耦合高压斯特林制冷效应的复合磁制冷循环的数值模拟

本文借助计算流体力学软件, 对复合磁制冷机进行整机数值模拟分析. 以复合磁制冷机为建模原形, 分别计算了主动式磁制冷循环以及复合磁制冷循环. 利用模型计算分析了利用系数, 工作频率对主动式磁制冷的制冷效果影响, 同时模拟计算了不同相位角、不同频率下的复合磁制冷机的制冷效果, 计算得到适合复合磁制冷循环的最佳匹配相位角. 模拟计算结果对后续实验台的设计搭建有很好的指导作用.     

Temperature dependence of indirect-exciton luminescence in in-plane magnetic field

We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one.     

Effect of magnetic field on Mott's variable-range hopping parameters in multiwall carbon nanotube mat

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott’s variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott’s T0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism.     

Magnetic moment of a nanotube with helical symmetry

The magnetic properties of a nanotube are investigated using a model in which the helical symmetry of the nanotube is governed by an extended δ potential. The magnetic moment of the system is studied as a function of the magnetic flux. It is demonstrated that the equilibrium magnetic moment at a large amplitude of the δ potential is a smooth function of the magnetic flux. An expression is derived for the magnetic moment induced in the given system for the case in which the electron current in a ballistic regime passes through the system.     

Cyclotron resonance maser as a solar flare trigger

We propose a new mechanism of solar flaring, which is based on explosive phenomena in magnetic traps in the presence of a two-component plasma composed of fast electrons with anisotropic velocity distribution and a dense cold background plasma with high ionization. It is assumed that such a plasma is generated in a coronal magnetic trap in a preflare stage. This system, which is essentially a cyclotron resonance maser, becomes unstable under certain conditions and gives rise to an explosive cyclotron instability, which develops at first in a very small local area and is accompanied by intense heating of the background plasma and release of fast electrons at the trap ends. The energy of fast particles is collected from the entire volume of the magnetic trap and is localized in the form of heat in the explosion area from which thermal and shock waves are propagated. The model makes it possible to explain the main solar flare effects.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 6, pp. 699–712, June, 1996.     

Dynamical view of pair creation in uniform electric and magnetic fields

Pair creation in a uniform classical electromagnetic field (Schwinger mechanism) is studied focusing on the time evolution of the distribution of created particles. The time evolution of the distribution in time-dependent fields is also presented as well as effects of back reaction. Motivated by the Glasma flux tube, which may be formed at the initial stage of heavy-ion collisions, we investigate effects of a magnetic field parallel to an electric field, and find that the magnetic field makes the evolution of a fermion system faster.     

金属壳体和电缆的系统电磁脉冲响应

 研究了金属壳体和电缆遇到高空核爆炸产生的脉冲X射线辐射时的系统电磁脉冲响应，讨论了系统电磁脉冲产生机理、幅值和对系统的耦合途径。通过对计算和试验数据的分析，获得了金属壳体模型和单电缆及双电缆的系统电磁脉冲响应数据，简要分析了壳体的系统电磁脉冲损伤薄弱环节。     

Multiple structure of two-pulse nuclear spin echo in cobalt films

The conditions for the formation of two-pulse echo signals from ⁵⁹Co nuclei in thin magnetic films at T=4.2 K are investigated. In the framework of the existing mechanisms, numerical simulation of the conditions for the formation of extra 3τ and 4τ echo signals (τ is the time delay between pulses) is carried out. It is shown that the multiple structure of the echo from ⁵⁹Co nuclei at T=4.2 K is due to a mechanism in which an additional hyperfine magnetic field proportional to nuclear magnetization is acting on the nuclear spin system.     

磁刺激穴位复杂脑功能网络构建与分析

本文采用互信息方法对磁刺激内关穴过程中的脑电信 号进行了两两通道间非线性时域关联特性分析, 构建了不同频率刺激前、刺激中、刺激后的脑功能网络, 并基于复杂网络理论对脑功能网络的特征进行了深入研究. 结果表明, 磁刺激频率为3 Hz 时, 大脑功能网络的平均度、平均聚类系数和全局效率与刺激前相比均有显著升高, 平均路径长度显著降低, 并且相应脑功能网络的"小世界"属性有所增强, 信息在大脑各区域间的传递更加高效. 本研究首次开展了磁刺激穴位复杂脑功能网络的构建与分析, 为探索磁刺激穴位对大脑神经调节的作用和机理提供新思路和新方法. 关键词： 复杂网络 磁刺激 脑功能网络 互信息     

Fe-Ni殷钢的穆斯堡尔谱研究

用穆斯堡尔谱研究了Fe-Ni殷钢的磁性特征,证实合金中存在两种不同磁态的Fe原子,并依此解释了殷钢效应。提出了测定合金中两种状态Fe原子磁矩的公式。报道了Fe-Ni合金在Ni32wt%附近的一些反常现象。 关键词：     

Vanishing magnetic interactions in ferromagnetic thin films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.     

Ground state of finite arrays of magnetic dots in the presence of an external magnetic field

The ground state of an array of magnetic particles (magnetic dots), which are ordered in a square 2D lattice and whose magnetic moment is perpendicular to the lattice plane, in the presence of an external magnetic field has been analyzed. Such a model is applicable for sufficiently small dots with perpendicular anisotropy that are in a single-domain state and for dots in a strongly inhomogeneous vortex state whose magnetic moment is determined by the vortex core. For the magnetic field perpendicular to the system plane, the entire set of the states has been analyzed from the chessboard antiferromagnetic order of magnetic moments in low fields to the saturated state of the system with the parallel orientations of the magnetic moments of all dots in strong fields. In the presence of the border, the destruction of the chessboard order first occurs at the edges of the system, then near the extended sections of the surface, and finally expands over the entire interior of the array. The critical field at which this simplest state is destroyed is much more weakly than the value characteristic of the ideal infinite system. In contrast to this scenario, the destruction of the saturated state with decreasing field always begins far from the borders. Despite such different behaviors, the magnetic structure in the intermediate range of fields that is obtained with both increasing and decreasing field for finite arrays strongly differs from that characteristic of the ideal infinite system. The role of simple stacking faults of the magnetic dot lattice (such as single vacancies or their clusters) in the remagnetization of the system has been analyzed. The presence of such faults is shown to give rise to the appearance of local destructions of the chessboard antiferromagnetic order at fields that are much weaker than those for an ideal lattice.     

Angular dependence of the magnetic-field driven superconductor-insulator transition in thin films of amorphous indium-oxide

A significant anisotropy of the magnetic-field driven superconductor-insulator transition is observed in thin films of amorphous indium-oxide. The anisotropy is largest for more disordered films which have a lower transition field. At higher magnetic fields the anisotropy reduces and even changes sign beyond a sample specific and temperature independent magnetic field value. The data are consistent with the existence of more than one mechanism affecting transport at high magnetic fields.     

Cluster of solar active regions and onset of coronal mass ejections

round-the-clock solar observations with full-disk coverage of vector magnetograms and multi-wavelength images demonstrate that solar active regions(ARs) are ultimately connected with magnetic field. Often two or more ARs are clustered, creating a favorable magnetic environment for the onset of coronal mass ejections(CMEs). In this work, we describe a new type of magnetic complex: cluster of solar ARs. An AR cluster is referred to as the close connection of two or more ARs which are located in nearly the same latitude and a narrow span of longitude. We illustrate three examples of AR clusters, each of which has two ARs connected and formed a common dome of magnetic flux system. They are clusters of NOAA(i.e., National Oceanic and Atmospheric Administration) ARs 11226 11227, 11429 11430, and 11525 11524. In these AR clusters, CME initiations were often tied to the instability of the magnetic structures connecting two partner ARs, in the form of inter-connecting loops and/or channeling filaments between the two ARs. We show the evidence that, at least, some of the flare/CMEs in an AR cluster are not a phenomenon of a single AR, but the result of magnetic interaction in the whole AR cluster. The observations shed new light on understanding the mechanism(s) of solar activity. Instead of the simple bipolar topology as suggested by the so-called standard flare model, a multi-bipolar magnetic topology is more common to host the violent solar activity in solar atmosphere.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

Yield stress in thin layers of ferrofluids

We present results of theoretical study of quasielastic behavior of ferrofluid filling a thin flat gap, placed into perpendicular magnetic field. When the field exceeds a certain critical magnitude, magnetic particles form dense discrete domains, elongated along the field, and linking the gap boundaries. Due to these bridges between the gap boundaries, the ferrofluid exhibits quasielastic properties with respect to shear strain in the plane of the gap. We estimated the elastic modules as well as the yield stress of the system, depending on magnetic field and concentration of magnetic particles in the ferrofluid. Analysis shows that there are at least two microscopical mechanisms of transition from the elastic to fluid behavior of the ferrofluid. The first one is connected with the loss of the mechanical equilibrium of the domains, slopped, under the shear stress, with respect to applied magnetic field. The second mechanism is connected with breakup of the “bridge” into two separate drops, when the shear strain exceeds some critical magnitude. Estimates show that for real ferrofluids the second mechanism is more probable.     

Anomalous ferromagnetism and non-Fermi-liquid behavior in the Kondo lattice CeRuSi<Subscript>2</Subscript>

The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi₂ are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity, heat conductivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, a non-Fermi-liquid behavior is observed at low temperatures, which is unusual for a ferromagnetic Kondo system. A comparison with other magnetic Kondo lattices is performed.     

Amplification of mean magnetic field and magnetic helicity production in hot lepton plasma of early universe

We argue that the magnetic helicity conservation is violated at the lepton stage in the evolution of early Universe owing to the parity violation in the Standard Model of electroweak interactions. As a result, a cosmological magnetic field which can be a seed for the galactic dynamo obtains from the beginning a substantial magnetic helicity which has to be taken into account in the magnetic helicity balance at the later stage of galactic dynamo. The particle physics mechanism suggested in our works depends neither on helicity of matter turbulence with plasma vortices resulting in the standard α effect in dynamo theory nor on general rotation. The mechanism can result in a self-exitation of an (almost) uniform cosmological magnetic field. The text was submitted by the authors in English.     

Interface study of ion irradiated Cu/Ni/Cu(0 0 2)/Si magnetic thin film by X-ray reflectivity

The irradiation effect of 1 MeV C⁺ on the interface and magnetic anisotropy of epitaxial Cu/Ni system with a perpendicular magnetic anisotropy was investigated by using magneto-optical Kerr effects, grazing incident diffraction and X-ray reflectivity. The magnetic easy-axis was altered from the direction along the surface normal to in-plane and the strain in the Ni layer was relaxed after ion irradiation. Though the interface between the top Cu layer and the Ni layer becomes rough, the contrast of electron densities of Cu and Ni layer increases and the grain-growth occurs during ion irradiation. These phenomena arise from thermo-chemical driving force, i.e. heat of formation, which may be a crucial factor in determining the interface shape in the case of indirect energy transfer mechanism. Therefore, the change of the magnetic anisotropy of the Ni/Cu system after ion irradiation is not due to the formation of the intermixed layer at the interface. The ion irradiation effects on the grain-growth and enhancement of the electronic contrast between Ni and Cu are explained by the interfacial atomic movement caused by thermo-chemical driving force.