Coherent Coupling of Double Quantum Dots Embedded in a Mesoscopic Ring

We theoretically study the properties of the ground state of a series-coupled double quantum dot embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently,which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possible candidate for future device applications.     

Mesoscopic Persistent Currents,Aharonov—Bohm Magnetic Flux and Time Reversal Symmetry

We discuss the effect of Aharonov-Bohm magnetic flux on the time reversal symmetric properties of mesoscopic metallic ring systems.It is usually believed that AB flux causes time reversal symmetry breaking.We analyse the case of mesoscopic persistent currents and find out that AB flux does not break time reversal symmetry.our arguments are supported by the general theory of mesoscopic persistent currents.     

Transmission of electron through an Aharonov－Bohm interferometer with a quantum dot in Coulomb blockade regime

We calculate conductance of an Aharonov－Bohm (AB) interferometer for which a single-level quantum dot in the Coulomb blockade regime is embedded in one of its arms. Using the Schr?dinger equations and taking into account the Coulomb interaction on the dot, we calculate conductance G as a function of flux φ threaded through the ring and as a function of gate voltage V applied to the dot. It is found that the AB oscillations of G(φ) depend on the particle occupation on the dot, controlled by V. If the system is closed, there is no loss of particles, G(φ) is periodic and G(φ)=G(-φ), satisfying the Onsager relation. In this case G(φ) can reach its maximum value, 2e^2/h, at the resonance. When the system is open, one has G(φ)≠G(-φ), G(φ) yields a phase shift which depends on the loss rate of electrons in this open system.     

Tunable Kondo Effect of a Three-Terminal Transport Quantum Dot Embedded in an Aharonov-Bohm Ring

We theoretically investigate the Kondo effect of a three-terminal transport quantum dot （QD） embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. We find that in this system, the Kondo effect depends sensitively oil the parity and size of the ring; the Kondo screening cloud can be tuned by tuning the coupling strength of the reservoir-dot. Thus this model might be a candidate for future device 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.     

Persistent Current in a Mesoscopic Ring Side-Attached with a Quantum Dot

We theoretically investigate properties of the ground state of a closed dot-ring system with a magnetic flux in the Kondo regime by means of a tight-banding Anderson Hamiltonian, using the Slave-Boson mean-field approach. The persistent current shows interesting dependences on the parity and on the size of the system. The signature of Kondo resonance at ζk/L = 0.15 is expected to be observed experimentally in the future. With the intensity of the coupling tD changes from weak to strong, the properties of the system changes largely, which are different from the in-line one because of the attached dot.     

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

We study Andreev tunneling through a ferromagnet/quantum-dot(QD)/superconductor system,By using nonequilibrum Green function method.the averaged occupation of electrons in QD and the Andreev tunneling current are 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 polaryzation of ferromagnetic lead,the Andreev reflection current decreases.(iii) However,even the ferromagnetic lead reaches full polarization,the averaged occupation of spin-dowm electrons is not zero,The physics of these changes is discussed.     

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.     

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.     

Effects of Induced Flux on Andreev Levels in a Quantum Point Contact Embedded in Superconducting Ring

We investigate the effects of flux induced by persistent current on the states of Andreev levels in a quantum point contact (QPC) embedded in a superconducting ring. The Andreev levels correspond to the bound states in the QPC created by multiple reflection at the boundaries. We self-consistently calculate the eigenstates and persistent current of such a system with different external fluxes threaded through the ring by using a tight-binding model with Bogoliubov-de Gennes potential. We show that the induced flux can significantly change the dependence of the Andreev levels and the persistent current on the external flux if the inductance of the system is large. By solving the time-dependent Schroedinger equation we obtain the dynamical behaviour of Andreev states in the presence of flux oscillations in the equivalent LC circuit.     

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.     

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.     

Giant Persistent Current in a Mesoscopic Ring with Parallel-Coupled Double Quantum Dots

We theoretically study the properties of the ground state of the parallel-coupled double quantum dots embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. Our results show that in this system, the persistent current depends sensitively on both the parity of this system and the size of the ring. Two dots can be coupled coherently, which is reflected in the giant current peak in the strong coupling regime. This system might be a candidate for future device applications.     

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.     

Spin Polarization Through Resonant unnelling in a Magnetic Double—Barrier and Double—Well Quantum Structure

Recent studies have indicated that spin polarization of the conductance shows strong osicllation and zero average for electrons resonant tunnelling throught a magnetic double-barrier and double-well quantum structure.Our studies indicate that strong oscillation of the polarization of the conductance is not physically intrinsic but inaccurate due to problems of numeration and insufficient numerical accuracy.We also indicate that its average of the spin polarization is not zero.The reasonable explanation is presented in details.     

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.     

Interface Roughness Scattering on Electronic Transport in a Quantum Well

Several theoretical models are established to simulate the interface roughness in a quantum well. The numerical result shows that the roughness correlation function always deviates from the extensively used Gaussian formto some extent, which depends on what a model is used. The influence of such a deviation on the electronic transport property is investigated by assuming several different analytical forms of the correlation function. It is found that the Fermi wavevector is crucial to determine whether the conductivity depends sensitively on the details of the correlation function.     

Quantum entanglement in a two—dimensional ion trap

In this paper,we investigate the quantum entanglement in a two-dimensional ion trap system.we discuss the quantum entanglement between the ion and phonons by using reduced entropy,and that between two degrees of freedom of the vibrational motion along x and y directions by using quantum relative entropy.We discuss also the influence of initial state of the system on the quantum entanglement and the relation between two entanglements in the trapped ion system.     

The Critical Properties of a Modulated Quantum sine—Gordon Model

A new procedure of trial variational wave functional is proposed for invostigating the mass reuormalizationand the local structure of the ground state of a one-dimensional quantum sine-Gorrdonm model with linear spatial modu-lation, whose ground state differs from that without modulation. Thc phase diagram obtained in parameters (αA 2,β2)plane shows that the vertical part of the boundary between soliton lattice phase and iucommensurate (IC) phase withvanishing gap sticks at β2 = 4, the IC phase can only appear forβ2 ＞ 47r and thc IC phase regime is enlarged withincreasing spatial modulation in the case of definite parameter αA-2. The transition is of the continuous type on thevertical part of the boundary, while it is of the first order on the boundary forβ2 ＞ 47r.     

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.