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.

Magneto-optical properties of diluted magnetic semiconductor quantum dots

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magnetic semiconductor quantum dot (QD), with the Coulomb interaction and the sp-d exchange interaction included. The MP energy decreases rapidly with increasing magnetic field at low magnetic field and saturates at high magnetic field for small QDs, and the dependences of the MP energy on magnetic field are quite different for different QD radii due to the different carrier-induced magnetic fields B-MP. The competition between the sp-d exchange interaction and the band gap shrinkage results in there being a maximum exhibited by the MP energy With increasing temperature. Our numerical results are in good agreement with experiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, Forchel A, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

Collective and single-particle excitations in coupled quantum wires in magnetic fields

The full spectra of magnetoplasmons and single-particle excitations are obtained of coupled one-dimensional electron gases in parallel semiconductor quantum wires with tunneling. We show the effects of the interwire Coulomb interaction and the tunneling, as well as the magnetic-field-induced localization on the elementary excitations in symmetric and asymmetric coulped quantum wire structures. The interacton and resonance between the plasmon and the intersubband single-particle excitations are found in magnetic fields.

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.

Oscillating magnetoresistance in diluted magnetic semiconductor barrier structures

Ballistic spin polarized transport through diluted magnetic semiconductor single and double barrier structures is investigated theoretically using a two-component model. The tunneling magnetoresistance (TMR) of the system exhibits oscillating behavior when the magnetic field is varied. An interesting beat pattern in the TMR and spin polarization is found for different nonmagnetic semiconductor/diluted magnetic semiconductor double barrier structures which arises from an interplay between the spin-up and spin-down electron channels which are split by the s-d exchange interaction.

Quantum-confined magneto-Stark effect in diluted magnetic semiconductor coupled quantum wells

The magneto-Stark effect in a diluted magnetic semiconductor (DMS) coupled quantum well (CQW) induced by an in-plane magnetic field is investigate theoretically. Unlike the usual electro-Stark effects, in a DMS CQW the Lorenz force leads to a spatially separated exciton. The in-plane magnetic field can shift the ground state of the magnetoexciton from a zero in-plane center of mass (CM)/momentum to a finite CM momentum, and render the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. (C) 2002 American Institute of Physics.

Static and dynamic electric field domain formation in a doped GaAs/AlAs superlattice

We have observed the transition from static to dynamic electric field domain formation induced by a transverse magnetic field and the sample temperature in a doped GaAs/AlAs superlattice. The observations can be very well explained by a general analysis of instabilities and oscillations of the sequential tunnelling current in superlattices based solely on the magnitude of the negative differential resistance region in the tunnelling characteristic of a single barrier. Both increasing magnetic field and sample temperature change the negative differential resistance and cause the transition between static and dynamic electric field domain formation. (C) 2000 Elsevier Science B.V. All rights reserved.

Oscillatory magneto-conductance in quantum waveguides with lateral multi-barrier structures

The magneto-transport properties of a narrow quantum waveguide with lateral multibarrier modulation are investigated theoretically. It is found that the magnetoconductance as a function of Fermi energy or magnetic field exhibits square-wave-like oscillations. In the presence of magnetic field, the edge states are formed near each barrier and the boundaries. Therefore, the number of edge states increases with the number of lateral barriers, leading to the increase of the propagating modes. On the other hand, owing to the tunneling effect a pair of edge states around the barrier region with opposite moving directions may be coupled and formed a circulating localized state, leading to the quenching of the related propagating states. The resulting dispersion relation exhibits oscillation structures superimposed on the bulk Landau levels. These novel conductance characteristics may provide potential applications to the fabrication of new quantum devices.

Magnetic properties of ZnO-doped cobalt ferrite

The cobalt ferrites with chemical composition Co1+xZnxFe2-2xO4 (r=0.0, 0.1, 0.2, 0.4) were obtained with conventional solid reaction. The ZnO-doped samples have lower lattice constant than CoFe2O4 by adjusting Co ions to the octahedral sites. The results show that doping ZnO could extremely improve the magnetic properties. In comparison with pure CoFe2O4, the little ZnO-doped sample has higher permeability and much lower coercivity at the condition of a little decrease of magnetization saturation. Sample with x=0.1 shows evident magnetostrictive effect at the magnetic field of 30-60 mT while pure cobalt ferrite sample does not, though the saturation magnetostriction decreases. These indicate that ZnO-doping improves the magnetostrictive sensitivity of the cobalt ferrites and have potential applications in magnetoelectric devices and magnetic detector.

Electronic states of InAs verfically-assembled quantum disks in magnetic fields and two-electron quantum-disk qubit

We have studied the single-electron and two-electron vertically-assembled quantum disks in an axial magnetic field using the effective mass approximation. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate the six criergy levels of single-electron quantum disks and the two lowest energy levels of two-electron quantum disks in an axial magnetic field. The change of the magnetic field as an effective potential strongly modifies the electronic structures. leading to splittings and crossings between levels The results demonstrate the switching between the around states with the total spins S = 0 and S = 1. The switching results in a qubit allowed to fabricate by current growth techniques.

Controllable negative and positive group delay in transmission through a single quantum well at finite magnetic fields

We investigate the controllable negative and positive group delay in transmission through a single quantum well at the finite longitudinal magnetic fields. It is shown that the magneto-coupling effect between the longitudinal motion component and the transverse Landau orbits plays an important role in the group delay. The group delay depends not only on the width of potential well and the incident energy, but also on the magnetic-field strengthen and the Landau quantum number. The results show that the group delay can be changed from positive to negative by the modulation of the magnetic field. These interesting phenomena may lead to the tunable quantum mechanical delay line. (c) 2007 Elsevier B.V. All rights reserved.

Electron tunneling through double magnetic barriers on the surface of a topological insulator

We study electron tunneling through a planar magnetic and electric barrier on the surface of a three-dimensional topological insulator. For the double barrier structures, we find (i) a directional-dependent tunneling which is sensitive to the magnetic field configuration and the electric gate voltage, (ii) a spin rotation controlled by the magnetic field and the gate voltage, (iii) many Fabry-Perot resonances in the transmission determined by the distance between the two barriers, and (iv) the electrostatic potential can enhance the difference in the transmission between the two magnetization configurations, and consequently lead to a giant magnetoresistance. Points (i), (iii), and (iv) are alike with that in graphene stemming from the same linear-dispersion relations.

Magnetic barrier on strained graphene: A possible valley filter

We put forward a two-terminal valley filter based on a bulk graphene sheet under the modulations of both a local perpendicular magnetic field and a substrate strain. When only one of the two modulations is present, no valley polarization can be generated. A combination of the two modulations leads to a different (but not opposite) shifts of the K and K' valleys, which could be utilized to generate a valley-polarized current. The degree of the valley polarization can be tuned by the strain strength and the inclusion of a scalar potential. The valley polarization changes its polarity as the local magnetic field switches its direction.

Ultrafast Kerr rotations and zero-field dephasing time of electron spins in InAs/GaAs quantum disks

Time-resolved Kerr rotation (TRKR) measurements based on pump-probe arrangement were carried out at 5 K on the monolayer fluctuation induced InAs/GaAs quantum disks grown on GaAs substrate without external magnetic field. The lineshape of TRKR signals shows an unusual dependence on the excitation wavelength, especially antisymmetric step-shaped structures appearing when the excitation wavelength was resonantly scanned over the heavy- and light-hole subbands. Moreover, these step structures possess an almost identical decay time of similar to 40 Ps which is believed to be the characteristic spin dephasing time of electrons in the extremely narrow InAs/GaAs quantum disks.