有限深抛物势量子盘中极化子的激发态性质

量子点作为一种重要的低维纳米结构, 近年来在单光子光源和新型量子点单光子探测器的研究引起了人们的广泛关注, 对各种势阱中量子点性质的研究已取得了重要成果. 但是大多理论研究都局限于无限深势阱, 而有限深势阱更具有实际意义. 利用平面波展开、幺正变换和变分相结合的方法研究了有限深势阱中极化子激发态能量及激发能随势阱形状和量子盘大小的变化规律. 数值计算结果表明: 极化子的激发态能量、激发能随势垒高度或宽度的增大而增大, 原因是势垒愈高、愈宽, 电子穿透势垒的可能性愈小, 电子在阱内运动的可能性愈大, 进而导致极化子的激发态能量和激发能均随势垒高度和宽度的增大而增大; 极化子的激发态能量和激发能随量子盘半径的增大而减小, 表明量子盘具有显著的量子尺寸效应; 极化子的激发态能量随有效受限长度的增加而减小, 原因是有效受限长度愈大, 有效受限强度愈小, 电子受到的束缚愈弱、振动愈慢、势能愈小, 进而导致基态能量、激发态能量减小; 同时由于激发态能量较基态能量减小慢, 使得激发能随之增加. 研究结果对量子点的应用具有一定的理论指导意义.     

量子点中束缚极化子性质的温度依赖性

采用线性组合算符和幺正变换方法研究抛物量子点中弱耦合束缚极化子性质的温度依赖性,导出了弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的变化关系。取ZnS晶体为例进行数值计算,结果表明:量子点中弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的升高而增大。     

Formula for the absorption coefficient for multi-wall nanotubes

We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A spatially nonlocal, dynamical self-consistent field theory is obtained by calculating the electrostatic potential produced by the charge density fluctuations as well as the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. In this Letter, we numerically calculate the plasmon contribution to the absorption spectrum when an external electric field is applied. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak is determined by the plasmon wavelength and energy. For a chosen wave number, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength of the excited modes. Some of the low-frequency plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale as the modes with larger energy of excitation. We plot the peak positions of the plasmon excitations for a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. No effects due to inter-tubule hopping are included in these calculations.     

Pump–probe analysis of polaron decay in InAs/GaAs self-assembled quantum dots

We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T₁ polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.     

Study of interference in a double-slit without walls by plasmon tomography techniques

We investigate the free propagation of two parallel surface plasmon polaritons (SPP) beams using plasmon tomography. In the Fourier-plane images, we observed interference features that are not in correspondence with the images of SPPs on the sample's surface. We clearly observed that the interference maxima and minima are distributed over an arc of a circle. We explain the characteristics of the observed interference patterns assuming that each SPP beam can be considered as a “slit without walls”. We discuss important implications of this work for SPP tomography and interferometric plasmonic sensors.     

Mass of a lattice polaron from an extended Holstein model using the Yukawa potential

Renormalization of the mass of an electron is studied within the framework of the Extended Holstein model at strong coupling regime and nonadiabatic limit. In order to take into account an effect of screening of an electron-phonon interaction on a polaron it is assumed that the electron-phonon interaction potential has the Yukawa form and screening of the electron-phonon interaction is due to the presence of other electrons in a lattice. The forces are derived from the Yukawa type electron-phonon interaction potential. It is emphasized that the early considered screened force of (Kornilovitch (1998), Spencer et al. (2005), Hague et al. (2006), Hague and Kornilovitch (2009)) Refs. [7], [18], [19] and [22] is a particular case of the force deduced from the Yukawa potential and is approximately valid at large screening radiuses compared to the distances under consideration. The Extended Holstein polaron with the Yukawa type potential is found to be a more mobile than polaron studied in early works at the same screening regime.     

The mass of a large polaron

We show that, beginning with the works of L. D. Landau and S. I. Pekar, the effective mass of a large polaron has been determined with a crucial error. Since all such research ignored the spatial dispersion of the lattice polarizability, the maximum group velocity of phonons is found to be zero, so that the phonon “cloud” of a polaron is unable to follow the polaron. We allow for the spatial dispersion of the lattice polarizability and derive an expression for the effective polaron mass valid over the entire velocity range in which a polaron can exist: from zero to the maximum group velocity of phonons. According to this expression, the polaron mass depends not only on the phonon frequency, reciprocal effective dielectric constant, and the carrier mass but also on the maximum group velocity of phonons interacting with the carrier and on the polaron velocity. Zh. éksp. Teor. Fiz. 112, 278–283 (July 1997)     

非对称量子点中弱耦合极化子的相互作用能

采用改进的线性组合算符和幺正变换方法,研究非对称量子点中弱耦合极化子的性质.导出了非对称量子点中弱耦合极化子的振动频率和相互作用能随量子点的横向和纵向有效受限长度和电子-声子耦合强度的变化关系.数值计算结果表明:非对称量子点中弱耦合极化子的振动频率和相互作用能随量子点的横向和纵向有效受限长度的减小而迅速增大,表现出奇特的量子尺寸效应.     

On the possibility of electric transport mediated by long living intrinsic localized solectron modes

We consider a polaron Hamiltonian in which not only the lattice and the electron-lattice interactions, but also the electron hopping term is affected by anharmonicity. We find that the one-electron ground states of this system are localized in a wide range of the parameter space. Furthermore, low energy excited states, generated either by additional momenta in the lattice sites or by appropriate initial electron conditions, lead to states constituted by a localized electron density and an associated lattice distortion, which move together through the system, at subsonic or supersonic velocities. Thus we investigate here the localized states above the ground state which correspond to moving electrons. We show that besides the stationary localized electron states (proper polaron states) there exist moving localized solectron states which can be easily excited. The evolution of these localized states suggests their potential as new carriers for fast electric charge transport.     

电子相互作用和高聚物中的极化子

各种电致发光聚合物具有非简并基态.当电子和空穴注入后,形成的载流子是极化子.极化子的大小和能量决定于电子相互作用.目前大家采用Hubbard model来描述聚合物中的电子相互作用,所得到的结果与实验不符.Hubbard model和extended Hubbard model对窄能带体系是好的近似,但聚合物是宽能带,反映价键关联的非对角相互作用（在extended Hubbard model中被略去）有重要贡献,特别是在强屏蔽情况下,价键关联与Hubbard关联相互竞争,可纠正现有理论的缺陷. 关键词：     

极性晶体中表面极化子的温度效应

有不少的极性晶体,电子与体纵光学声子的耦合弱,但与表面光学声予的耦合强.本文讨论电子和体纵光学声子耦合弱,与表面光学声子耦合强时对表面极化子的温度特性的影响,用线性组合算符法研究表面极化子的振动频率、诱生势和有效质量的温度依赖性.对AgBr晶体进行了数值计算,结果表明极化子的振动频率,诱生势和有效质量随温度的升高而减小.     

Polaron effects in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowires

By the fractal dimension method, the polaron properties in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are explored. In this study, the polaron effects in GaAs/Al_xGa_(1-x)As core-shell nanowire at different values of shell width and aluminum concentration are discussed. The polaron binding energy, polaron mass shift and fractal dimension parameter are numerically worked out each as a function of core radius. The calculation results show that the binding energy and mass shift of the polaron first increase and then decrease as the core radius increases, forming their corresponding maximum values for different aluminum concentrations at a given shell width. Polaron problems in the cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are solved simply by using the fractal dimension method to avoid complex and lengthy calculations.     

磁场中强耦合表面极化子的性质

本文研究磁场中强耦合表面极化子的性质,采用线性组合算符法导出表面极化子的基态能量和振动频率.对AgCl和KCl晶体进行了数值计算.结果表明,磁场中强耦合表面极化子的振动频率随磁场的增加而增大,基态能量随磁场的增加而减少.     

抛物量子阱中束缚极化子的极化势和结合能

利用改进的Lee-Low-Pines(LLP)方法,用变分法计算了无限深抛物量子阱中同时考虑与体纵光学声子和界面纵光学声子相互作用的束缚极化子的极化势和结合能.数值计算得出:阱宽较大时极化势很小,阱宽较小时极化势较大,所以对于较窄的抛物阱必须考虑极化势.对于给定阱宽的抛物阱,随着远离阱中心极化势迅速减小,当到达阱的界面附近极化势又开始增大.阱宽较小时,束缚极化子的结合能随着阱宽L的增大而急剧减小;阱宽较大时,结合能减小的非常缓慢,最后接近体材料中的三维值.     

晶格热振动对准二维强耦合极化子有效质量的影响

采用Tokuda改变的线性组合算符法和改进的LLP变分法，研究了晶格热振动对无限势垒量子阱中电子与界面光学声子强耦合、与体纵光学声子弱耦合系统的影响，推导出作为阱宽和温度函数的极化子有效质量的表达式. 尤其得到了量子阱中极化子的振动频率及其随阱宽和温度变化的规律. 对KI/AgCl/KI量子阱进行了数值计算，结果表明，极化子的振动频率和有效质量随阱宽的增加而减小、随温度的升高而减小，但不同支声子与电子相互作用对极化子的振动频率和有效质量的贡献以及它们随阱宽和温度的变化情况大不相同. 关键词： 量子阱 强耦合极化子 振动频率 有效质量 温度依赖性     

Non-linear Doppler shift of the plasmon resonance in a grating-coupled drifting 2DEG

We report experimental measurements and computer calculations of the plasmon resonances of two dimensional electron gases in the far-infrared which show the effects of laterally drifting the 2DEG. Coupling to radiation is achieved using an overlaid metal grating of submicron period, and its periodic screening effect splits the plasmon into upper and lower energy modes. For a symmetric grating profile the higher energy mode is non-radiative for a stationary 2DEG and a splitting is not observable, but when the 2DEG is laterally drifted under the grating, coupling to both modes can occur, and their Doppler shifts produce an observable splitting which increases with drift velocity. These Doppler shifts are not linear with drift velocity for low velocities, but approach asymptotically the expected linear shift with increasing drift velocity. Experimental results on 2DEGs at GaAs/AlGaAs heterojunctions compare well with theoretical calculations.     

Stationary polaron motion in a polymer chain at high electric fields

We study the stationary motion of a polaron in a conducting polymer in the presence of a high electric field. Using the Su-Schrieffer-Heeger model plus an electric field, we find that at polaron velocities not exceeding the sound velocity, the dissipation of the electronic energy into the lattice occurs via emission of phonons with single selected wave vector. For this case the corresponding contribution to the polaron mobility can be calculated analytically. We discuss the issue of the polaron stability with respect to dissociation in a very high field at supersonic velocities.     

Core excitons in the electronic polaron model

Summary The electronic polaron model of the exciton is used to study the dielectric response of a medium to the excitation of a ?core? level, by adopting the method of direct solution of the Eulerian functional variational equations. The dynamical response of the electronic polarization affects the electron-hole attraction and the exciton binding energy, in a way which depends on the basic parameters of the crystal (dielectric constant, effective masses, lattice parameter) through the core exciton radius and the polaron radius. When the former is much larger than the latter, static dielectric screening results. When the exciton radius is comparable to the polaron radius, the screening is reduced and the binding energy is increased. Core exciton binding energies are computed in a number of substances using the effective-mass approximation. Space dispersion of the dielectric function coupled to intervalley interaction may, however, contribute in some cases to reducing the excitonic radius and bringing about an instability to a deep state that would invalidate the effective-mass approximation. Based on work supported by the Italian Research Council (C.N.R.) through a contract G.N.S.M.     

Polaron dynamics in a system of coupled conjugated polymer chains

The motion of excitations such as polarons is believed to be of fundamental importance for the transport properties of conjugated polymers for the use in, e.g., polymer based LED's. We have investigated polaron dynamics in a system of coupled polymer chains in the presence of an external electric field. In particular, we focus on how a polaron migrates through the polymer lattice, i.e., the situation in which a polaron reaches a chain end and is scattered to the surrounding chains. We show that the outcome of this event strongly depends on the strength of the electric field, and we identify three different cases for the polaron migration.     

Energy of a bound polaron in a magnetic field in asymmetric polar semiconductor heterostructures

In this paper, a new modified Hamiltonian of a polaron bound to a donor impurity in asymmetric step quantum wells (QWs) in the presence of an arbitrary magnetic field is given, in which the coupling of an electron with confined bulk-like LO phonons, half-space LO phonons and interface phonon modes is included. Especially, the interaction of the impurity with all possible optical-phonon modes is also considered. The ionization energy of a bound polaron in a magnetic field for asymmetric step QWs are studied by using a modified Lee-Low-Pines (LLP) variational method. The effects of the finite electronic confinement potential and the subband nonparabolicity are also considered. The relative importance of the donor impurity located at the well and the step is analyzed. Our results show the interaction between the impurity and the phonon field in screening the Coulomb interaction has a significant influence on the binding energy of bound polaron. The influence of subband non-parabolicity is appreciable on the bound polaron effects for the narrow well. The binding energy of bound polaron given in this paper are excellent agreement with the experimental measurement.