Spin-polarized transport in one-dimensional waveguide structures with spatially periodic electric fields

We investigate theoretically spin-polarized transport in a one-dimensional waveguide structure under spatially periodic electric fields. Strong spin-polarized current can be obtained by tuning the external electric fields. It is interesting to find that the spin-dependent transmissions exhibit gaps at various electron momenta and/or gate lengths, and the gap width increases with increasing the strength of the Rashba effect. The strong spin-polarized current arises from the different transmission gaps of the spin-up and spin-down electrons. (c) 2006 American Institute of Physics.

Spin-polarized transport in a lateral two-dimensional diluted magnetic semiconductor electron gas

The transport property of a lateral two-dimensional paramagnetic diluted magnetic semiconductor electron gas under a spatially periodic magnetic field is investigated theoretically. We find that the electron Fermi velocity along the modulation direction is highly spin dependent even if the spin polarization of the carrier population is negligibly small. It turns out that this spin-polarized Fermi velocity alone can lead to a strong spin polarization of the current, which is still robust against the energy broadening effect induced by the impurity scattering. (c) 2006 American Institute of Physics.

Spin-polarized transport in one-dimensional waveguide structures with spatially-periodic electronic and magnetic fields

We investigate theoretically the spin-polarized transport in one-dimensional waveguide structure with spatially-periodic electronic and magnetic fields. The interplay of the spin-orbit interaction and in-plane magnetic field significantly modifies the spin-dependent transmission and the spin polarization. The in-plane magnetic fields increase the strength of the Rashba spin-orbit coupling effect for the electric fields along y axis and decrease this effect for reversing the electric fields, even counteract the Rashba spin-orbit coupling effect. It is very interesting to find that we may deduce the strength of the Rashba effect through this phenomenon. (c) 2005 Elsevier Ltd. All rights reserved.

Electron transport properties of MM-HEMT with varied channel indium contents

Transport properties of two-dimensional electron gas (2DEG) are crucial to metamorphic high-electron-mobility transistors (MM-HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov-de Hass oscillations., and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM-HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.

Quantum waveguide theory for hole transport in mesoscopic structures

A quantum waveguide theory is proposed for hole transport in the mesoscopic structures, including the band mixing effect. We found that due to the interference between the 'light' hole and 'heavy' wave, the transmission and reflection coefficients oscillate more irregularly as a function of incident wave vector geometry parameters. Furthermore conversion between the heavy hole and light hole states occurs at the intersection. (C) 2003 Elsevier Ltd. All rights reserved.

Electron transport through coupled quantum dots

The transmission through coupled quantum dots (CQDs) is calculated using the coupled-channel recursion method. Our results reveal that the conductance peaks move to high energy as the CQDs radius decreases or the period increases. If we increase the transverse momentum the conductance peaks move to high energy. Applying this characteristic, we can design a switch device using CQDs by applying a static electric field perpendicular to transmission direction. The theoretical results qualitatively agree with the available experimental data. Our calculated results may be useful for the application of CQDs to photoelectric devices. (C) 2003 American Institute of Physics.

Transport properties through quantum dot in a vertical electric field

The transport properties through a quantum dot are calculated using the recursion method. The results show that the electric fields can move the conductive peaks along the high- and low-energies. The electric field changes the intensity of conductance slightly. Our theoretical results should be useful for researching and making low-dimensional semiconductor optoelectronic devices. (C) 2002 Elsevier Science B.V. All rights reserved.

Influence of transverse interdot coupling on transport properties of an Aharonov-Bohm structure composed by two dots and two reservoirs

We derive the modified rate equations for an Aharonov-Bohm (AB) ring with two transversely coupled quantum dots (QD's) embedded in two arms in the presence of a magnetic field. We find that the interdot coupling between the two QD's can cause a temporal oscillation in electron occupation at the initial stage of the quantum dynamics, while the source-drain current decays monotonically to a stationary value. On the other hand, the interdot coupling equivalently divides the AB ring into two coupled subrings. That also destroys the normal AB oscillations with a period of 2pi, and generates new and complex periodic oscillations with their periods varying in a linear manner as the ratio between two magnetic fluxes (each penetrates one AB subring) increases. Furthermore, the interference between two subrings is also evident from the observation of the perturbed fundamental AB oscillation.

Electron and hole transport through quantum dots

The transmission through quantum dots (QDs) is calculated using the recursion method. In our calculation, the effect of finite offset is taken into account. The results show that the shapes of the QDs determine the number of resonant tunneling peaks and the distances between the peaks decrease as the radii of the QDs increase. The intensities of the conductance are strongly dependent on the barrier widths. The conductance peaks are split when transmitting through two QDs. The theoretical results qualitatively agree with the available experimental data. Our calculated results should be useful for the application of QDs to photoelectric devices. (C) 2002 American Institute of Physics.

A partially incoherent rate theory of long-range charge transfer in deoxyribose nucleic acid

A quantum chemistry based Green's function formulation of long-range charge transfer in deoxyribose nucleic acid (DNA) double helix is proposed. The theory takes into account the effects of DNA's electronic structure and its incoherent interaction with aqueous surroundings. In the implementation, the electronic tight-binding parameters for unsolvated DNA molecules are determined at the HF/6-31G* level, while those for individual nucleobase-water couplings are at a semiempirical level by fitting with experimental redox potentials. Numerical results include that: (i) the oxidative charge initially at the donor guanine site does hop sequentially over all guanine sites; however, the revealed rates can be of a much weaker distance dependence than that described by the ordinary Ohm's law; (ii) the aqueous surroundings-induced partial incoherences in thymine/adenine bridge bases lead them to deviate substantially from the superexchange regime; (iii) the time scale of the partially incoherent hole transport through the thymine/adenine pi stack in DNA is about 5 ps. (C) 2002 American Institute of Physics.

Electronic structure and transport properties of quantum rings in a magnetic field

The electronic structure of quantum rings is studied in the framework of the effective-mass theory and the two dimensional hard wall approximation. In cases of both the absence and presence of a magnetic field the electron momenta of confined states and the Coulomb energies of two electrons are given as functions of the angular momentum, inner radius, and magnetic-field strength. By comparing with experiments it is found that the width of the real confinement potential is 14 nm, much smaller than the phenomenal width. The Coulomb energy of two electrons is calculated as 11.1 meV. The quantum waveguide transport properties of Aharonov-Bohm (AB) rings are studied complementarily, and it is found that the correspondence of the positions of resonant peaks in AB rings and the momentum of confined states in closed rings is good for thin rings, representing a type of resonant tunneling.

Highly coherent solid-state quantum bit from a pair of quantum dots

In this letter, we propose a scheme to buildup a highly coherent solid-state quantum bit (qubit) from two coupled quantum dots. Quantum information is stored in the state of the electron-hole pair with the electron and hole located in different dots, and universal quantum gates involving any pair of qubits are realized by effective coupling interaction via virtually exchanging cavity photons. (C) 2002 American Institute of Physics.

Longitudinal spin transport in diluted magnetic semiconductor superlattices: The effect of the giant Zeeman splitting

Longitudinal spin transport in diluted magnetic semiconductor superlattices is investigated theoretically. The longitudinal magnetoconductivity (MC) in such systems exhibits an oscillating behavior as function of an external magnetic field. In the weak magnetic-field region the giant Zeeman splitting plays a dominant role that leads to a large negative magnetoconductivity. In the strong magnetic-field region the MC exhibits deep dips with increasing magnetic field. The oscillating behavior is attributed to the interplay between the discrete Landau levels and the Fermi surface. The decrease of the MC at low magnetic field is caused by the s-d exchange interaction between the electron in the conduction band and the magnetic ions. The spin polarization increases rapidly with increasing magnetic field and the longitudinal current becomes spin polarized in strong magnetic field. The effect of spin-disorder scattering on MC is estimated numerically for low magnetic fields and found to be neglectible for our system.

Electrical transport through individual DNA molecules

A theoretical model is presented to describe electrical transport through individual DNA molecules. By contacting the proposed model with the experimentally measured data, a variety of valuable quantities are identified. The partially decoherent nature on the guanine-cytosine (GC) pairs of DNA is also elaborated in contrast to the completely incoherent hopping mechanism discussed in the context of charge transfer experiments. (C) 2001 American Institute of Physics.

Scattering matrix approach to electronic dephasing in long-range electron transfer

Based on the Buttiker dephasing model, we propose an analytical scattering matrix approach to the long-range electron transfer phenomena. The present efficient scheme smoothly interpolates between the superexchange and the sequential hopping mechanisms. Various properties such as the drastic dephasing-assisted enhancement and turnover behaviors are demonstrated in good agreement with those obtained via the dynamical reduced density-matrix methods. These properties are further elucidated as results of the interplay among the dephasing strength, the tunneling parameter, and the bridge length of the electron transfer system. (C) 2001 American Institute of Physics.