Interactions of a Charged Particle with Parallel Two-Dimensional Quantum Electron Gases

By using the linearized quantum hydrodynamic (QHD) theory, electronic excitations induced by a charged particle moving between or over two parallel two-dimensional quantum electron gases (2DQEG) are investigated. The calculation shows that the influence of the quantum effects on the interaction process should be taken into account. Including the quantum statistical and quantum diffraction effects, the general expressions of the induced potential and the stopping power are obtained. Our simulation results indicate that a V-shaped oscillatory wake potential exists in the electron gases during the test charge intrusion. Meanwhile, double peaks will occur in the stopping power when the distance of two surfaces is smaller and the test charge gets closer to any one of the two sheets.     

基底性质对二维层状电子气感应电场的影响

采用线性化量子流体动力学模型,研究了载能粒子与二维层状电子气相互作用时的集体静电激发现象。在适当的边界条件下推导出感应电势,入射粒子所受的阻止力和侧向力的一般表达式,讨论了存在绝缘基底情况下基底介电常数对感应电势、阻止力及侧向力的影响。结果表明:当入射粒子速度较小时,基底的极化电场较弱,其介电常数的变化对感应电场几乎无影响,而当速度较高时因极化电场较强而影响很大;入射粒子距离与基底相连的电子气平面越近,基底介电常数对各物理量的影响越大。     

Wake effects and stopping power for a charged particle moving above two-dimensional quantum electron gases

Electronic excitations induced by a charged particle moving above two-dimensional electron gases are studied by means of the linearized quantum hydrodynamic (QHD) theory. In this calculation, we show that the influence of the quantum effects on the interaction process should be taken into account. The induced potential and the perturbed density of the electron gases as well as the stopping power of the particles are derived as functions of the projectile velocity, the particle position and the density function when including the quantum statistical and quantum diffraction effects. The dependence relations of the induced potential and the particle speed around the peak position at which the stopping power takes the maximum value are also discussed in this work.     

External forces outside of a layered electron gas

We calculate the external forces outside of the surface of a layered electron gas (LEG). The LEG is a model of a metal where the electrical current is carried in parallel layers, and there is no current between layers. It describes the high-temperature cuprate superconductors and many other layered solids. We calculate the image potential from an external charge, the van der Waals potential from a neutral atom and the Casimir force between the parallel surfaces of two LEGs. Our theory does not use dielectric functions. We write down the quantum mechanical Hamiltonian, calculate the exact ground state energy and deduce the forces from the energy. We also show that the LEG has no surface plasmon.     

基底对离子束与层状二维电子气作用的影响

采用线性化量子流体动力学（Quantum Hydrodynamics QHD）模型和Poisson方程相结合,研究了离子束与存在基底的双层二维电子气平面相互作用时的静电激发现象。选择恰当边界条件,推导出相互作用过程中的感应电子气密度、感应电势及阻止本领的一般表达式。结果表明,随着基底介电常数的增加,两个平面内的感应电子气密度以及空间感应电势振荡的波长明显变长。同时,由于两个电子气平面间的耦合作用,阻止本领随速度变化曲线出现明显的双峰结构,随着介电常数的增大,双峰结构逐渐消失。     

基底对离子束与层状二维电子气作用的影响

采用线性化量子流体动力学（Quantum Hydrodynamics QHD）模型和Poisson方程相结合，研究了离子束与存在基底的双层二维电子气平面相互作用时的静电激发现象。选择恰当边界条件，推导出相互作用过程中的感应电子气密度、感应电势及阻止本领的一般表达式。结果表明，随着基底介电常数的增加，两个平面内的感应电子气密度以及空间感应电势振荡的波长明显变长。同时，由于两个电子气平面间的耦合作用，阻止本领随速度变化曲线出现明显的双峰结构，随着介电常数的增大，双峰结构逐渐消失。     

Validity of the semi-classical approach for calculation of the surface excitation parameter

The problem of surface plasmon excitation by moving charges has been elaborated by several different approaches, mainly based on dielectric response theory within either semi-classical or quantum mechanical frameworks. In this work, a comparison of the surface excitation effect between two different frameworks is made by calculation of the differential inverse inelastic mean free path (DIIMFP) and a Monte Carlo simulation of reflection electron energy loss spectroscopy (REELS) spectra. A semi-classical modeling of the interaction between electrons and a solid surface is based on analyzing the work done by moving electrons; the stopping power and inelastic cross section are derived with the induced potential. On the other hand, a quantum mechanical approach is based on derivation of the complex inhomogeneous self-energy of the electrons. The numerical calculation shows that the semi-classical model presents almost the same values of DIIMFP as by the quantum model except at the glancing condition. The simulation of REELS spectra for Ag and SiO(2) as well as a comparison with experimental spectra also confirms that a good agreement with the spectral shape is found among the two simulation results and the experimental data.     

Wake potential and stopping power for a charged particle moving outside a nanosphere

The interaction of a charged particle with a nanosphere is studied based on the dielectric response theory. We obtain the analytical expressions of the induced potential and stopping power, as the charged particle moving outside the nanosphere with a constant velocity. From our results, since the spherical shape limitation, the well-known V-shaped wake effect tracing the particle cannot be observed clearly no matter at the nanosphere surface or in the bulk. Besides, we also find that the particle can even gain energy from the electron polarization as the particle moves to the nanosphere at relatively low velocity.     

二维方形量子点体系等离激元的量子化

量子点体系等离激元的研究是光电子学领域的热点.为进一步加深和完善对等离激元的量子效应的认识,本文利用紧束缚近似和线性响应理论研究了二维方形量子点体系对外场的集体响应.结果表明,当外场频率等于等离激元的频率时,量子点体系会有强烈的电荷振荡,并伴随着能量的极大吸收和近场的增强.在量子点中,等离子体存在分立的元激发.等离子体元激发的个数将随着量子点尺寸和电子个数的增加而增加.随量子点尺寸的增加,分立的等离激元将逐步呈现准连续的特性,即过渡为经典连续的等离激元,其频谱曲线演化为经典的色散曲线.结果还表明:随量子点尺寸的增加,等离激元的频率会红移,等离激元的激发强度会增大;随量子点中电子数的增加,等离激元的频率会蓝移,等离激元的激发强度会增大.     

Synthesis and charge storage properties of double-layered NiSi nanocrystals

Based on bidirectional diffusion of Ni atoms, double-layered nickel silicide (NiSi) nanocrystals (NCs) for multilevel charge storage were fabricated, and their charge storage properties were examined. The double layer was produced by long-term thermal annealing (for 4 h at 900 °C) of a sandwich structure comprised of a thin Ni film of 0.3 nm sandwiched between two silicon-rich oxide (SiO_1.36) layers. Transmission electron microscopic image clearly exhibits a distinct NiSi nanocrystal double layer with a gap of about 7 nm between the mean positions of particle distribution in each NC layer. Capacitance–voltage measurements on the metal/oxide/semiconductor (MOS) capacitors with the double-layered NiSi nanocrystals are shown to have the apparent two plateaus of charge storage, the large memory window of about 9 V and the improved charge retention stability.     

Quantum kinetic theory of trapped particles in a strong electromagnetic field

The idea of treating quantum systems by semiclassical representations using effective quantum potentials (forces) has been successfully applied in equilibrium by many authors, see e.g. [D. Bohm, Phys. Rev. 85 (1986) 166 and 180; D.K. Ferry, J.R. Zhou, Phys. Rev. B 48 (1993) 7944; A.V. Filinov, M. Bonitz, W. Ebeling, J. Phys. A 36 (2003) 5957 and references cited therein]. Here, this idea is extended to nonequilibrium quantum systems in an external field. A gauge-invariant quantum kinetic theory for weakly inhomogeneous charged particle systems in a strong electromagnetic field is developed within the framework of nonequilibrium Green’s functions. The equation for the spectral density is simplified by introducing a classical (local) form for the kinetics. Nonlocal quantum effects are accounted for in this way by replacing the bare external confinement potential with an effective quantum potential. The equation for this effective potential is identified and solved for weak inhomogeneity in the collisionless limit. The resulting nonequilibrium spectral function is used to determine the density of states and the modification of the Born collision operator in the kinetic equation for the Wigner function due to quantum confinement effects.     

带电粒子系统在双外场中的高温弱简并效应

用Thomas-Fermi半经典统计理论和方法,研究处于匀强电场加谐振场约束的带电粒子系统的热力学性质.首先导出外场约束下带电粒子系统的量子状态数密度,进而给出高温弱简并情况下带电粒子系统的化学势、内能和热容量的解析式,并分析电场和谐振场对系统化学势、内能和热容量的具体影响. 研究结果表明,带电粒子系统在双外场约束下具有高温弱简并效应. 关键词： 带电粒子 匀强电场 谐振场 高温弱简并效应     

Wake effect in doped graphene due to moving external charge

We use the dielectric-response formalism to evaluate the induced density of charge carriers in supported graphene due to an external moving charged particle in terms of its velocity and distance from graphene for several equilibrium charge carrier densities due to graphene doping. We show that, when the particle speed exceeds a threshold value, an oscillatory wake effect develops in the induced charge density trailing the particle. Strong effects are observed in the wake pattern due to finite size of the graphene–substrate gap, as well as due to strong coupling effects, and plasmon damping of graphene?s π electrons.     

Dynamic image forces near a semiconductor-vacuum interface: The role of quantum-mechanical corrections

The energy of dynamic image forces acting on a charged particle moving normally to the semiconductor-vacuum interface or in a vacuum gap between two semiconductors is calculated in the framework of the perturbation theory. The dielectric approach allows for spatial and time dispersions of the dielectric functions of electrodes. It is shown that the quantum-mechanical character of the screening should be taken into account. In particular, the dynamic corrections to the static image forces appear to be less than those in the quasi-classical model. The perturbation method used in this work is applicable in wider ranges of external electrostatic fields and particle energies.     

反比相关的双曲余弦平方势与电子的面沟道辐射

带电粒子在晶体沟道中的运动行为决定于粒子-晶体相互作用势.常用的粒子晶体相互作用势有Lindhard势、Moliere和正弦平方势.当超相对论电子沿着晶体的低晶面指数方向入射时,电子和晶体之间的相互作用势可用反比相关的双曲余弦平方势描写.在量子力学框架内,利用这一相互作用势成功地将系统的Schrodinger方程化为超几何方程,从而简化了系统本征值和本征态问题的计算和讨论.考虑到质量的相对论效应和频率的Doppler效应,导出了实验室坐标系中电子的能级分布和辐射谱分布.并以电子的Si(110)面沟道辐射为例,选定一组与入射粒子有关的参数和一组与晶体有关的参数,计算了能量为E=0.5GeV的电子在低位能级之间的跃迁,导出了电子面沟道辐射能量ΔE=49.1MeV,得到了与实验符合的结果.     

Electrodynamics of effective charge

A portion of the charge induced in an isotropic plasma by an injected charged particle is shown to combine with the particle to form a charged quasi-particle, the effective charge, that is different from the conventional “dressed” charge. The remainder of the induced charge is found in a wake behind the effective charge. Properties of effective charge are described, and the application of this concept to coherent ion acceleration and stopping power is discussed.     

Formation and manipulation of regular metallic nanoparticle arrays on bacterial surface layers: an advanced TEM study

The template-directed formation of regular nanoparticle arrays on two-dimensional crystalline protein layers after their treatment with metal salt complexes was studied by transmission electron microscopy. For these investigations, bacterial surface layers (S layers), recrystallized in vitro into sheets and tube-shaped protein crystals with typical dimensions in the micrometer range, were used as the template. As identified by electron holography and scanning force microscopy, the S-layer tubes form alternating double layers when deposited onto a solid substrate surface. Two distinct pathways for the metal particle formation at the templates have been found: the site-specific growth of metal clusters by chemical reduction of the metal salt complexes, and the electron-beam induced growth of nanoparticles in the transmission electron microscope. Both mechanisms lead to regular arrays with particle densities > 6×10¹¹ cm ^-2. Nanoparticle formation by electron exposure takes exclusively place in the flat-lying double-layered protein tubes, where a sufficient amount of metal complexes can be accumulated during sample preparation. Received 6 December 2000     

调制n型掺杂ZnSe/BeTe Ⅱ型量子阱结构的发光特性

报道了调制n型掺杂ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-II QW)在极低温至室温(14—296K)条件下的各种光学性质. 反射光谱显示了对于非掺杂样品,激子(X)的跃迁起着支配作用,而只有在掺杂样品的光谱里展示了一个典型的负的带电激子(X^-)的跃迁特征. PL光谱及其直线偏振度P_l都显著地依赖于n型掺杂量和平行于QW生长方向的外加电场. 这个特征被认为是由n型掺杂导致了内秉电场(built 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

The Larmor nutation

The classical non-relativistic problem of the motion of a charged particle in an external central force field and a weak uniform magnetic field is revisited to show that the motion of the kinetic angular momentumL = r × p of the particle, in the so-called Larmor approximation, is not a simple precession but is actually a composite motion involving precession as well as a high frequency nutation. The precession-nutation motion ofL is discussed in the Larmor approximation when the Larmor-frame-orbit of the charged particle is an ellipse (or a circle) for the case of the two central forces namely the Coulomb and the Hooke-law-force, which are the only two central forces known to permit closed orbits.     

Knight-field-enabled nuclear spin polarization in single quantum dots

We demonstrate dynamical nuclear-spin polarization in the absence of an external magnetic field by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our experiments constitute a first step towards a quantum measurement of the Overhauser field.