Investigation of Dynamical Parameters of Exciton Confined in CdS Spherical Quantum Dots with a B—Spline Technique

Exciton energies as a function of radii of quantum dots in the range of 5-35 A are calculated based on effective mass approximation model with the B-spline technique and compared with experimental and other theoretical data for the CdS dots. This method leads to accurate and fast convergent exciton energy, which are in good agreement with experimental data in the whole confinement regime. The effect of penetration of wave function from the inside to the outside of the dots and the effect of dielectric constants are taken into account. The magnitudes of dynamical parameters are discussed. It is found that the different materials surrounding the CdS quantum dot affect not only the potential energy and Coulomb interaction energy of the system, but also the effective masses. The comparison shows that the effective mass approximation model can describe very well the quantum size effects observed experimentally on the exciton ground state energy.

医用增感屏组合使用的研究与应用

X线摄影是近代医疗诊断不可缺少的一个重要手段,但X线辐射对人体有很大的损害。X光胶片对可见的蓝紫光具有强烈吸收作用,但对X射线的吸收量却很少。在X线摄影时,如使用增感屏,则增感屏上发生的荧光。

Effects of Impurity on the Ground—State Transitions of a Quantum Dot in Strong Magnetic Field

The low-lying spectra of parabolic quantum dots with or without an impurity at the center are investigated. While it has been known that the electron-electron interaction leads to ground-state transitions on magic values of angular momentum in a magnetic field. We show, in this paper, that the implantation of an impurity ion at the center can either enhance or suppress such transitions, depending on whether it is an acceptor or a donor ion.

Effect of the Electron—Phonon Coupling on Barrier D^— Quantum Dots in Magnetic Fields

The influence of the electron-phonon coupling on the energy of low-lying states of the barrier D- center,which consists of a positive ion located on the z-axis at a distance from the two-dimensional quantum dot plane and two electrons in the dot plane bound by the ion, is investigated at arbitrary strength of magnetic field by making use of the method of few-body physics. Discontinuous ground-state energy transitions induced by the magnetic field are reported.The dependence of the binding energy of the D- ground state on the quantum dot radius is obtained. A considerable enhancement of the binding is found for the D- ground state, which results from the confinement of electrons and electron-phonon coupling.

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 ofa 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 ground-state of the 1D- center on the dot radius for a few values of the magnetic field strength is obtained and compared with other results.

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.

Implementation of Quantum Fourier Transform and Its Applications via Quantum-Dot Spins and Microcavity

A scheme for implementing discrete quantum Fourier transform is proposed via quantum dots embeddedin a microcavity,and then some of its applications are investigated,i.e.,Deutsch-Jozsa.algorithm and Shot's quantumfactoring.In particular,the detailed process of implementing one-qubit Deutsch Jozsa algorithm and the factorization ofN=15 are given.The microcavity mode is only virtually excited in the whole interaction,so the effective decoherent hasslight effect on the current scheme.These schemes would be an important step to fabricate a solid quantum computer.

In—Phase Resonances with Generic Transmission Zeros and Eigenvectors of Hamiltonian in Models of Single Channel Transport

We investigate the phase coherent transport in a single channel system. The theory that the transmission zeros lead to abrupt phase change and in-phase resonances is confirmed numerically in two tight-binding models. After calculating the eigenvalues and eigenvectors of the Harniltonians we also confirmed that the same symmetry of the eigenvectors also leads to the abrupt phase change and in-phase resonances that equal the transmission zero.

Andreev Tunneling Through a Ferromagnet／Quantum－Dot／Superconductor System

We study Andreev tunneling through a ferromagnet/quantum-dot (QD)/superconductor system. By usingnonequilibrum Green function method, the averaged occupation of electrons in QD and the Andreev tunneling currentare studied. Comparing to the norma-metal/quantum-dot/superconductor, the system shows significant changes: (i)The averaged occupations of spin-up and spin-down electrons are not equal. (ii) With the increase of the polarizationof ferromagnetic lead, the Andreev reflection current decreases. (iii) However, even the ferromagnetic lead reaches fullpolarization, the averaged occupation of spin-down electrons is not zero. The physics of these changes is discussed.

The Role of Silion Oxide Layers in Luminescence of Ensembles of Silicon Quantum Dots

Based on the quantum confinement-luminesecence center model,to ensembles of spherical silicon nanocrystals (nc-Si)containg two kinds of luminescence centers(LCs) in the SiOx layers surrounding the nc-Si,the relationship between the photolumincescence(PL) and the thickness of the SiOx layer is studied with the excitation energy flux density as a parameter.When there is no SiOx layer surrounding the nc-Si,the electron-heavy hole pair can only recombine inside the nc-Si,then the PL bluehift with reducing particle sizes roughly accords with the rule predicted by the quantum confinement model of Canham.When there presences a SiOx layer,some of the carriers may tunnel into it and recombine outside the nc-Si at the LCs to emit visible light.The thicker the SiOx layer is,the higher the radiative recombination rate occurred outside the nc-Si will be.When the central Scale of the nc-Si is much smaller than the critical scale,the radiative recombination rate outside the nc-Si dominates,and visible PL will be possible for some nc-Si samples with big average radius,greater than 4nm,for example.When there is only one kind of LC in the SIOx layer,the PL peak position does not shift with reducing particle sizes.All these conclusions are in accord with the experimental results.When there are two or more kinds of LCs in the SiOx layer,the PL peak position energy and intensity swing with reducing particle sizes.

Effect of the Electron－LO－Phonon Coupling on an Exciton Quantum Dot

The influence of the electron-LO-phonon coupling on energy spectrum of the low-lying states ofan exciton inparabolic quantum dots is investigated as a function of dot size. Calculations are made by using the method of few-bodyphysics within the effective-mass approximation. A considerable decrease of the energy in the stronger confinement rangeis found for the low-lying states of an exciton in quantum dots, which results from the confinement of electron-phononcoupling.

Weakly Coupled Three—Layer Quantum Dot with a Charged Impurity in Magnetie Field

The states of a weakly coupled 3-quantum-dot system with an external charged impurity located on the z-axis are studied in a magnetic field.The evolutions of the true ground state with the magnetic field B are botained for various impurity cases.It is found that the negative charge impurity would promote the phase transition of the true ground state.

Asymmetric Double Quantum Dots as a Subminiature Mesoscopic Cell

A subminiature mesoscopic cell, consisting of asymmetric double quantum dots capacitively coupled to a nearby mesoscopic circuit, is proposed, which can transform disordered noise energy to ordered electric energy. Two schemes, the noises originating from the nearby mesoscopic circuit and from the electromagnetic wave disturbance in external environment, are investigated. We found that the proposed cell can manifest as a good constant current source and the output current may not reach its largest value even if the circuit is shorted.