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

The influence of the electron-phonon coupling of the energy of low-lying states of the barrier D^- center,which consists of a positive ion located on the z-azis 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 maguetic field by mading 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.     

Effects of Electric Field on the Valence—Bond Property of an Electron in a Quantum—Dot Molecule

The electronic structure of the quantum-dot molecules in an electric field is investigated by the finite element method with the effective mass approximation.The numerical calculation results show that the valence bond of the quantum-dot molecule alternates between covalent bonds and ionic bonds as the electric field increases.The valence-bond property can be reflected by the oscillator strength of the intraband transition.The bound state with the highest energy level in the quantum-dot molecule gradually changes into a quasibound state when the electric field increases.     

Exciton—Phonon Scattering in CdSe／ZnSe Quantum Dots

A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantum dots with a ZnCdSe quantum well.We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantum dots and two-dimensional exciton in the CdSe wetting layer.The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantum dots in comparison with that in the two-dimensional wetting layer,which indicates the decrease of exciton and optical phonon coupling in the CdSe quantum dots.     

Polar Interface Optical Phonon Modes and Froehlich Electron－Phonon Interaction Hamiltonians in an Arbitrary Layer－Number Quantum Well System

By using determinant method as in our recent work, the IO phonon modes, the orthogonal relation for polarization vector, electron-IO phonon F~6hlich interaction Hamiltonian, the dispersion relation, and the electron-phonon coupling function in an arbitrary layer-number quantum well system have been derived and investigated within the framework of dielectric continuum approximation. Numerical calculation on seven-layer AlxGal-xAs/GaAs systems have been performed. Via the numerical results in this work and previous works, the general characters of the IO phonon modes in an n-layer coupling quantum well system were concluded and summarized. This work can be regarded as a generalization of previous works on IO phonon modes in some fLxed layer-number quantum well systems, and it provides a uniform method to investittate the effects of IO phonons on the multi-layer coupling quantum well systems.     

Rabi Oscillation of Exciton Dressed by Phonons In a Quantum Dot

The effect of strong exciton-phonon interaction on the excitonic Rabi oscillations in a coherently driven quantum dot in a high-Q single mode cavity is investigated theoretically. We show that the Rabi oscillation of exciton dressed by phonons can persists with the Rabi frequency ge^-λ/2 at absolute zero temperature, where g is the single-photon Rabi frequency and λ is the Huang-Rhys factor. The results also present that such coherent oscillations can be modified by manipulating the Rabi frequency of the driving field.     

Electron States of Few—Electron Quantum dots

We study few-electron semiconductor quantum dots using the unrestricted Hartree-Fock-Roothaan method hased on the Gaussian basis.Our emphasis is on the energy level calculation for quantum dots.The confinement potential in a quantum dot is assumed to be in a form of three-dimensional spherical finite potential well.Some valuable results,such as the rearrangement of the energy level,have been obtained.     

Effects of Magnetic Field on the Valence Bond Property of the Double—Quantum—Dot Molecule

The effects of the magnetic field on the valence bond property of the double-quantum-dot molecule are numerically studied by the finite element method and perturbation approach because of the absence of cylindrical symmetry in the horizontally coupled dots.The calculation results show that the energy value of the ground state changes differently from that of the first excited state with increasing magnetic field strength,and they cross under a certain magnetic field.The increasing magnetic field makes the covalent bond state change into an ionic bond state,which agrees qualitatively with experimental results and and makes ionic bond states remain.The oscillator strength of transition between covalent bond states decreases distinctly with the increasing magnetic field strength,when the molecule is irradiated by polarized light.Such a phenomenon is possibly useful for actual applications.     

Effects of Conduction Electron Band Structure on Transport of Quantum Dot Systems

We study the effects of the energy band structure of conduction-electron on the transport properties of an interacting quantum dot system.By applying the nonequilibrium Keldysh Green function technique,we show that the finte width of electron band in leads causes the negative differential conductance in some regions of the applied voltage.We also show that the van Hove singulartites in the density of stated of conduction-electreon do not qualitatively change the differential conductance of the system,and hence can be safely ignored.Therefore,the wide band approximation used in the previous investigations is partially justified.     

Electron—Phonon Coupling in Anion Metallic Solids and Superconducting MgB2

We analyse the Hopfield factor of the newly found superconductor MgB2 using the linear muffin-tin orbital method.Based on a uniform transferred charge density model raised,it is shown from our calculation that the high difference of electronegativity between boron and magnesium favours the high l(l 1) angular momentum hybridization and then the Hopfield factor.Our analysis is consistent with experimental results.Comparisons with cuprate superconductors are also discussed.     

Strain and Phonon Confinement in Self－Assembled Ge Quantum Dot Superlattices Superlattices ＊

Raman scattering measurements were carried out in self-assembled Ge quantum dot superlattices grown bymolecular beam epitaxy. The characteristics of the Ge-Ge, Si-Ge, Si-SiLoc and Si-Si peaks were investigated,especially the Ge-Ge optical phonon frequency shift was emphasized, which was tuned by the phonon confinementand strain effects. The experimentally observed frequency shift values of the Ge-Ge peak frequency caused byoptical phonon confinement and strain in Ge quantum dots were discussed with quantitative calculations.     

Dynamic Localization of a One—Dimensional Quantum Dot Array in an ac Electric Field

We investigate the dynamics of two interaction electrons confined to one-dimensional quantum dot array in an ac electric field.We find that initially localized electrons will remain localized in the absence of coulomb interaction if the ratio of the ac field magnitude to the frequency is a root of the ordinary zero-order Bessel function.In contrast to the case without Coulomb interaction,no matter what the value is,the electrons are delocalized and the delocalization effect depends on the ratio U/ω and eaE/ω,where U is the strength of Coulomb interaction,a is the lattice constant,and E and ω are the ac field amplitude and frequency,respectively.     

Transition of the True Ground State in a Coupled Three—Layer Quantum Dot

Low-lying states of a vertically coupled three-layer quantum-dot system are studied.Each layer contains one electron,and the tunnelling of electrons between layers is neglected.Effects of the interlayer separation d and the external magnetic field B are evaluated by numerical calculations.In the strong coupling case (i.e.d is small),as in a single dot,transitions of the angular momentum L of the true ground states occur when B increases, whereas in the weak coupling case the transition does not occur and L remains zero.Furthermore,it is found that the variation of d may also induce the L transition.As a result,a phase diagram of L of the true ground state is given in the d-B plane.     

The Electron－Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupled quantum dot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.     

Role of Interactions in Electronic Structure of a Two-Electron Quantum Dot Molecule

We have studied a two-electron quantum dot molecule in a magnetic field. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate two lowest energy levels of the two-electron quantum dot molecule in a magnetic field. Our results show that the electron interactions are significant, as they can change the total spin of the two-electron ground state of the system by adjusting the magnetic field between S = 0 and S = 1. The energy difference AE between the lowest S = 0 and S = 1 states is shown as a function of the axial magnetic field. We found that the energy difference between the lowest S = 0 and S = 1 states in the strong-B S = 0 state varies linearly. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.     

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.     

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.     

Electron-Phonon Interactions on the Optical Kerr Effect in a Quantum Disc

The optical Kerr effect in a quantum disc is investigated with the density-matrix approach, most emphasis is devoted to the electron-phonon interactions on the optical Kerr effect. The numerical results are presented with characteristic parameters of a GaAs quantum disc. It is shown that the optical Kerr effect increases as the disc thickness D decreases. LO-phonons impose greater influence on the optical Kerr effect than SO-phonons. The smaller the disc thickness D, the sharper the peak, and the larger the peak intensity. When the disc thickness is greater than 30, the peak will disappear gradually.     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Quantum—Confined Stark Effect of Vertically Stacked Self—Assembled Quantum Discs

We study the electronic energy levels and probability distribution of vertically stacked self-assembled InAs quantum discs system in the presence of a vertically applied electric field. This field is found to increase the splitting between the symmetric and antisymmetric levels for the same angular momentum. The field along the direction from one disc to another affects the electronic energy levels similarly as that in the opposite direction because the two discs are identical. It is obvious from our calculation that the probability of finding an electron in one disc becomes larger when the field points from this disc to the other one.     

柱形量子线中极化子的电子与LO声子之间相互作用能

采用变分法，研究了柱形量子线中在考虑电子与LO声子相互作用的情况下，极化子在基态时系统的能量以及电子-LO声子之间的相互作用能。数值计算结果表明：随着柱形量子线截面半径的减小，基态能量和电子-LO声子相互作用能的绝对值都增大。