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.     

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.     

Dynamical Localization in an Asymmetric Two-Electron Quantum Dot Molecule by an Alternating-Current Electric Field

We investigate the dynamics of two interacting electrons in an asymmetric double coupled quantum dot under an ac electric field. The numerical results demonstrate that dynamical localization and Rabi oscillation still exist in such a system under the stronger electron correlation. The two electrons can be regarded as a quasiparticle, which move together between two dots similarly to a boson. The dynamics of two electrons in such a quantum system are mainly confined in a Q subspace, which is constructed by two double-occupied states.     

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 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 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.     

Dynamic Localization Condition of Two Electrons in a Strong dc-ac Biased Quantum Dot Molecule

We present a perturbation investigation of dynamic localization condition of two electrons in a strong dc-ac biased quantum dot molecule.By reducing the system to an Hubbard-type effective two-site model and by applying Floquet theory,we find that the dynamical localization phenomenon occurs under certain values of the large strength of the dc and ac field.This demonstrates the possibility of using appropriate dc-ac fields to manipulate dynamical localized states in mesoscopic devices,which is an essential component of practical schemesfor quantum information processing.Our conclusion is instructive to the field of quantum function devices.     

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.     

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 of an exciton in parabolic quantum dots is investigated as a function of dot size.Calculations are made by using the method of few-body physics within the effective-mass approximation.A considerable decrease of the energy in the stronger confinement range is found for the low-lying states of an exction in quantum dots.Which results from the confinement of electron-phonon coupling.     

Dynamical Localization in a Two—Electron Quantum Dot Molecule Biased by a dc Voltage

We study the dynamics of two interacting electrons in a coupled-quantum-dot system with a time-dependent external electric field. The numerical results of the two-particle states reveal that the dynamical localization still exists under appropriate dc and ac voltage amplitudes. Such localization is different from the stationary localization phenomenon. Our conclusion is instructive for the field of quantum function devices.     

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.     

Quantum Dynamics of One—Dimensional Nanocrystalline Solids

A novel ballistic-nonballistic dynamic transition in one-dimensional nanocrystalline solids is found upon varying the strength of the composition modulation and the grain-boundary effect.This can contribute to the understanding of the strange electronic transport porperties of nanostructured systems.     

Coherent Transport Through a Quantum Dot Embedded in a Double—Slit—Like Aharonov—Bohm Ring

Coherent transport through a quantum dot embedded in one arm of a double-slit-like Aharononv-Bohm(AB) ring is studied using the green‘s function approach.We obtain experimental obsevations such as continous phase shift along a single resonance peak and sharp inter-resonance phase drop.The AB oscillations of the differential conductance of the whole device are calculated by using the nonequilibrium Keldysh formalism.It is shown that the oscillating conductance has a continuous bias-voltage-dependent phase shift and is asymmetric in both linear and nonlinear response regimes.     

Binding Energies of Negatively Charged Donors in a Gaussian Potential Quantum Dot

We investigate a negatively charged donor centre (D^-) trapped by a quantum dot, which is subjected to a Gaussian potential confinement. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian within the effective-mass approximation. The dependence of the ground state of the negatively charged donor on the dot size and the potential depth is studied. The same calculations performed with the parabolic approximation of the Gaussian potential lead to the results that are qualitatively and quantitatively different.     

Photon—Storage in Optical Memory Cells Based on a Semiconductor Quantum Dot—Quantum Well Hybrid Structure

We report a new type of photonic memory cell based on a semiconductor quantum dot (QD)--quantum well (QW) hybrid structure, in which photo-generated excitons can be decomposed into separated electrons and holes, and stored in QW and QDs respectively. Storage and retrieval of photonic signals are verified by time-resolved photoluminescence experiments. A storage time in excess of l00ms has been obtained at a temperature of 10K while the switching speed reaches the order of ten megahertz.     

An Exciton bound to a Neutral Donor in Quantum Dots

The binding energies for an exciton(X) trapped in a two-dimensional quantum dot by a neutral donor have been calculated using the method of few-body physics for the heavy hole(σ=0.196) and the light hole (σ=0.707).We find that the (D^0,X) complex confined in a quantum dot has in general a larger binding energy than those in a two-dimensional quantum well and a three-dimensional bulk semiconductor,and the binding energy than those in a two-dimensional quantum well and a three-dimensional bulk semiconductor,and the binding energy increases with the decrease of the dot radius.At dot radius R→∞,we compare our calculated result with the previous results.     

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.     

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.     

Quantum Poincare Section of a Two—Dimensional Hamiltonian in a Coherent State Representation

We study the quantum behaviour of a quasi-integrable Hamiltonian.The unperturbed Hamiltonian displays degeneracies of energy levels,which become avoided crossing under a nonintegrable perturbation.In this two-dimensional system,the quantum Poincare section plot is constructed in the coherent state representation with the restriction that the centres of the wavepackets are confined at thd classical surface of constant energy.It is found that the quantum Poincare section plot obtained in this way provides an evident counterpart of the classical system.     

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.