Time evolution of electron temperature and hot LO-phonon distribution function in laser excited polar semiconductors

The numerical analysis is presented of the time evolution of the conduction electron temperature and of the hot LO-phonon population in polar semiconductors GaAs, CdTe and ZnS excited by a short laser pulse. The paper is based on the equations describing the evolution of the nonequilibrium electron-phonon system including the electron energy collision broadening and the electron screening of the electron-phonon interaction. The connection between the hot phonon population in the range of small values of the wave vector and the wave vector dependence of the screening function is pointed out.     

Electron-confined LO-phonon scattering in GaAs-Al0.45Ga0.55As superlattice

We develop a theoretical model to the scattering time due to the electron-confined LO-phonon in GaAs-Al_xGa_1-xAs superlattice taking into account the sub-band parabolicity. Using the new analytic wave function of electron miniband conduction of superlattice and a reformulation slab model for the confined LO-phonon modes, an expression for the electron-confined LO-phonon scattering time is obtained. In solving numerically a partial differential equation for the phonon generation rate, our results show that forx = 0.45, the LO-phonon in superlattice changes from a bulk-like propagating mode to a confined mode. The dispersion of the relaxation time due to the emission of confined LO-phonons depends strongly on the total energy.     

Calculation of the inverse bremsstrahlung absorption in unmagnetized plasmas

Inverse bremsstrahlung is one important way to deliver laser energy to the plasma in inertial confinement fusion. In this article, we study the collisional absorption rate as obtained from the Fokker–Planck treatment of an unmagnetized plasma in harmonic laser field. The electron–ion collision rate is considered in the Krook approximation, and the electron distribution function is considered a Maxwellian function. We evaluate the inverse bremsstrahlung absorption near the irradiated plasma surface in the critical layer. We observe that absorption increases with shorter laser wavelength and lower electron temperatures. When the maximum electron velocity in the limit of q → 1 reaches infinity, the q-non-extensive distribution function reduces to the standard Maxwell-Boltzmann distribution.     

低压氩气电感耦合等离子体特性分析及光谱诊断

电感耦合等离子体具有电子密度高、放电面积大、工作气压宽、结构简单等特点,在等离子体隐身领域具有突出的潜在优势。相对于开放式等离子体,闭式等离子体更适应于飞行器表面空气流速高、气压变化大的特殊环境。研究着眼于飞行器关键部件的局部隐身应用,设计了一种镶嵌于不锈钢壁中的圆柱形石英腔体结构,利用电感耦合放电的方式在腔体中产生均匀的平板状等离子体。由于增加了接地金属,降低了腔体内的钳制电位,同之前的纯石英腔体相比,该结构显著改善了等离子体的均匀性。研究了该闭式腔体内氩气电感耦合等离子体（ICP）的放电特性和发射光谱,实验中放电功率达到150 W时,可以明显观察到ICP的E-H模式转换,此时发射光谱和电子密度都呈现阶跃式增长。氩气发射光谱强度随放电功率升高显著增加,但是不同谱线强度增加幅度并不一致,分析认为是受不同的跃迁概率和激发能的影响。根据等离子体的发射光谱,利用玻尔兹曼斜率法对电子激发温度进行诊断,得到电子激发温度在2 000 K以上,并且随功率升高而降低,因为功率增大使电子热运动增强,粒子间的碰撞加剧,碰撞导致的能量消耗也更大。电子激发温度沿腔体径向呈近似均匀分布,分布趋势受功率影响不大。针对利用发射光谱诊断电子密度误差较大、计算繁琐的问题,引入Voigt卷积函数,经过拟合滤除多余展宽项的影响,得到准确的Stark展宽半高宽。最终利用发射光谱Stark展宽法计算了电子密度,腔体中心处的峰值密度可以达到7.5×1017 m-3。随着放电功率增大,线圈中容性分量降低,耦合效率增大,电子密度随之增大,但空间分布趋势基本不受功率影响。     

弱电离大气等离子体电子碰撞能量损失的理论研究

在前期计算电子能量分布函数的基础上, 求出弱电离大气等离子体中各碰撞反应过程的电子能量损失. 由于在弹性碰撞中电子-重粒子能量交换很少, 同时氮气、氧气分子又有很多能量阈值较低的转动、振动能级存在, 因此在大气等离子体中弹性碰撞电子能量损失所占份额很小(直流电场下小于6%). 研究发现, 弱电离大气等离子体中在不同能量区间占主导的能量损失过程不同. 随着有效电子温度(或约化场强)增加, 占主导的电子能量损失过程依次为转动激发、振动激发、电子态激发、碰撞电离、加速电离产生的二次电子. 在约化场强E/N=1350 Td (或有效电子温度为14 eV)附近, 平均电离一个电子所需的能量最小, 约为57 eV. 因此可以根据不同的需求调节电场强度, 从而达到较高的能量利用率. 关键词： 弱电离大气等离子体 碰撞反应过程 电子能量损失     

The dependence of the quantum oscillation amplitude on spin splitting

The electron density of states of a semiconductor in magnetic field exhibits an oscillatory behaviour. The amplitude of these oscillations is studied theoretically as a function of the electron spin splitting. The amplitude is shown to be strongly reduced when the spin splitting is equal to the half of the orbital splitting ?ω, provided that the collision broadening of the electronic levels is large enough. This fact should be reflected in a similar reduction of the amplitude of various quantum oscillatory phenomena, e.g. Shubnikov-de Haas effect.     

非均匀等离子体湍流的重正化准线性理论——Misguich-Balescu理论的推广

本文把Misguich和Balescu建立的均匀等离子体湍流的重正化准线性理论推广到非均匀等离子体湍流。在弱耦合和弱非均匀性近似下,得到传播算符和平均湍性碰撞算符的明晰表达式。非均匀性除对算符中的扩散贡献、共振加宽或频移、Dupree衰减和平均分布函数在速度空间的陡度效应等作出一定的修正之外,还引出新的指数微分算符:在传播算符中是涨落电场关联函数非均匀性导致的新的速度微分算符,在平均湍性碰撞算符中是平均分布函数非均匀性导致的新的空间微分算符。平均分布函数非均匀性还使得在平均湍性碰撞算符中出现的两个互为逆运算的自由流传播算符不能抵消,从而使包含在(直接作用于平均分布函数的)传播算符中的非马尔可夫效应显露出来。 关键词：     

Effect of structure anisotropy on low temperature spin dynamics in quantum wells

Spin dynamics of two-dimensional electron gas confined in an asymmetrical quantum well is studied theoretically in the regime where the scattering frequency is comparable with the spin precession frequency due to the conduction band spin splitting. The spin polarization is shown to demonstrate quantum beats. If the spin splitting is determined by both bulk and structural asymmetry mechanisms the beats are damped at zero temperature even in the absence of a scattering. We calculate the decay of spin beats due to the thermal broadening of the electron distribution function and electron scattering. The magnetic field applied along the structure growth axis is shown to increase the frequency of the beats and shift system towards the collision dominated regime.     

Multiplication of high-energy electrons in irradiated materials studied using the Boltzmann kinetic equation

Processes involved in the formation of electron collision cascades created by nonrelativistic high-energy electrons, which can develop in materials exposed to electron and gamma radiation fluxes, have been considered. The problem is solved using the Boltzmann kinetic equation for high-energy electrons moving in a medium. A model scattering indicatrix is constructed for this equation with an arbitrary potential of interaction between colliding particles. Using this scattering indicatrix, the distribution of the particle energies is obtained. Based on this energy distribution (with an arbitrary interparticle interaction potential), a cascade function is found that describes the multiplication of knock-out electrons (electron cascade) generated when a high-energy electron with a certain energy is scattered on the electron subsystem of the irradiated material. The cascade function has been calculated for the Coulomb potential of the interaction between a high-energy electron and atomic-shell electrons.     

用并行扫描方法开展的表面等离激元电子能谱成像

本文报道了高定向热解石墨上银纳米结构表面等离激元的电子能谱成像研究. 本文利用现有扫描探针电子能谱仪的角度色散特性,发展了一种多道测量模式. 通过沿一个方向扫描,可以并行获得银纳米结构表面等离激元电子能谱的二维分布. 实验确定的能谱成像空间分辨为80 nm左右.     

谱线非均匀展宽条件下原子激发的计算方法

原子吸收谱线展宽对原子光激发有较大的影响.在速率方程理论中人们通常将原子谱线的均匀展宽和非均匀展宽都归于原子谱线的线型函数.本文中提出了一种将均匀展宽和非均匀展宽区别处理的原子激发计算方法,并指出当激发光与原子作用时间短于原子热运动平均碰撞时间时,这种区别对待均匀展宽和非均匀展宽的做法是十分必要的.文中还将本文提出的新方法应用于一个二能级原子跃迁过程的计算,计算结果显示两种方法具有很大差别.     

Influence of “travelling velocities” on molecular autoionization electron spectra

We present electron energy spectra that arise in Li⁺ —He collisions in the laboratory collision energy range from 1.4 to 3 keV, and that are observed at 0° and 180° with respect to the Li⁺ beam. The spectra are composed of a component due to atomic autoionization of doubly excited He in the field of Li⁺, and a component due to molecular autoionization of the collision system. The variation of the latter component with collision energy and observation angle is analyzed in terms of a newly developed spectral shape function for molecular autoionization that implies a “distribution of travelling velocities” of the collision system during electron emission. From the comparison of calculated and measured spectra it is concluded that the physical interpretation on which the analysis is based is correct. Two parameters of the distribution of travelling velocities are determined.     

Mechanisms for formation of the electron distribution function in the positive column of discharges under Langmuir-paradox conditions

The form of the electron distribution function in the positive column of low-pressure discharges is examined under conditions such that the electron mean free path exceeds the vessel radius. Its formation is analyzed taking all major factors into account, including elastic and inelastic collisions, radial and axial electric fields, and the loss of fast electrons to the wall. It is shown that the main mechanism controlling the fast part of the distribution function is the loss of electrons to the wall, which is determined by the scattering of electrons into a comparatively small loss cone that depends on the relationship between the axial and radial components of the velocity. Since the elastic collision rate for all elements has a weak dependence on the energy beyond the ionization threshold, ultimately the high-energy part of the electron energy distribution function in the positive column of low-pressure discharges is nearly Maxwellian. The subthreshold portion of the distribution function, in turn, is determined by the energy diffusion, in a comparatively strong field, of Maxwellian electrons which arrive after inelastic collisions. The final electron distribution function is well approximated by an exponential with a single slope over the entire energy range. Only within a narrow range of scattering angles is the electron distribution function strongly depleted by the loss of electrons to the vessel walls. In the end, it is concluded that this phenomenon, like the Langmuir paradox, may be related to aspects of the physics of the formation of the electron distribution function owing to a combination of already known mechanisms, rather than to a hypothetical mechanism for thermalization of the electrons, as assumed up to now in the literature. A comparison of solutions of the model kinetic equation given here with published Monte Carlo calculations and experimental data shows that they are in good agreement. Zh. Tekh. Fiz. 69, 34–41 (November 1999)     

Polarised hot electron luminescence in p-GaAs: Electric field effects

Electrons photo-excited to high-energy conduction band states of GaAs exhibit complex energy and momentum distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs a fraction of these hot electrons combine with localised acceptor states, producing a hot electron luminescence (HEL) spectrum with a cascade of peaks corresponding to discrete energy losses resulting from LO-phonon emission. The highest peak involves unscattered electrons, and their energy distribution is due to warping of the initial heavy-hole (HH) bands. We report measurements of the line shape of this 0-HH peak, and its polarisation profile which identifies emission from electrons along particular directions. An applied electric field of 1 kV cm⁻¹ distorts the hot electron momentum distribution, and this is reflected in the polarisation profile. These line shapes and profiles, with and without field, are calculated using a computer model incorporating a band structure and optical matrix elements, the effect of electric field being included using a k-broadening model. The data and model are in good quantitative agreement assuming an electron lifetime of 100 fs, and confirm the expected differences in the profiles for different excitation polarisation states and applied field directions.     

Infrared catastrophe in a two-quasiparticle collision integral

Relaxation of a nonequilibrium state in a disordered metal with a spin-dependent electron energy distribution is considered. The collision integral due to the electron-electron interaction is computed within the approximation of a two-quasiparticle scattering. It is shown that the spin-flip scattering processes with a small energy transfer may lead to the divergence of the collision integral for a quasi one-dimensional wire. This divergence is present only for a spin-dependent electron energy distribution that corresponds to the total electron spin magnetization M = 0 and only for nonzero interaction in the triplet channel. In this case, a nonperturbative treatment of the electron-electron interaction is needed to provide an effective infrared cutoff.     

Longitudinal broadening of quenched jets in turbulent color fields

The nearside distribution of particles at intermediate transverse momentum, associated with a high momentum trigger hadron produced in a high energy heavy-ion collision, is broadened in rapidity compared with the jet cone. This broadened distribution is thought to contain the energy lost by the progenitor parton of the trigger hadron. We show that the broadening can be explained as the final-state deflection of the gluons radiated from the hard parton inside the medium by soft, transversely oriented, turbulent color fields that arise in the presence of plasma instabilities. The magnitude of the effect is found to grow with medium size and density and diminish with increasing energy of the associated hadron.     

Inreased Power Concentration and its Effect on the Discharge Parameters on the Low Pressure Hg-Rare Gas Positive Column III. Effects of an Axial Rod

After the investigation of the influence of current and tube radius on the power concentration of the Hg-rare gas discharge in the two previous parts, part III deals with the corresponding effects of additional recombination surfaces. The plasma parameters (radiant flux of the resonance lines, field strength, electron density, electron energy distribution function) are measured in a discharge tube with an axial glass rod. With increasing rod diameter and increasing distance from the tube axis radial radiation intensity distribution becomes asymmetric. The field strength increases with the radius of the rod. The dependence of the field strength and that of the radiant flux on the parameters of the discharge do not basically change. The measured UV-radiation efficiency of the positive column is higher than that of discharges without axial rod for equal tube radius. This is attributed primarily to the changed radial electron density distribution and its effect on the collision processes.     

Supercurrent-induced temperature gradient across a nonequilibrium SNS Josephson junction

Using tunneling spectroscopy, we have measured the local electron energy distribution function in the normal part of a superconductor-normal metal-superconductor (SNS) Josephson junction containing an extra lead to a normal reservoir. In the presence of simultaneous supercurrent and injected quasiparticle current, the distribution function exhibits a sharp feature at very low energy. The feature is odd in energy and odd under reversal of either the supercurrent or the quasiparticle current direction. The feature represents an effective temperature gradient across the SNS Josephson junction that is controllable by the supercurrent.     

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为了研究靶材料对快电子能量分布的影响,采用电子谱仪测量了飞秒激光与Cu和CH靶相互作用中在靶前和靶后产生的快电子能谱。结果显示,在靶前Cu和CH靶的快电子能谱相似,反应了快电子发射对靶材料的依赖性较弱;在靶后Cu和CH靶的快电子能谱具有明显的差异,说明电子的输运过程与靶材料密切相关。冷电子环流以及自生磁场是导致Cu靶快电子能谱"软化"的原因,而对于CH靶麦克斯韦分布的快电子能谱主要由碰撞机制决定。