Energy loss rate of two-dimensional electron gas in GaInAs/AlInAs, InSb/AlInSb and GaSb/AlGaAsSb heterostructures

Energy loss rates of two-dimensional electron gas in GaInAs/AlInAs, InSb/AlInSb and GaSb/AlGaAsSb heterostructures are theoretically investigated over a wide range of temperature based on the electron–one-phonon and electron–two-phonon interactions. Calculations are presented for electron acoustic one-phonon interaction via deformation potential and piezoelectric coupling and electron–LO phonon interaction with hot phonon effect. In addition, energy loss rate due to electron-two-zone edge transverse acoustic (TA) phonons is also presented. A very good agreement is obtained between the calculations and experimental data in GaInAs/AlInAs structure with the inclusion of electron–two-zone edge TA phonon interaction. In all these three structures energy loss is dominated by (i) acoustic one-phonon scattering at low temperatures, (ii) two-TA zone edge phonons at intermediate temperatures and (iii) LO phonons at high temperatures. It is observed that, hot phonon effect reduces the energy loss rate considerably in these structures.     

Low-temperature heat transfer in nanowires

A new regime of low-temperature heat transfer in suspended nanowires is predicted. It takes place when (i) only "acoustic" phonon modes of the wire are thermally populated and (ii) phonons are subject to the effective elastic scattering. Qualitatively, the main peculiarities of heat transfer originate due to the appearance of the flexural modes with high density of states in the wire phonon spectrum. They give rise to the T(1/2) temperature dependence of the wire thermal conductance. Experimental situations where the new regime is likely to be detected are discussed.     

Interaction of an electromagnetic wave with oscillations in solids

A series of effects arising in a solid as a result of the nonlinear decay of a pumping wave is considered. The growth rate of the phonon subsystem instability caused by the decay process as photon → photon + phonon and the threshold value of the power flow of the pumping at which the stimulated scattering on the electron-hole sound plasmons starts are determined. A variational of Mandelstamm-Brillouin scattering characteristic for the presence of an interaction of the acoustic wave with the Langmuir plasmons is discussed.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Composition of propagated,reflected and transmitted waves in arbitrary and continuously stratified environments

First, a solution is presented for a canonical problem in wave propagation. Second, illustrations and applications of the results are carried out to study cases which are relevant to the propagation problem in the ocean and atmosphere.The canonical problem consists of a plane wave incident on an arbitrary and continuously stratified region with planar boundaries. The explicit composition of the reflected, transmitted and propagated waves are derived. The solution is systematic and allows for (i) discontinuities in the acoustic properties at boundaries and arbitrary variation within, (ii) attenuation, (iii) all angles of incidence. The general expressions are obtained by using an alternate procedure to one recently devised [1]. The present approach is straightforward and plainly amenable to physical interpretation of its auxiliary mathematical constants. The discontinuities at the boundaries are satisfied at the outset. The reflected and transmitted waves are directly and explicitly specified. Comparison to widely used techniques in both analytical and numerical works is made to demonstrate the viability of the present approach.A series of cases relevant to the problem at hand are considered. These cases illustrate the mechanics involved in use of the method, and expand its application to problems that appear to be at variance with the formulation of the canonical problem. The illustrations include attenuation in the medium, effect on the solution of different acoustic discontinuities at the boundaries, and use of an inhomogeneous background profile with known independent solutions. The expanded applications treat formally three types of problems: (i) the exact solution for plane waves in continuously stratified media where the well-used ray theory or W-K-B approximation serves only as a first approximation in a correct iterative solution; (ii) the scattering of a plane wave by non-planar boundaries, i.e., spherical or cylindrical acoustic lens with the stratification along the radial direction; (iii) the field due to a point source in a continuously stratified wave guide, like the ocean or atmosphere.     

Ultrafast Fano resonance between optical phonons and electron-hole pairs at the onset of quasiparticle generation in a semiconductor

Based on the many-body time-dependent approach applied to the ultrafast time region, we investigate the dynamics of creation of an optical phonon incorporating with the electron-hole continuum in a semiconductor. In the transient Fano resonance, due to an interference between those sharp (optical phonon) and continuum (electron-hole pair) quasiparticles, we find the robust destructive interference at birth of them, i.e., tau approximately 0 if the created phonon is coherent under the irradiation of ultrashort optical pulses. The origin is found to be the potential scattering of the electron-hole pair by the q=0 coherent phonon. This finding agrees well with the recent experiment.     

Tomographic Description of Stimulated Brillouin Scattering of Light

The process of stimulated Brillouin scattering is described by the twodimensional oscillator model. Photons of the Stokes mode are described by one mode of the oscillator, and acoustic phonons are described by the other mode. The interaction of photons and acoustic phonons is assumed to be quadratic in the creation and annihilation operators of photons and phonons. The laser is considered as a classical light source and its depletion is neglected. New timedependent integrals of motion and the photon–phonon probability distribution function are found. The mean Stokes photon number and the mean acoustic phonon number are expressed as functions of the medium parameters (initial dispersions) and the interaction parameter (coupling constant). The classical propagator for stimulated Brillouin scattering and tomograms of the photon and phonon states are investigated within the framework of the symplectictomography scheme.     

Determination of the LO phonon energy by using electronic and optical methods in AlGaN/GaN

The longitudinal optical (LO) phonon energy in AlGaN/GaN heterostructures is determined from temperature-dependent Hall effect measurements and also from Infrared (IR) spectroscopy and Raman spectroscopy. The Hall effect measurements on AlGaN/GaN heterostructures grown by MOCVD have been carried out as a function of temperature in the range 1.8-275 K at a fixed magnetic field. The IR and Raman spectroscopy measurements have been carried out at room temperature. The experimental data for the temperature dependence of the Hall mobility were compared with the calculated electron mobility. In the calculations of electron mobility, polar optical phonon scattering, ionized impurity scattering, background impurity scattering, interface roughness, piezoelectric scattering, acoustic phonon scattering and dislocation scattering were taken into account at all temperatures. The result is that at low temperatures interface roughness scattering is the dominant scattering mechanism and at high temperatures polar optical phonon scattering is dominant.     

Sound wave propagation in weakly polydisperse granular materials

Dynamic simulations of wave propagation are performed in dense granular media with a narrow polydisperse size-distribution and a linear contact-force law. A small perturbation is created on one side of a static packing and its propagation, for both P- and S-waves, is examined. A size variation comparable to the typical contact deformation already changes sound propagation considerably. The transmission spectrum becomes discontinuous, i.e., a lower frequency band is transmitted well, while higher frequencies are not, possibly due to attenuation and scattering. This behaviour is qualitatively reproduced for (i) Hertz non-linear contacts, for (ii) frictional contacts, (iii) for a range of smaller amplitudes, or (iv) for larger systems. This proves that the observed wave propagation and dispersion behaviour is intrinsic and not just an artifact of (i) a linear model, (ii) a frictionless packing, (iii) a large amplitude non-linear wave, or (iv) a finite size effect.     

C掺杂FePt铁磁薄膜光诱导超快退磁动力学研究

FePt合金薄膜由于具有较强的磁各向异性而在磁信息和磁光信息存储中具有重要的应用.C掺杂可精确调控薄膜的磁各向异性,从而可有效地改变薄膜的矫顽场.通过超短激光脉冲与铁磁薄膜相互作用,可以获得非平衡状态下电子、自旋和晶格等自由度之间的动态耦合参数,这是研究超快磁记录材料的物理基础.本文基于瞬态磁光Kerr效应,研究了两种C掺杂浓度下FePt薄膜的超快磁光响应.实验结果表明:瞬态Kerr信号与外加磁场正相关,磁场反向,Kerr信号反号,而瞬态反射率与外加磁场无关;不同C掺杂的FePt薄膜的矫顽场不同,软磁的退磁时间显著小于硬磁薄膜的退磁时间.我们还观测到超快激光在铁磁薄膜中诱导频率约为49 GHz的相干声学声子,该声子的频率与外加磁场无关.实验结果为设计和研制新型磁光薄膜提供了实验依据.     

Coupling of orthogonally polarized waves in BaTiO3

The first observation of anisotropic diffraction (with a 90|Mo change of the polarization of the diffracted wave) in BaTiO₃ from a grating recorded by two orthogonally polarized waves, an ordinary and an extraordinary wave, is reported. Three possible origins of this effect are considered: (i) direct one-step diffraction from a grating recorded anisotropically by spatially oscillating photovoltaic currents, (ii) indirect, sequential diffraction from two gratings, each recorded by one incident wave and an additional wave due to light-induced scattering, (iii) direct one-step diffraction from a grating with the grating vector equal to the difference of the grating vectors of two conjugated noisy gratings. The experimental evidence indicates that the process (iii) contributes most, and process (i) more than 10% to the overall diffraction efficiency, whereas process (ii) seems to be of minor importance.     

Propagation of sound at continuous structural phase transitions

Structural phase transitions of second order can be divided into two groups: (i) distortive phase transitions, with a soft (ultimately overdamped) optic mode, and (ii) elastic phase transitions, with an acoustic soft mode or no soft phonon for shear or isostructural transitions, respectively. The propagation of sound shows significantly different features in these two cases. We consider the theory of the critical variation of the velocity of ultrasonic modes as well as the damping and dispersion near transitions of second order.Talk given at the Conference on Transport and Propagation in Nonlinear Systems, Los Alamos, May 21–25, 1984.     

Anomalous Hall effect in the phonon thermal conductivity of paramagnetic dielectrics

A theory of the phonon Hall effect during heat transfer in a paramagnetic dielectric discovered by Strohm et al. [Phys. Rev. 95, 155901 (2005)] is developed. The heat flux emerging in the direction perpendicular to the magnetic field and to the temperature gradient is associated with the interaction of magnetic ions with the oscillating crystal field. In crystals with an arbitrary phonon spectrum, this interaction induces elliptic polarization of phonons. On the other hand, for any type of scattering, the temperature gradient forms part of the phonon density matrix, which is nondiagonal in modes. The combined action of these factors leads to the anomalous Hall effect.     

Topological charges in monopole theories

Let us consider a monopole theory with a compact, simply connected gauge group and the Higgs field in the adjoint representation. Using root theory we show that.(i) The homotopy class of the Higgs field is ap-tuple of integers wherep is the dimension of the centre of the residual symmetry group. These Higgs charges can be expressed as surface integrals of differential forms.(ii) To any invariant polynomial on the Lie algebra is associated a topological invariant which turns out to be a combination of the Higgs charges.(iii) Electric charge is quantized. The monopole's magnetic charge is a combination — with the Higgs charges as coefficients — ofp basic magnetic charges which satisfy generalized Dirac conditions.The example ofG=SU(N) is worked out in detail.     

The Three-Component Defocusing Nonlinear Schrödinger Equation with Nonzero Boundary Conditions

We present a rigorous theory of the inverse scattering transform (IST) for the three-component defocusing nonlinear Schrödinger (NLS) equation with initial conditions approaching constant values with the same amplitude as \({x\to\pm\infty}\). The theory combines and extends to a problem with non-zero boundary conditions three fundamental ideas: (i) the tensor approach used by Beals, Deift and Tomei for the n-th order scattering problem, (ii) the triangular decompositions of the scattering matrix used by Novikov, Manakov, Pitaevski and Zakharov for the N-wave interaction equations, and (iii) a generalization of the cross product via the Hodge star duality, which, to the best of our knowledge, is used in the context of the IST for the first time in this work. The combination of the first two ideas allows us to rigorously obtain a fundamental set of analytic eigenfunctions. The third idea allows us to establish the symmetries of the eigenfunctions and scattering data. The results are used to characterize the discrete spectrum and to obtain exact soliton solutions, which describe generalizations of the so-called dark-bright solitons of the two-component NLS equation.     

Suppression of electron-phonon interaction in narrow-band crystals

Taking the inelastic nature of the interaction of electrons with acoustic phonons into account is crucial in crystals having a conduction band width Δɛ comparable to the maximum acoustic phonon energy . In view of this, the laws of conservation of energy and wave vector impose stringent constraints on possible electron scattering processes, and single-phonon scattering becomes impossible for . The phonon contribution to the resistance may be negligible, therefore, in narrow-band conductors. Novosibirsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, p. 78–82, August, 1997.     

Resonant magnetic excitations at high energy in superconducting YBa2Cu3O6.85

A detailed inelastic neutron scattering study of the high temperature superconductor YBa2Cu3O6.85 provides evidence of new resonant magnetic features, in addition to the well-known resonant mode at 41 meV: (i) a commensurate magnetic resonance peak at 53 meV with an even symmetry under exchange of two adjacent CuO2 layers, and (ii) high-energy incommensurate resonant spin excitations whose spectral weight is around 54 meV. The locus and the spectral weight of these modes provides unrevealed insight about the momentum shape of the electron-hole spin-flip continuum of d-wave superconductors.     

Contribution of the electron-phonon interaction to the broadening of cyclotron resonance line of a two-dimensional electron gas in multiple quantum well structures

Summary The theory of cyclotron resonance (CR) lineshape of a two-dimensional electron gas (2 DEG) due to the electron-phonon interaction in multiple-quantum-well structures (MQWS) is investigated. The contribution of the deformation potential acoustic and piezoelectric phonon scattering to the broadening of the cyclotron resonance spectra (CRSB) of such a system is calculated fro GaAs/AlAs. The piezoelectric phonon scattering contribution to the linewidth is smaller as compared to the deformation potential acoustic phonon scattering but is significantly comparable. The magnetic-field dependence of CRSB due to the deformation potential acoustic and piezoelectric phonons isB ^1/2 andB ^1/4, respectively, and the frequency shift Δ _N,p =0 for both interactions in the elastic-scattering approximation. Observed numerical values of the CRSB indicate that at low temperatures acoustic and piezoelectric phonons are dominant scatterers and interact strongly with 2 DEG in MQWS where the impurity scattering is suppressed due to the modulation doping. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Influence of the quantum dot shape on the determination of the electronic structure and electron decoherence

Theoretical calculations of electron–phonon scattering rates in AlGaN/GaN quantum dots (QDs) have been performed by means of effective mass approximation in the frame of finite element method. The influence of a symmetry breaking of the carrier's wave function on the electron dephasing time is investigated for various QDs shapes. In a QD system the electron energy increases when the QD shape changes from a spherical to a non-spherical form. In addition, the influence of the QD shape upon the electronic structure can be modulated by external magnetic fields. We also show that the electron–acoustic phonon scattering rates strongly depend upon both the QD shape and the applied magnetic field. As an additional parameter, the QD shape can be used to modify the electron–acoustic phonon interaction in a wide range. Moreover, the scattering rate of different transitions, such as Δm=0(1), presents distinct magnetic field dependency.