Bifurcation of a periodic instanton and quantum－classical transition in the biaxial nano－ferromagnet with a magnetic field along hard axis

Crossover from classical to quantum regimes of the barrier transition rate in a biaxial ferromagnetic magnet with a magnetic field applied along hard anisotropy axis is investigated. We show that the type of action-temperature diagrams can he determined by counting the number of bifurcation points. The model possesses not only the known type I and Ⅱ, hut also the interesting type Ⅲ and Ⅳ of transition which do not occur in general.     

Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

疏松型螺线管电流轴线上磁场的数值计算

应用数值模拟方法，细致地分析了疏松型螺线管电流轴线上的磁场随螺距和轴线上位置的变化关系，计算结果表明，当螺距较大时，其磁场分布与密绕螺线管电流的磁场分布相差很大，磁场沿轴线和垂直于轴线的方向都有分量，螺距越大，垂直于轴线方向的磁场分量越大；但当螺距较小或距螺线管中心较近外，磁场几乎与密切螺线管电流产生的磁场相当，可用表述密绕螺线管电流的轴线上磁场的简单公式来近似。     

Dispersion properties of a periodic semiconductor structure in a magnetic field directed along the periodicity axis

The band spectrum of natural waves in a periodic structure formed by alternating dielectric and semiconductor layers is investigated for the propagation of waves at some angle with respect to a magnetic field applied along the periodicity axis. A method is presented for deriving a dispersion equation, and its properties are analyzed numerically. It is shown that, in the absence of dissipation, there are two independent spectra of natural waves in the structure being considered, and the regions of existence of various types of such waves are classified. It is established that the transmission bands of the two spectra may supplement one another or overlap. It is found that, for the chosen magnetic-field direction, there exist numerous bands of transmission of cyclotron waves.     

Algebraic soliton in a ferromagnet in the presence of the magnetic field directed along the anisotropy axis

Magnetic solitons in a one-dimensional ferromagnet with easy-plane anisotropy are considered. Algebraic solitons are discussed and the conditions of existence of such solitons are investigated.     

Resonant tunneling in weakly coupled GaAs/AlGaAs superlattices in a transverse magnetic field: A probe of electronic distribution function

A method is proposed to get information about carrier distribution function in superlattices and multiple quantum-well structures from the analysis of the vertical transport experiments in a transverse magnetic field. The method was applied to the GaAs/AlGaAs superlattices with wide quantum wells in strong (B=0–7 T) magnetic fields. It was shown that the distribution function of electron is nonequilibrium Boltzmann-like, with electronic temperature T=10–20 K.     

Mott-Hubbard transition in a magnetic field

We study the density of states (DOS) as a function of the interaction U in the half-filled simplified Hubbard model in a magnetic field. This model is considered on the Bethe lattice in the limit of high dimensions. We show that the DOS can be calculated exactly, and that many of its properties have an astonishingly simple form. In particular, the DOS can be investigated explicitly in the limits of weak and strong coupling and near the metal-insulator transition. E.g., we find an explicit result for the critical value U_c, at which the metal-insulator transition occurs, as a function of the magnetization. The relation between the magnetization and the magnetic field is calculated numerically. An important result is that the metal-insulator transition, occurring in the model with B = 0, is continuously connected to the metal-insulator transition in the subspace of single spin flips.     

Exciton spectra of semiconductor superlattices in a parallel magnetic field

Low-temperature photoluminescence and photoluminescence excitation spectra of GaAs/AlGaAs semiconductor superlattices having different potential barrier widths (b=20, 30, 50, and 200 Å), i.e., degrees of tunnel coupling between quantum wells, are studied in magnetic fields up to 5 T oriented parallel and perpendicular to the layers of the structure. The changes in the qualitative character of the photoluminescence excitation spectra observed in a parallel magnetic field with increasing tunnel transparency of the barrier correspond to a transition from a quasi-two-dimensional to a quasi-three-dimensional electronic spectrum as a miniband develops in the superlattice. In the photoluminescence excitation spectra of the superlattice with b=50 Å, as the parallel magnetic field is increased, a new line appears in the violet wing of the spatially indirect exciton excitation line, which is absent in a perpendicular field. A similar line was also observed to arise in the photoluminescence spectra. It is shown that the indirect exciton luminescence line can be suppressed by both parallel and perpendicular magnetic fields.     

Parallel magnetic field induced transition in transport in the dilute two-dimensional hole system in GaAs

A magnetic field applied parallel to the two-dimensional hole system in the GaAs/AlGaAs heterostructure, which is metallic in the absence of an external magnetic field, can drive the system into insulating at a finite field through a well-defined transition. The value of resistivity at the transition is found to depend strongly on density.     

带开口磁环的周期永磁聚焦系统的轴向磁场

基于磁环的元电流模型,分析了永磁磁环的轴向磁场分布规律。理论计算和测试结果表明:轴向磁化磁环中心轴上的磁感应强度不存在径向分量,只有轴向分量;但磁环开口后,轴向磁场降低,而径向磁场增加,其中心轴上的合成磁感应强度体现出非轴对称性,不利于其在周期永磁（PPM）聚焦系统中的应用。根据磁场的轴向分量和径向分量的分布特点,设计和优化了带开口磁环的PPM系统,实现了轴向磁场的周期化。     

带开口磁环的周期永磁聚焦系统的轴向磁场

 基于磁环的元电流模型,分析了永磁磁环的轴向磁场分布规律。理论计算和测试结果表明:轴向磁化磁环中心轴上的磁感应强度不存在径向分量,只有轴向分量;但磁环开口后,轴向磁场降低,而径向磁场增加,其中心轴上的合成磁感应强度体现出非轴对称性,不利于其在周期永磁（PPM）聚焦系统中的应用。根据磁场的轴向分量和径向分量的分布特点,设计和优化了带开口磁环的PPM系统,实现了轴向磁场的周期化。     

Kinetic-anisotropy-induced ordering-orientation transition in epitaxial growth: a method to synthesize ordering-orientation superlattices

The epitaxial growth has a distinct kinetic feature that the lateral surface diffusion is faster than the longitudinal bulk diffusion. We show there is an ordering-orientation transition of alloy films with the change of growth rate due to this kinetic anisotropy. As an example, we have calculated the epitaxial growth of CoPt alloy films on the Pt buffer layer. We show the ordered structure of CoPt films changes from the L1(0) [001] (a compositional modulation along the [001] direction) variant to the L1(0) [100] variant with the increase of growth rate. This ordering-orientation transition also occurs with the decrease of temperature at adequate growth rate. Based on this mechanism, we propose a simple method to synthesize the ordering-orientation superlattices.