Optical and electrical investigation of low dimensional self-assembled InAs quantum dot field effect transistors

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 mu m at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current-voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

Nonlinear optical properties of semiconductor quantum wells under intense terahertz radiation

The nonlinear optical properties of semiconductor quantum wells driven by intense in-plane terahertz electric fields are investigated theoretically by employing the extended semiconductor Bloch equations. The dynamical Franz-Keldysh effect of the optical absorption near the band edge is analyzed with Coulomb correlation among the carriers included. The in-plane terahertz field induces a variety of behavior in the absorption spectra, including terahertz replicas of the (dark) 2p exciton and terahertz sidebands of the 1s exciton. The dependence of these interesting features on the intensity, frequency, and phase of the terahertz field is explored in detail.

Magnetoexcitonic optical absorption in semiconductors under strong magnetic fields and intense terahertz radiation in the Voigt configuration

The magnetoexcitonic optical absorption of a GaAs bulk semiconductor driven by a terahertz (THz) field is investigated numerically. The method of the solution of the initial-value problem, in combination with the perfect matched layer technique, is used to calculate the optical susceptibility, with Coulomb interaction, Landau quantization, and THz fields involved nonperturbatively. It shows that there appear replicas and sidebands of magnetoexciton of different Landau levels, which greatly enrich the magneto-optical spectrum in the presence of a driving THz field. Copyright (C) EPLA, 2008.

用计算机模拟与X-射线衍方法研究几种卤化物熔盐的结构

传统的熔盐结构理论多以“伪晶格模型”为基础,虽能解释熔盐结构的某些规律，但“伪晶格模型”与熔盐的真实结构相差甚远。迄今为止较为止较为严格的熔盐溶液理论是径向分布函数理论。本工作以径向分布函数理论为基础，利用Monte Carlo计算机模拟与X-射线衍射方法计算和测定了几种卤化物熔盐的径向分布函数（RDF），并在此基础上对若干熔盐微观结构和某些热力学性质的问题进行了讨论。1、LaCl_3-KCl与La-LaCl_3-KCl熔体结构的Monte Carlo计算机模拟研究；2、高温液态X-射线衍射实验方法的建立；3、用高温液态X-射线衍射方法研究几种卤化物熔盐的结构。研究结果表明，熔盐的结构对摩尔体积的变化很敏感，由温度的降低而引起的熔盐摩尔体积的收缩会导致熔盐中相同离子对间的距离缩短。由于熔盐是由阴、阳离子组成的离子熔体，其离子间的主要作用是Coulomb作用，因此熔盐中阳离子的尺寸及其所带电荷的多少是熔盐结构变化的主要影响因素。

Asymmetric stark shifts of exciton in InAs/GaAs pyramidal quantum dots

Within the framework of the single-band effective-mass envelope-function theory, the effect of electric field on the electronic structures of pyramidal quantum dot is investigated. Taking the Coulomb interaction between the heavy holes and electron into account, the quantum confined Stark shift of the exciton as functions of the strength and direction of applied electric field and the size of the quantum dot are obtained. An interesting asymmetry of Stark shifts around the zero field is found. (C) 1997 Elsevier Science Ltd.

Exciton states in isolated quantum wires

The exciton states in isolated and semi-isolated quantum wires are studied. It is found that the image charges have a large effect on the effective Coulomb potential in wires. For the isolated wire the effective potential approaches the Coulomb potential in vacuum at large z distance. For the semi-isolated wire the effective potential is intermediate between the Coulomb potential in vacuum and the screened Coulomb potential at large distance. The exciton binding energy in the isolated wire is about ten times larger than that in the quantum well, and that in the semi-isolated wire is also intermediate between those in the isolated wire and in the quantum well. When the lateral width increases the binding energy decreases further, and approaches that in the quantum well. The real valence-band structure is taken into account, the exciton wave functions of the ground state in the zero-order approximation are given, and the reduced mass is calculated. The effect of the coupling between the ground and excited states are considered by the degenerate perturbation method, and it is found the coupling effect is small compared to the binding energy.

First-principles investigation of the electronic structure and magnetism of eskolaite

The electronic structure and magnetism of eskolaite are studied by using first-principles calculations where the on-site Coulomb interaction and the exchange interaction are taken into account and the LSDA+U method is used.The calculated energies of magnetic configurations are very well fitted by the Heisenberg Hamiltonian with interactions in five neighbour shells; interaction with two nearest neighbours is found to be dominant. The Neel temperature is calculated in the spin-3/2 pair-cluster approximation. It is found that the measurements are in good agreement with for the values of U and J that are close to those obtained within the constrained occupation method.The band gap is of the Mott-Hubbard type.

纳米硅薄膜中的量子点特征

从电性和结构上论证了纳米硅薄膜中的细微晶粒(3～6nm)具有量子点(Q.D)特征.在其 电导曲线中呈现出随晶粒尺寸减小而增大的小尺寸效应.使用薄层(～20nm厚)纳米硅膜制成了隧道二极管,已在液氮温区(≈77K)在其I-V及σ-V曲线上呈现出Coulomb台阶.对实验结果做了初步分析讨论.

Phase character of the electron transport through a double quantum dot in a mesoscopic ring

We theoretically study the electron transport through a double quantum dot (QD) in the Coulomb blockade regime and reveal the phase character of the transport by embedding the double QD in a mesoscopic Aharonov-Bohm ring. It is shown that coherent transport through the double QD is preserved in spite of intradot and interdot Coulomb interactions.

基于连续介质力学的块体单元离散弹簧法研究

提出一种基于连续介质力学的块体单元离散弹簧法,并将其应用于地质体渐进破坏过程的数值模拟研究。该方法以连续介质力学的基本理论为基础,以八节点正六面体单元为离散对象,根据有限元的形函数理论及正六面体单元刚度矩阵的具体表达式,将正六面体单元离散成为12根棱弹簧,并推导各弹簧的弹簧力计算公式,给出各弹簧的弹簧力分量的物理意义、对应的弹簧刚度及相关联的位移。在弹性部分的计算结果与传统的有限元计算结果一致的基础上,在弹簧中引入Mohr-Coulomb准则及拉伸破坏准则,进行弹簧破坏的判断。如果块体内的弹簧全部断裂,便将其视为散体。在判断破坏时采用双重判断模式,块体单元的判断主要用于选取弹簧潜在的破坏面,离散弹簧的判断用于计算弹簧上的真实弹簧力。最后,通过相关算例验证引入破坏准则后所得计算结果的合理性。块体单元离散弹簧法的实质是通过12根离散弹簧将单元转化为结构,因此可以通过分析各弹簧的断裂情况研究块体的内部破坏特征,更可以通过各弹簧的先后断裂过程来研究地质体的渐进破坏过程。

基于连续介质力学的块体单元离散弹簧法研究

提出一种基于连续介质力学的块体单元离散弹簧法,并将其应用于地质体渐进破坏过程的数值模拟研究。该方法以连续介质力学的基本理论为基础,以八节点正六面体单元为离散对象,根据有限元的形函数理论及正六面体单元刚度矩阵的具体表达式,将正六面体单元离散成为12根棱弹簧,并推导各弹簧的弹簧力计算公式,给出各弹簧的弹簧力分量的物理意义、对应的弹簧刚度及相关联的位移。在弹性部分的计算结果与传统的有限元计算结果一致的基础上,在弹簧中引入Mohr-Coulomb准则及拉伸破坏准则,进行弹簧破坏的判断。如果块体内的弹簧全部断裂,便将其视为散体。在判断破坏时采用双重判断模式,块体单元的判断主要用于选取弹簧潜在的破坏面,离散弹簧的判断用于计算弹簧上的真实弹簧力。最后,通过相关算例验证引入破坏准则后所得计算结果的合理性。块体单元离散弹簧法的实质是通过12根离散弹簧将单元转化为结构,因此可以通过分析各弹簧的断裂情况研究块体的内部破坏特征,更可以通过各弹簧的先后断裂过程来研究地质体的渐进破坏过程。

Pion Production in Heavy-ion Collisions in the 1 A GeV region

Within the framework of the improved isospin dependent quantum molecular dynamics (ImIQMD) model, the pion emission in heavy-ion collisions in the region 1AGeV is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances ∆(1232) and N∗(1440). The in-medium dependence and Coulomb effects of the pion production are included in the calculation. Total pion multiplicity and π−/π+ yields are calculated for the reaction 197Au+197Au in central collisions for selected Skyrme parameters SkP, SLy6,Ska, SIII and compared them with the measured data by the FOPI collaboration.

Fusion dynamics of symmetric systems near barrier energies

Within the framework of the improved isospin-dependent quantum molecular dynamics (ImIQMD) model,he fusion dynamics of symmetric reaction systems are investigated systematically. Calculations show that the number of nucleon transfer in the neck region is appreciably dependent on the incident energies, but strongly on he reaction systems. A comparison of the neck dynamics is performed for the symmetric reactions 58Ni+58Niand 64Ni+64Ni at energies in the vicinity of the Coulomb barrier. An increase of the ratios of the neutron to proton in the neck region at initial collision stage is observed and obvious for the latter system, which reduces the fusion barrier of two colliding nuclei. The distribution of the dynamical fusion barriers and the fusion excitation functions are calculated and compared with the available experimental data.

Pion Production in Heavy-Ion Collisions in the 1 A GeV Region

Within the framework of the improved isospin dependent quantum molecular dynamics (ImIQMD) model, pion emission in heavy-ion collisions in the region 1 A GeV is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances Delta(1232) and N*(1440). The in-medium dependence and Coulomb effects of pion production are included in the calculation. Total pion multiplicity and pi(-)/pi(+) yields are calculated for the reaction Au-197+(197) Au in central collisions for selected Skyrme parameters SkP, SLy6, Ska, SIII and compared with the measured data of the FOPI collaboration.

Sputtering induced by highly charged Pb-208(q+) bombardment on Al surface

Highly charged ions (HCls) carrying high Coulomb potential energy (E-p) could cause great changes in the physical and chemical properties of material surface when they bombard on the solid surface. In our work, the secondary ion yield dependence on highly charged Pbq+ (q = 4-36) bombardment on Al surface has been investigated. Aluminum films (99.99%) covered with a natural oxide film was chosen as our target and the kinetic energy (E-k) was varied between 80 keV and 400 keV. The yield with different incident angles could be described well by the equation developed by us. The equation consists of two parts due to the kinetic sputtering and potential sputtering. The physical interpretations of the coefficients in the said equation are discussed. Also the results on the kinetic sputtering produced by the nuclear energy loss on target Surface are presented.