Effect of Shape and Size on Electron Transition Energies for Nanoscale InAs/GaAs Quantum Rings

In this paper, we study the impact of the sizes and the shapes of nanoscale semiconductor quantum rings on the electron and hole energy states. A three-dimensional effective one band Schrödinger equation is solved numerically for semiconductor quantum rings with disk, cut-bottom-elliptical, and conical shapes. For small InAs/GaAs quantum rings we have found a sufficient difference in the ground state and excited state (l = ?1) electron energies for rings with the same volume but different shapes. Volume dependence of the electron and hole energies can vary over a wide range and depends significantly on the ring shapes. It is found that a non-periodical oscillation of the energy band gap between the lowest electron and hole states as a function of external magnetic fields.     

Numerical Calculation of Electronic Structure for Three-Dimensional Nanoscale Semiconductor Quantum Dots and Rings

In this paper the electronic structure of nanoscale ellipsoid-torus-shaped semiconductor quantum dot and quantum ring is investigated of utilizing a unified model. This three-dimensional model considers the effective one-band Hamiltonian, the position- and energy-dependent effective mass approximation and Landé factor, the finite hard wall confinement potential, and the Ben Daniel-Duke boundary conditions. It is solved numerically without any fitting parameters by using a computationally cost effective nonlinear iterative method. It is found that the penetration of magnetic fields into non-simply connected topology of structures leads to substantial difference in the transition energy between InAs/GaAs quantum dot and quantum ring. The quantum ring exhibits non-periodical electron-hole transition energies when the magnetic field increases. Contrary to the one-dimensional periodical argument on the ring's energy spectra, our examination into the nanoscale semiconductor quantum ring agrees with the experimental result. The energy band gap of quantum dot is an increasing function of the magnetic field. For quantum rings the energy band gap oscillates non-periodically and the oscillation period is strongly controlled by the inner radius of structures. The magnetization of quantum ring not only jumps non-periodically but also saturates eventually when the magnetic field increases.     

Magnetization and Magnetic Susceptibility in Nanoscale Vertically Coupled Semiconductor Quantum Rings

In this paper we computationally examine the magnetization and the magnetic susceptibility for vertically coupled quantum rings (VCQRs) under applied magnetic fields. The theoretical model of VCQRs considers a three-dimensional (3D) effective one-electronic-band Hamiltonian with the position- and energy-dependent effective mass, the finite hard-wall confinement potential, and the Ben Daniel-Duke boundary condition. The nonlinear iterative method is applied to solve the problem in the structure of VCQRs. For the structure formed with nanoscale disk-shaped InAs/GaAs quantum rings, the the tunable states of structure as well as the electron transition energy is dominated by the radius of ring (R) and the inter-distance (d) between quantum rings. The electron energy oscillates non-periodically among the lowest electron states as a function of external magnetic fields due to the penetration of magnetic fields into the inter-regions of VCQRs. The magnetization of VCQRs at zero temperature is non-periodical oscillation and the period of jump is governed by R. Therefore, the differential susceptibility of VCQRs has delta-like paramagnetic peaks. When d is increased, the peak is decreased which is contrary to conventional mesoscopic arguments. Our investigation is constructive for studying the magneto-optical phenomena of the nanoscale semiconductor artificial molecules.     

The influence of the zigzag and armchair leads on quantum transport through the graphene based quantum rings

The conductivity of a graphene ring with two semi-infinite, armchair and zigzag leads has been investigated. We have performed numerical calculations based on the nearest neighbor tight-binding Hamiltonian and Dirac point approximation. A Non-Equilibrium Green’s Function (NEGF) approach has been employed to calculate the electric current under an applied bias voltage, in this two terminal mesoscopic system. We have studied the effect of the external magnetic field on transport characteristics of this graphene-based quantum ring. Coherent transport features of the system have been studied. It was shown that there is significant distinction between the I–V characteristics (and also the Aharonov-Bohm effect) of the graphene rings depending on the edge structure of the leads.     

A multi-purpose Schrödinger-Poisson Solver for TCAD applications

We present the Vienna Schrödinger-Poisson Solver (VSP), a multi-purpose quantum mechanical solver for investigations on nano-scaled device structures. VSP includes a quantum mechanical solver for closed as well as open boundary problems on fairly arbitrary one-dimensional cross sections within the effective mass framework. For investigations on novel gate dielectrics VSP holds models for bulk and interface trap charges, and direct and trap assisted tunneling. Hetero-structured semiconductor devices, like resonant tunneling diodes (RTD), can be treated within the closed boundary model for quick estimation of resonant energy levels. The open boundary model allows evaluation of current voltage characteristics.     

The theory of non-Markovian gain in semiconductor lasers

In this paper, non-Markovian optical gain of a semiconductor laser is derived from recently developed time convolutionless (TCL) quantum kinetic equations for electron-hole pairs, including the many body effects. Plasma screening and excitonic effects are taken into account using an effective Hamiltonian in the time-dependent Hartree-Fock approximation. To calculate the optical gain, equation of motion for the interband pair amplitude is integrated directly. It is shown that the line shape of optical gain spectra is Gaussian for the simplest, non-Markovian quantum kinetics, and the optical gain is enhanced by the excitonic effects caused by the attractive electron-hole Coulomb interaction and the interference effects (renormalized memory effects) between the external driving field and the stochastic reservoir of the system. Enhancement of optical gain by the memory effects suggests the violation of strict energy conservation on a very short time scale, as compared with the correlation time of the system governed by non-Markovian quantum kinetics     

Oscillator strengths of the intersubband electronic transitions in the multi-layered nano-antidots with hydrogenic impurity

In this study, we have obtained the exact solutions of the Schr?dinger equation for a multi-layered quantum antidot (MLQAD) within the effective mass approximation and dielectric continuum model for the spherical symmetry. The MLQAD is nano-structured semiconductor system that consists of a spherical core (e.g. Ga_1?x Al _x As) and a coated spherical shell (e.g. Ga_1?y Al _y As) as the whole anti-dot is embedded inside a bulk material (e.g. GaAs). The dependence of the electron energy spectrum and its radial probability density on nano-system radius are studied. The numeric calculations and analysis of oscillator strength of intersubband quantum transition from the ground state into two first allowed excited states at the varying radius, for both the finite and infinite confining potential (CP) as well as constant shell thickness, are performed. It is shown that, in particular, the binding energy and the oscillator strength of the hydrogenic impurity of a MLQAD behave differently from that of a single-layered quantum antidot (SLQAD). For a MLQAD with finite core and shell CPs, the state energies and the oscillator strengths of the impurity are found to be dependent on the shell thickness. At the large core radius and very small shell thickness, our results are closer to respective values for a SLQAD that previously reported.     

Coulomb Blockade in Silicon Devices: Electronic Structure of Quantum Dots

This article presents a self-consistent 3D Poisson-Schrödinger calculation based on the density functional theory for silicon quantum dot under bias voltage. For various shapes and sizes of quantum dot surrounded by silicon dioxide, the energy levels and density are calculated as a function of the applied voltage and the number of stored electrons. The potential properties of such nanostructures for Coulomb blockade operation are deduced.     

EnergyDispersion Relations for Holes in Silicon Quantum Wells and Quantum Wires

We calculate the energy dispersion relations in Si quantum wells (QW), E(k _2D), and quantum wires (QWR), E(k _1D), focusing on the regions with negative effective mass (NEM) in the valence band. The existence of such NEM regions is a necessary condition for the current oscillations in ballistic quasineutral plasma in semiconductor structures. The frequency range of such oscillations can be extended to the terahertz region by scaling down the length of structures. Our analysis shows that silicon is a promising material for prospective NEM-based terahertz wave generators. We also found that comparing to Si QWRs, Si QWs are preferable structures for NEM-based generation in the terahertz range.     

±800kV直流二联复合绝缘子耐张串均压环参数设计

为了给±800kV直流输电线路复合绝缘子耐张串提供合理的均压环配置方案,采用场域分解的方法将三维无界场域分解成有界子区域,使用有限元方法计算了±800kV直流输电线路二联复合绝缘子耐张串的电场分布,并分别进行了均压环参数优化设计。应用ANSOFT软件建立±800kV直流线路带杆塔、导线、绝缘子耐张串的三维模型,研究了均压环的管径、环径和抬高距对耐张串电场分布的影响规律,基于控制电场强度的考虑得到了均压环结构参数的优化配置方案。有限元计算结果表明,安装了设计的均压环后,复合绝缘子护套、金具、均压环表面最大电场强度均能满足要求。