Influence of Local Magnetic Fields on P-Doped Si Floating Zone Melting Crystal Growth in Microgravity

A two-dimensional axisymmetric numerical model is presented to study the influence of local magnetic fields on P-doped Si floating zone melting crystal growth in microgravity. The model is developed based on the finite difference method in a boundary-fitted curvilinear coordinate system. Extensive numerical simulations are carried out, and parameters studied include the curved growth interface shape and the magnetic field configurations. Computed results show that the local magnetic field is more effective in reducing the impurity concentration nonuniformity at the growth interface in comparison with the longitudinal magnetic field. Moreover, the curved growth interface causes more serious impurity concentration nonuniformity at the growth interface than the case with a planar growth interface.

The mechanism of energy storage by shearing the solar magnetic field

In this paper, a complete set of MHD equations have been solved by numerical calculations in an attempt to study the dynamical evolutionary processes of the initial equilibrium configuration and to discuss the energy storage mechanism of the solar atmosphere by shearing the magnetic field. The initial equilibrium configuration with an arch bipolar potential field obtained from the numerical solution is similar to the configuration in the vicinity of typical solar flare before its eruption. From the magnetic induction equation in the set of MHD equations and dealing with the non-linear coupling effects between the flow field and magnetic field, the quantitative relationship has been derived for their dynamical evolution. Results show that plasma shear motion at the bottom of the solar atmosphere causes the magnetic field to shear; meanwhile the magnetic field energy is stored in local regions. With the increase of time the local magnetic energy increases and it may reach an order of 4×10^25 J during a day. Thus the local storage of magnetic energy is large enough to trigger a big solar flare and can be considered as the energy source of solar flares. The energy storage mechanism by shearing the magnetic field can well explain the slow changes in solar active regions.

Guiding and confining fast electrons by transient electric and magnetic fields with a plasma inverse cone

The fast electron propagation in an inverse cone target is investigated computationally and experimentally. Two-dimensional particle-in-cell simulation shows that fast electrons with substantial numbers are generated at the outer tip of an inverse cone target irradiated by a short intense laser pulse. These electrons are guided and confined to propagate along the inverse cone wall, forming a large surface current. The propagation induces strong transient electric and magnetic fields which guide and confine the surface electron current. The experiment qualitatively verifies the guiding and confinement of the strong electron current in the wall surface. The large surface current and induced strong fields are of importance for fast ignition related researches.

Generation of strong quasistatic magnetic fields in interactions of ultraintense and short laser pulses with overdense plasma targets

An analytical fluid model is proposed for the generation of strong quasistatic magnetic fields during normal incidence of a short ultraintense Gaussian laser pulse with a finite spot size on an overdense plasma. The steepening of the electron density profile in the originally homogeneous overdense plasma and the formation of electron cavitation as the electrons are pushed inward by the laser are included self-consistently. It is shown that the appearance of the cavitation plays an important role in the generation of quasistatic magnetic fields: the strong plasma inhomogeneities caused by the formation of the electron cavitation lead to the generation of a strong axial quasistatic magnetic field B-z. In the overdense regime, the generated quasistatic magnetic field increases with increasing laser intensity, while it decreases with increasing plasma density. It is also found that, in a moderately overdense plasma, highly intense laser pulses can generate magnetic fields similar to 100 MG and greater due to the transverse linear mode conversion process.

Long-term exposure to extremely low-frequency magnetic fields impairs spatial recognition memory in mice

1. In the present study, we investigated the short- and long-term effects of extremely low-frequency (ELF) magnetic fields on spatial recognition memory in mice by using a two-trial recognition Y-maze that is based on the innate tendency of rodents to exp

Anisotropy of magnetic properties in ferrimagnetic ytterbium iron garnet under high magnetic fields

The magnetic anisotropy in ytterbium iron garnet (YbIG) is theoretically investigated under high magnetic fields (up to 160 kOe). According to the crystal field effect in ytterbium gallium garnet (YbGaG), a detailed discussion of crystal-field interaction in YbIG is presented where a suitable set of crystal-field parameters is obtained. Meanwhile, the influences of nine crystal-field parameters on the crystal-field energy splitting are analyzed. On the other hand, considering the ytterbium-iron (Yb-Fe) superexchange interaction of YbIG, the spontaneous magnetization is calculated at different temperatures for the [111] direction. In particular, we demonstrate that the Wesis constant lambda is the function of 1/T in YbIG. In addition, the field dependences of the magnetization for the [110] and [111] directions are theoretically described where a noticeable anisotropy can be found. Our theory further confirms the great contribution of anisotropic Yb-Fe superexchange interaction to the anisotropy of the magnetization in YbIG. Moreover, our theoretical results are compared with the available experiments.

Observation of the current-voltage plateau-like structure of resonant tunnelling diode with prewells under different magnetic fields

We have grown resonant tunnelling diodes (RTDs) with different sized emitter prewells and without a prewell. The current-voltage (I-V) characteristics of them in different magnetic fields were investigated. Two important phenomena were observed. First, a high magnetic field can destroy the plateau-like structure in the I-V curves of the RTD. This phenomenon is ascribed to the fact that the high magnetic field will demolish the coupling between the energy level in the main quantum well and that in the emitter quantum well or in the prewell. Secondly, the existence and size of the prewell are also important factors influencing the plateau-like structure.

Coherent tunnelling in coupled quantum wells under a uniform magnetic fields

Coherent tunnelling is studied in 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.

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.

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.

Strong three-level resonant magnetopolaron effect due to the intersubband coupling in heavily modulation-doped GaAs/AlxGa1-xAs single quantum wells at high magnetic-fields

Electron cyclotron resonance CR) measurements have been carried out in magnetic fields up to 32 T to study electron-phonon interaction in two heavily modulation-delta -doped GaAs/Al0.3Ga0.7As single-quantum-well samples. No measurable resonant magnetopolaron effects were observed in either sample in the region of the GaAs longitudinal optical (LO) phonons. However, when the CR frequency is above LO phonon frequency, omega (LO)=E-LO/(h) over bar, at high magnetic fields (B>27 T), electron CR exhibits a strong avoided-level-crossing splitting for both samples at frequencies close to (omega (LO)+ (E-2-E-1)1 (h) over bar, where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large with the minimum separation of 40 cm(-1) occurring at around 30.5 T. A detailed theoretical analysis, which includes a self-consistent calculation of the band structure and the effects of electron-phonon interaction on the CR, shows that this type of splitting is due to a three-level resonance between the second Landau level of the first electron subband and the lowest Landau level of the second subband plus one GaAs LO phonon. The absence of occupation effects in the final states and weak screening or this three-level process yields large energy separation even in the presence of high electron densities. Excellent agreement between the theory and the experimental results is obtained.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

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.