CORRELATION EFFECTS ON THE GROUNDSTATES OF THREE-ELECTRON QUANTUM DOTS IN A STRONG MAGNETIC FIELD

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NEGATIVE DONOR CENTER QUANTUM DOTS IN MAGNETIC FIELDS

The method of few-body physics is applied to treat a D^{-<\sup> center quantum dot system in a magnetic field. The magnetic field is applied in the z direction. Using this method, we investigate the energy spectra of low-lying states of D-<\sup> center quantum dots as a function of magnetic field. The dependence of the binding energies of the ground-state of the D-<\sup> center are calculated as a function of the dot radius with a few values of the magnetic field strength and compared with other results.}     

用B样条技术研究半导体微晶中激子的量子受限效应

在有效质量近似（EMA）下，采用B样条技术和变分方法，分别研究较大CdTe球量子点(25—35nm)和较小CdS球量子点(025—35nm)中激子的量子受限效应，计算出CdTe和CdS球量子点中受限激子的基态能和束缚能随参数的变化规律，比较两种计算结果得到：（1）较大量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数不敏感，但较小量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数比较敏感.（2）在较强受限区域，大量子点与小量子点的激子基态能和束缚能的变化规律完全不同.（3）B样条技术对于研究这种具有边界的束缚态系统是很精确的方法，这种方法特别适合用于多层结构量子点系统的精确计算. 关键词： B样条技术 量子受限效应 有效质量近似     

Vortex formation in quantum dots in high magnetic fields

We study electronic structures of two-dimensional quantum dots in high magnetic fields using the density-functional theory (DFT) and the exact diagonalization (ED). With increasing magnetic field, beyond the formation of the totally spin-polarized maximum density droplet (MDD) state, the DFT gives the ground-state total angular momentum as a continuous function with well-defined plateaus. The plateaus agree well with the magic angular momenta of the ED calculation. By constructing a conditional wave function from the Kohn–Sham states we show that vortices enter the quantum dot one-by-one at the transition to the state with the adjacent magic angular momentum. Vortices are also observed outside the high-density region of the quantum dot. These findings are compared to the ED results and we report a significant agreement. We also study interpretations and limitations of the density functional approach in these calculations.     

Off—Center D—Centers in a Quantum Dot in the Presence of a Perpendicular Magnetic Fields

We investigate the effect of the position of the donor in quantum dots on the energy spectrum in the presence of a perpendicular magnetic field by using the method of few-body physics,As a function of the magnetic field,we find,when D^- centers are placed sufficiently off-center,discontinuous ground-state transitions which are similar to those found in many-electron parabolic quantum dots.Series of magic numbers of angular momentum which minimize the ground-state electron-electron interaction energy have been discovered.The dependence of the binding energy of the gound-state of the D^- center on the dot radius for a few values of the magnetic field strength is obtained and compared with other results.     

Impurity-related electronic properties in quantum dots under electric and magnetic fields

This paper presents a systematic study of the ground-state binding energies of a hydrogenic impurity in quantum dots subjected to external electric and magnetic fields.The quantum dot is modeled by superposing a lateral parabolic potential,a Gaussian potential and the energies are calculated via the finite-difference method within the effectivemass approximation.The variation of the binding energy with the lateral confinement,external field,position of the impurity,and quantum-size is studied in detail.All these factors lead to complicated binding energies of the donor,and the following results are found:(1) the binding energies of the donor increase with the increasing magnetic strength and lateral confinement,and reduce with the increasing electric strength and the dot size;(2) there is a maximum value of the binding energies as the impurity placed in different positions along the z direction;(3) the electric field destroys the symmetric behaviour of the donor binding energies as the position of the impurity.     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

Spezifische Wärme der Nitride Seltener Erden

The specific heat of rare earth nitrides has been measured at temperatures between 1.7 ans 270°K by means of an adiabatic calorimeter. The various terms contributing to the total specific heat have been separated. The total splitting of the ground-state multiplet of the trivalent RE-ions in the octahedral crystal field has been determined. The experimental values agree reasonably well with those calculated with the point charge model. The spontaneous magnetization and the magnetic specific heat below the ordering temperature are calculated with the molecular field theory by taking into account all the 2J+1 energy levels of the ground-state multiplet and including exchange interaction and crystal field splitting. Comparision is made with the experimental results, and the values obtained for the exchange energies are listed.     

Two-Electron Energy Spectrum in a Parabolic Quantum Dot Under a Magnetic Field

Two interacting electrons in a harmonic oscillator potential under the influence of a perpendicular homo-geneous magnetic field are considered. The energies of two-electron quantum dots with the electron-LO-phonon couplingas a function of magnetic field are calculated. Calculations are made by using the method of few-body physics withinthe effective-mass approximation. Our results show that the electron-LO-phonon coupling effect is very important insemiconductor quantum dots.     

Magnetoexcitons in core/shell quantum dots

The ground-state energies of the “bright” and “dark” excitons formed by an electron and a hole localized in a thin spherical shell subjected to a high magnetic field are calculated. This model corresponds to a core/shell quantum dot. The high-field condition implies that the magnetic length is much shorter than the radius of the sphere. It is found that the ground-state energy of the bright exciton exhibits an unusual magnetic-field dependence: E ₀ ～ H ^2/3.     

Ground State Transitions in Vertically Coupled Four-Layer Single Electron Quantum Dots

We study a four-electron system in a vertically coupled four-layer quantum dot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupled quantum dots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.     

Ground State Transitions in Vertically Coupled Four-Layer Single Electron Quantum Dots

We study a four-electron system in a vertically coupled four-layer quantum dot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupled quantum dots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.     

Theoretical study of quantum confined Stark shift in InAs／GaAs quantum dots

The quantum confined Stark effect (QCSE) of the self-assembled InAs/GaAs quantum dots has been investigated theoretically. The ground-state transition energies for quantum dots in the shape of a cube, pyramid or “truncated pyramid” are calculated and analysed. We use a method based on the Green function technique for calculating thestrain in quantum dots and an efficient plane-wave envelope-function technique to determine the ground-state electronic structure of them with different shapes. The symmetry of quantum dots is broken by the effect of strain. So the properties of carriers show different behaviours from the traditional quantum device. Based on these results, we also calculate permanent built-in dipole moments and compare them with recent experimental data. Our results demonstrate that the measured Stark effect in self-assembled InAs/GaAs quantum dot structures can be explained by including linear grading.     

透镜型量子点中类氢杂质基态能的计算

通过有效质量近似和变分法，研究了垂直磁场下透镜型量子点中类氢杂质基态能量，并与球型量子点进行了比较.研究表明：对于球型量子点，基态能仅与杂质的偏离距离有关，与垂直和水平偏离无关；而对于透镜型量子点，由于水平方向和垂直方向束缚势的非对称性，电子基态能不仅与杂质的偏离距离有关，还与杂质偏离方向有关. 关键词： 透镜型量子点 基态能 变分法     

压力下GaN/Ga1-xAlxN量子点中杂质态的界面效应

考虑界面处导带弯曲,流体静压力以及有效质量随量子点位置的依赖性,采用变分法以及简化相干势近似,研究了无限高势垒GaN/Ga_1-xAl_xN球形量子点中杂质态的界面效应,计算了杂质态结合能随量子点尺寸、电子面密度以及压力的变化关系。结果表明,结合能随压力的增大呈线性增加的趋势,有效质量位置的依赖性以及导带弯曲对结合能有不容忽视的影响。     

Magneto-optics of zero-dimensional electron systems

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a -layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground-state energy of interacting electrons in the quantum dots. For the strain-confined 0DESs we have mapped the magneto-dispersion of the quantum confined electron states to reveal 15 electrons per dot.     

Strain and optical transitions in InAs quantum dots on (001) GaAs

To investigate the strain characteristics of InAs quantum dots grown on (001) GaAs by solid source molecular beam epitaxy we have compared calculated transition energies with those obtained from photoluminescence measurements. Atomic force microscopy shows the typical lateral size of the quantum dots as 20–22 nm with a height of 10–12 nm, and photoluminescence spectra show strong emission at 1.26 μ m when the sample is capped with a GaAs layer. The luminescence peak wavelength is red-shifted to 1.33 μ m when the dots are capped by an In_0.4Ga_0.6As layer. Excluding the strain it is shown that the theoretical expectation of the ground-state optical transition energy is only 0.566 eV (2.19 μ m), whereas a model with three-dimensionally-distributed strain results in a transition energy of 0.989 eV (1.25 μ m). It has thus been concluded that the InAs quantum dot is spatially strained. The InGaAs capping layer reduces the effective barrier height so that the transition energy becomes red-shifted.     

Spectroscopy of molecular states in a few-electron double quantum dot

Semiconductor quantum dots, so-called artificial atoms, have attracted considerable interest as mesoscopic model systems and prospective building blocks of the “quantum computer”. Electrons are trapped locally in quantum dots, forming controllable and coherent mesoscopic atom- and moleculelike systems. Electrostatic definition of quantum dots by use of top gates on a GaAs/AlGaAs heterostructure allows wide variation of the potential in the underlying two-dimensional electron gas. By distorting the trapping potential of a single quantum dot, a strongly tunnel-coupled double quantum dot can be defined. Transport spectroscopy measurements on such a system charged with N=0,1,2,… electrons are presented. In particular, the tunnel splitting of the double well potential for up to one trapped electron is unambiguously identified. It becomes visible as a pronounced level anticrossing at finite source drain voltage. A magnetic field perpendicular to the two-dimensional electron gas also modulates the orbital excitation energies in each individual dot. By tuning the asymmetry of the double well potential at finite magnetic field the chemical potentials of an excited state of one of the quantum dots and the ground state of the other quantum dot can be aligned, resulting in a second level anticrossing with a larger tunnel splitting. In addition, data on the two-electron transport spectrum are presented.     

Electronic phase diagrams of carriers in self-assembled quantum dots: violation of Hund's rule and the Aufbau principle for holes

We study the ground-state orbital and spin configurations of up to six electrons or holes loaded into self-assembled quantum dots. We use a general phase-diagram approach constructed from single-particle pseudopotential and many-particle configuration interaction methods. The predicted hole charging energies agree with recent charging experiments, but offer a different interpretation: we find that while the charging of electrons follows both Hund's rule and the Aufbau principle, the charging of holes follows a nontrivial charging pattern which violates both the Aufbau principle and Hund's rule.     

“Horizontal” and “vertical” quantum-dot molecules

An analysis is made of a two-electron system of two adjacent quantum dots (QD) with a two-dimensional parabolic lateral confining potential, and of two coupled or double, spatially separated QDs (a “horizontal” and a “vertical” QD molecule), and of their behavior in an external transverse magnetic field. The ground-state energies and energy spectra of the system have been determined by various methods (Heitler-London, molecular-orbital, variational approach, and numerical diagonalization of Hamiltonian), with inclusion of electron-electron interaction, and for a broad range of confining-potential steepness, QD-center spacing (interlayer distance), and external magnetic field. Fiz. Tverd. Tela (St. Petersburg) 40, 2127–2133 (November 1998)