitivity affects the sonic mode by making the process isothermal instead of adiabatic. Porosity of the medium is effective only in case of perpendicular direction to magnetic field for perfectly conducting plasma as it reduces the stabilizing effect of magnetic field. For longitudinal wave propagation, though FLR corrections have no effect on sonic mode but it changes the growth rate for Alfvén mode. For transverse wave propagation FLR corrections and porosity affect the Jeans' expression in case of nonviscous medium but viscosity of the medium removes the effect of FLR and porosity on Jeans' condition.     

Ordinary and extraordinary cyclotron waves in metals

Dispersion equations for the ordinary and extraordinary cyclotron waves propagating perpendicular to the magnetic field in metals in the critical region where the wavelength is comparable to the electron Larmor radius are derived as an infinite but rapidly converging power series expansion in δ( = Ω/Ω-M). Numerical studies for the cyclotron wave propagation near the first seven resonances are carried out. The non-local behaviour of those waves in the critical region 01 ⩽ kR ⩽ 3-0 is studied. For the ordinary waves the first few resonances show significant dispersion than those near higher resonances which are dispersion-free. Only one extraordinary wave propagates near the fundamental cyclotron frequency. For the higher resonances, two modes propagate near each of the resonant frequencies, of which one mode remains constant for all values ofkR whereas the second mode shows significant dispersion. But beyond the fifth resonance both the modes are dispersion free.     

Parametric instability in a two-hole species semiconductor-plasma

The parametric excitation of a low frequency wave has been investigated analytically in a two-hole species semiconductor-plasma in the region of kl ? 1 using the hydrodynamic model of the plasmas in the presence of a high frequency oscillatory electric field (E₀ cos ω₀t applied along the X-axis) and a d.c. magnetic field B₀ normal to the electric field (along the Z-axis), the low frequency wave propagating in the X–Z plane making a very small angle θ with the X-axis. The system supports a purely growing unstable mode. The variation of the growth rate of the unstable mode has been studied over a wide range of system parameters for the specific case of an intrinsic GaAs crystal at 300 K. The oscillatory electric field can be obtained by irradiating the crystal with a 119μm H₂O laser.     

Equations of motion for the magnetization in an optically oriented spin system

The spin system is present in conditions of optical orientation and magnetic resonance. The equations of motion are derived for coherent and incoherent excitation. In the latter case (light propagating along the magnetic field) the equations take the form of Bloch's equations and contain the relaxation times ₁ and ₂ as well as the equilibrium z component m_oz, these being dependent on the light intensity. Coherent excitation (circularly polarized light propagating perpendicular to the magnetic field, or two rays perpendicular and parallel to the magnetic field, or two rays perpendicular and parallel to the field) produces equations containing the equilibrium transverse magnetizations produced by optical orientation. In both cases the frequency of the magnetic resonance is shifted by an amount proportional to the light intensity.     

Wave function mapping of self-assembled quantum dots by capacitance spectroscopy

We use frequency-dependent capacitance–voltage spectroscopy to study the dynamic charging of self-assembled InAs quantum dots. With increasing frequency, the AC charging becomes suppressed, beginning with the low-energy states. By applying an in-plane magnetic field, we generate an additional magnetic confinement that alters the tunneling barrier and hence the charging dynamics. In traveling through the potential barrier, the electrons acquire an additional momentum k₀, proportional to the magnetic field B. As the tunneling is enhanced, when k₀ matches the maximum of the electronic wave function Ψ (in momentum representation), we are able to map out the shape of Ψ by varying B.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.