Effects of Electric Field on Electronic States in a GaAs/GaAlAs Quantum Dot with Different Confinements

Binding energies of shallow hydrogenic impurity in a GaAs/GaAlAs quantum dot with spherical confinement, parabolic confinement and rectangular confinement are calculated as a function of dot radius in the influence of electric field. The binding energy is calculated following a variational procedure within the effective mass approximation along with the spatial depended dielectric function. A finite confining potential well with depth is determined by the discontinuity of the band gap in the quantum dot and the cladding. It is found that the contribution of spatially dependent screening effects are small for a donor impurity and it is concluded that the rectangulax confinement is better than the parabolic and spherical confinements. These results are compared with the existing literature.     

Electric field effects on the states of an exciton in square cross-section quantum well wires

The Letter studies the role of the external electric field on the binding energy of the exciton states in square cross-section quantum well wires. Using the effective-mass approximation within a variational scheme and expanding the wave function into Fourier series, we calculate the binding energies of the ground state as well as that of the excited states as the functions of the geometry and the strength of the applied electric field. In the presence of an electric field, it is found that for the ground state the Stark effect is redshift, and for the first and the second excited state the binding energy are split into two levels which will change in contrary situation along with the increasing of the strength of the applied electric field.     

Time-Resolved Photoluminescence Spectroscopy： A Novel Technique for Determination of Luminescence of Quantum Dots

The time-resolved photoluminescence （PL） spectroscopy measured by the gradually increasing start delay time is utilized as a tool for the determination of the luminescence of quantmn dots （QDs）. The luminescence evolution of self-assembled CdSe QDs during the luminescence decay is fully revealed in terms of the experiment technique. The characteristic narrow luminescence lines of self-assembled CdSe QDs are obtained with increasing start delay time.     

Photon-Assisted Conductance Structure for a Quantum Dot

We investigate theoretically the electron transport of a two-level quantum dot irradiated under a weak laser field at low temperatures in the rotating wave approximation. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the conductance for the system with photon polarization perpendicular to the tunnelling current direction. It is demonstrated that by analytic analysing and numerical examples, a feature of conductance peak splitting appears, and the dependence of conductance on the incident laser frequency and self-energy are discussed.     

Spin Accumulation in a Double Quantum Dot Aharonov-Bohm Interferometer

We investigate the spin accumulation in a double quantum dot Aharonov-Bohm （AB） interferometer in which both the Rashba spin-orbit （RSO） interaction and intradot Coulomb interaction are taken into account. Due to the existence of the RSO interaction, the electron, flowing through different arms of the AB ring, will acquire a spin-dependent phase factor in the tunnel-coupling strengths. This phase factor will induce various interesting interference phenomena. It is found that the electrons of the different spin directions can accumulate in the two dots by properly adjusting the bias and the intradot level with a fixed RSO interaction strength. Moreover, both the magnitude and direction of the spin accumulation in each dot can be conveniently controlled and tuned by the gate voltage acting on the dot or the bias on the lead.     

Quantum Phase Transition and Ferromagnetic Spin Correlation in Parallel Double Quantum Dots

We investigate electronic transport through a parallel double quantum dot （DQD） system with strong on-site Coulomb interaction, as well as the interdot tunnelling. By applying numerical renormalization group method, the ground state of the system and the transmission probability at zero temperature are obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin-1 triplet state. The linear conductance will reach the unitary limit （2e^2/h） due to the Kondo effect at low temperature. As the interdot tunnel coupling increases, there is a quantum phase transition from ferromagnetic to anti-ferromagnetic spin correlation in DQDs and the linear conductance is strongly suppressed.     

Antiresonance Effect in Electronic Tunnelling through a One-Dimensional Quantum Dot Chain

Electronic tunnelling through a one-dimensional quantum dot chain is theoretically studied, when two leads couple to the individual component quantum dots of the chain arbitrarily. If there are some dangling quantum dots in the chain outside the leads, the electron tunnelling through the quantum dot chain is wholly forbidden while the energy of the incident electron is just equal to the molecular energy levels of the dangling quantum dots, which is known as the antiresonance effect. In addition, the influence of electron interaction on the antiresonance effect is discussed within the Hartree-Fock approximation.     

Scattering times in AlGaN/GaN two-dimensional electron gas from magnetotransport measurements

The various scattering times of two-dimensional electron gas were investigated in modulation-doped Al_0.22Ga_0.78N/GaN quantum wells by means of magnetotransport measurements. The ratio of transport and quantum scattering times, τ_t/τ_q∼1, shows that the dominant mobility-limiting mechanisms are short-range scattering potentials. The low-field magnetoresistance shows the weak antilocalization and localization phenomenon from which the spin-orbit scattering and inelastic scattering times are obtained. The inelastic scattering time is found to follow the T⁻¹ law, indicating that electron-electron scattering with small energy transfer is the dominant inelastic process.     

Beating patterns in the oscillatory magnetoresistance of an AlGaN/GaN heterostructure

Magneto-transport measurements have been carried out on a modulation-doped Al_0.22Ga_0.78N/GaN heterostructure in a temperature range between 1.5 and 25 K with a rather high carrier density, 1.1×10¹³ cm⁻². Striking beating patterns in magnetoresistance vs magnetic field are observed in the vicinity of a special temperature. Theoretical simulation is performed and the comparison between numerical simulations and the experimental data reveals that the beating patterns are due to the interference of the magneto-intersubband scattering and the SdH oscillator of first subband.     

Anisotropic Spin Splitting in Step Quantum Wells

By the method of finite difference, the anisotropic spin splitting of the AlxGa1-xAs/GaAs/Aly Ga1-yAs/AlxGal-xAs step quantum wells （QWs） are theoretically investigated considering the interplay of the bulk inversion asymmetry and structure inversion asymmetry induced by step quantum well structure and external electric field. We demonstrate that the anisotropy of the total spin splitting can be controlled by the shape of the QWs and the external electric field. The interface related Rashba effect plays an important effect on the anisotropic spin splitting by influencing the magnitude of the spin splitting and the direction of electron spin. The Rashba spin splitting presents in the step quantum wells due to the interface related Rashba effect even without external electric field or magnetic field.     

Correlation between Light Emissions from Amorphous-Si：H/SiO2 and nc-Si/SiO2 Multilayers

We investigate the properties of light emission from amorphous-Si：H/SiO2 and nc-Si/SiO2 multilayers （MLs）. The size dependence of light emission is well exhibited when the a-Si：H sublayer thickness is thinner than 4 nm and the interface states are well passlvated by hydrogen. For the nc-Si/Si02 MLs, the oxygen modified interface states and nanocrystalline silicon play a predominant role in the properties of light emission. It is found that the light emission from nc-Si/SiO2 is in agreement with the model of interface state combining with quantum confinement when the size of nc-Si is smaller than 4 nm. The role of hydrogen and oxygen is discussed in detail.     

Tuning Photoluminescence Energy and Fine Structure Splitting in Single Quantum Dots by Uniaxial Stress

We report a photoluminescence （PL） energy red-shift of single quantum dots （QDs） by applying an in-plane compressive uniaxial stress along the [110] direction at a liquid nitrogen temperature. Uniaxial stress has an effect not only on the confinement potential in the growth direction which results in the PL shift, but also on the cylindrical symmetry of QDs which can be reflected by the change of the full width at half maximum of PL peak. This implies that uniaxial stress has an important role in tuning PL energy and fine structure splitting of QDs.     

Electric Field Control of Interface Related Spin Splitting in Step Quantum Wells

Spin splitting of the Aly Ga1-y As/GaAs/A1x Ga1-x As/A1-y Ga1-y As （x ≠ y） step quantum wells （ Q Ws） has been theoretically investigated with a model that includes both the interface and the external electric field contribution. The overall spin splitting is mainly determined by the interface contribution, which can be well manipulated by the external electric field. In the absence of the electric field, the Rashba effect exists due to the internal structure inversion asymmetry （SIA）. The electric field can strengthen or suppress the internal SIA, resulting in an increase or decrease of the spin splitting. The step QW, which results in large spin splitting, has advantages in applications to spintronic devices compared with symmetrical and asymmetrical QWs. Due to the special structure design, the spin splitting does not change with the external electric field.     

Charge Injection and Transport in Metal/Polymer Chains/Metal Sandwich Structure

Using the tight-binding Su--Schrieffer--Heeger model and a nonadiabatic dynamic evolution method, we study the dynamic processes of the charge injection and transport in a metal/two coupled conjugated polymer chains/metal structure. It is found that the charge interchain transport is determined by the strength of the electric field and the magnitude of the voltage bias applied on the metal electrode. The stronger electric field and the larger voltage bias are both in favour of the charge interchain transport.     

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.     

Spin Transport in a Magnetic Superlattice with Broken Two-Fold Symmetry

We theoretically investigate the spin-dependent electron transport properties in a magnetic superlattice （MSL） with broken two-fold symmetry. An abnormal barrier in the MSL can break the two-fold symmetry of the system when it is not located at the two-fold symmetry center. A two-fold symmetry breaking factor is introduced to describe the two-fold symmetry breaking degree. Our numerical calculations show that the transmission, the conductance and the spin polarization are non-trivially dependent on the two-fold symmetry breaking factor. When the factor is large enough, the polarization almost approaches 100% in a proper Fermi energy range. However, for two mutually mirror-symmetric MSLs with the same factor, their polarizations may be either similar or distinct. These features provide some clues to the design and applications of MSL-based spin filters or spin-dependent tunneling electron devices.     

Coherent Tunnelling in Coupled Quantum Wells under a Uniform Magnetic Field

Coherent tunnelling is studied in the framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

Stark shifts of charged particles in semiconductor quantum wells

 We calculate the effect of a homogeneous electric field on electrons, holes and excitons confined in a quantum well structure consisting of alternate thin layers of well and barrier material. The electric field which acts perpendicular to the quantum well is taken as a perturbation on the quantum well structure confining the charges. The electron and hole energies in the conduction and valence subbands are calculated by solving a one-dimensional Schr?dinger equation. The exciton binding energy is calculated using an improved excitonic model. Results obtained indicate the importance of higher-order excitons in optical transitions at high electric fields. Received: 29 February 1996/Accepted: 19 August 1996     

Motional Stark Effect and Its Active Cancellation in Diamagnetic Spectrum of Barium

With time-of-flight and electric field ionization detection technique, we investigate the motional Stark effect for highly excited Rydberg barium in high magnetic field and its active cancellation experimentally. In the experiment, the atom beam is aligned at a small angle of 15° with respect to the magnetic field. The motional Stark effect cancellation is demonstrated on two sets of. circularly polarized spectra in static magnetic field B = 1.00000 Tesla and B = 1.70000 Tesla, respectively, although the effect is very small （- 3.5 Vcm^-1） in our apparatus configuration.