Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Ultrafast Photoinduced Multimode Antiferromagnetic Spin Dynamics in Exchange‐Coupled Fe/RFeO3 (R = Er or Dy) Heterostructures

Antiferromagnetic spin dynamics is important for both fundamental and applied antiferromagnetic spintronic devices; however, it is rarely explored by external fields because of the strong exchange interaction in antiferromagnetic materials. Here, the photoinduced excitation of ultrafast antiferromagnetic spin dynamics is achieved by capping antiferromagnetic RFeO₃ (R = Er or Dy) with an exchange‐coupled ferromagnetic Fe film. Compared with antiferromagnetic spin dynamics of bare RFeO₃ orthoferrite single crystals, which can be triggered effectively by ultrafast laser heating just below the phase transition temperature, the ultrafast photoinduced multimode antiferromagnetic spin dynamic modes, for exchange‐coupled Fe/RFeO₃ heterostructures, including quasiferromagnetic resonance, impurity, coherent phonon, and quasiantiferromagnetic modes, are observed in a temperature range of 10–300 K. These experimental results not only offer an effective means to trigger ultrafast antiferromagnetic spin dynamics of rare‐earth orthoferrites, but also shed light on the ultrafast manipulation of antiferromagnetic magnetization in Fe/RFeO₃ heterostructures.     

作大范围平动弹性梁的动力学性质

本文利用初始构形，中间构形和现时构形三种构形，给出了作大范围平动弹性结构的运动学描述方法，建立了作大范围平动弹性梁的刚－柔耦合动力学控制方程。利用Melnikov方法和数值仿真技术，讨论了作大范围平动弹性梁的全局分岔和混沌性质，通过分析表明，当大范围平动超过某一临界速度时，它对弹性结构动力学性质的影响有着决定性的作用。本文的研究将有利于柔性多体系统的刚－柔耦合动力学建模理论的发展。     

Study on the global chaotic dynamics and control of liquid-filled spacecraft with flexible appendage

This paper investigates the global chaotic attitude dynamics and control of completely viscous liquid-filled spacecraft with flexible appendage. The focus in this paper is on the way in which the dynamics of the liquid and flexible appendage vibration are coupled. The equations of motion are derived and then transformed into a form suitable for the application of Melnikov’s method. Melnikov’s integral is used to predict the transversal intersections of the stable and unstable manifolds for the perturbed system. An analytical criterion for chaotic motion is derived in terms of the system parameters. This criterion is evaluated for its significance to the design of spacecraft. In addition, the Melnikov criterion is compared with numerical simulations of the system. Numerical solutions to these equations show that the attitude dynamics of liquid-filled flexible spacecraft possesses characteristics common to random, non-periodic solutions and chaos. This paper demonstrated that the desired final polarity control is guaranteed by using a pair of thruster impulses. The control strategy for a reorientation maneuver is designed and the numerical simulation results are presented for both the uncontrolled and controlled spin transition.     

贮箱内液体小幅晃动的频率和阻尼计算

根据小幅晃动条件下利用速度势函数推导的计算液体晃动频率和内部Rayleigh阻尼的特征值方程,利用简化处理转化为一般的广义特征值问题,并针对这种非对称大型稀疏矩阵采用了Arnoldi迭代方法,求解得到晃动频率和内部阻尼.将利用Stokes边界层理论计算得到边界上的阻尼与内阻尼之和作为晃动的等效阻尼,通过两个算例比较计算结果接近实验结果.这种方法可以计算任意形状贮箱内液体的小幅晃动频率和晃动阻尼.     

Novel Aspects of Spin-Polarized Transport and Spin Dynamics

There is a renewed interest to study spin-polarized transport and spin dynamics in various electronic materials. Motivation to examine the spin degrees of freedom (mostly in electrons, but also in holes and nuclei) comes from various sources: ranging from novel applications which are either not feasible or ineffective with conventional electronics, to using the spin-dependent phenomena for exploring fundamental properties of solid-state systems. Taken in a broader context, term spintronics is addressing various aspects of these efforts and stimulating new interactions between different subfields of condensed matter physics. Recent advances in material fabrication made it possible to introduce the nonequilibrium spin in novel class of systems, including ferromagnetic semiconductors, high temperature superconductors, and carbon nanotubes—which leads to a question of how could such a spin be utilized. For this purpose it is important to extend the understanding of spin-polarized transport and spin dynamics to consider inhomogeneous systems, various heterostructures, and the role of interfaces. This article presents some views on novel aspects of spin-polarized transport and spin dynamics (referring also to the topics which were addressed at the conference Spintronics 2001) and suggests possible future research directions.     

绝对节点坐标列式流体单元建模方法及在液体晃动问题中的研究

计算流体力学的建模方法主要采用欧拉描述,而多体系统动力学的建模方法主要采用拉格朗日描述。与欧拉描述关注于流过空间固定点或固定体积上的流场状态不同,绝对节点坐标列式流体单元采用拉格朗日描述,能够跟踪流体物质点,建立流体与多体系统的统一描述。该文在绝对节点坐标列式流体单元方法基础上,提出和完善单元建模理论,并使用绝对节点坐标列式流体单元实现了对流体系统的建模,并首次将绝对节点坐标列式流体单元应用于液体晃动分析,初步验证了理论的正确性和可行性。仿真结果表明,在单元数量较少情况下,绝对节点坐标列式流体单元可以满足晃动计算需求。     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

A Critical Examination of the Spin Dynamics in High-Tc Cuprates

A critical examination of the spin dynamics in high-T _c cuprates is made in the light of recent inelastic neutron scattering results obtained by different groups. The neutron data show that incommensurate magnetic peaks in YBCO belong to the same excitation as the resonance peak observed at (/a, /a). Being observed only in the superconducting state, the incommensurability is rather difficult to reconcile with a stripe picture. We also discuss the link between the resonance peak spectral weight and the superconducting condensation energy.     

Drift Transport of Spin-Polarized Electrons in GaAs

We studied the transport properties of spin-polarized conduction electrons in GaAs by time-resolved photoinjection and photoluminescence polarization measurements. Under an electric field of a few kV/cm, the degree of the spin polarization considerably decreases during drift transport over the distance shorter than 4 m. Spin relaxation by D'yakonov–Perel' mechanism is suggested to be the cause of such a rapid spin depolarization for hot electrons under a moderate electric field at low temperatures.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Spin Injection: A Survey and Review

The study of the transport and relaxation of spin-polarized carriers in the solid state began about 30 years ago. Tunneling spectroscopy was applied to ferromagnet–insulator–superconductor junctions to demonstrate the polarization of interfacial currents. The use of a ferromagnetic material as an injector and/or detector of polarized carriers has since become a valuable tool, and spin injection has been applied to nonmagnetic metals, superconductors, and semiconductors. The spin injection phenomenology is reviewed in the context of two topics of continuing importance for basic and applied research: (i) the transmission of polarized carriers across ferromagnet/nonmagnetic material interfaces and (ii) carrier spin relaxation inside the nonmagnetic material.     

Dynamics of charged and conducting drops via the hybrid finite-boundary element method

A numerical study on the dynamic behaviour of a charged and conducting drop, with net electrical charge , is presented here, that is valid for arbitrary initial disturbances. It employs the integral form of Laplace's equation for the calculation of the velocity and electrostatic potentials, which only requires discretization and solution on the surface of the drop. Thus a hybrid method results with the integral equations solved via the boundary element technique, while the Galerkin finite element formulation is used for the kinematic and dynamic condition at the interface as well as for the net charge conservation equation. Recently, the authors followed this approach in their study on the free nonlinear oscillations of inviscid drops, and they were able to optimize time and space discretization as well as the treatment of the integral equation with excellent results.     

Surface tension measurements of metallic melts under microgravity

The surface tension of liquid metals and alloys was measured for the first time in microgravity using the oscillating drop technique. Data for pure gold, a congruently melting gold copper alloy, and an eutectic zirconium nickel alloy are presented. We find excellent agreement with available results obtained on Earth by the same technique, but only if the latter are corrected to account for gravity effects. This is not only shows the necessity for the correction of the surface tension data derived from Earth-bound oscillating drop experiments, but also proves its correctness.DeceasedPaper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover (SCO) complex [Fe(HB(tz)₃)₂] (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.     

Spin Dynamics in Disordered Solids

Modern state of studies in spin dynamics of statically disordered media is presented. Next four fundamental problems are attended mainly: 1) delocalization of nuclear polarization in subsystem of impurity nuclei (it is exemplified in model nuclear spin system ⁸Li-⁶ Li in the LiF single crystal) 2) nuclear relaxation via paramagnetic impurities in crystals of arbitrary space dimension d 3) free induction decay and EPR line form function at d ≤ 3; and 4) form function of the hole, burned on the wing of the dipolar EPR line.     

Molecular Dynamics Simulation of the Specific Heat of Undercooled Fe-Ni Melts

In this paper, a molecular dynamics simulation based on the embedded-atom method (EAM) is applied to calculate the specific heat capacity of the Fe-33% Ni alloy at temperatures above and below the melting temperature. The relationship between the specific heat of the alloy and undercooling is investigated. The heat capacity of the Fe-Ni alloy is a constant in the superheated and undercooled liquid states. Comparisons between previous work and the current study show that the heat capacity behavior of undercooled alloy strongly depends on the species of the alloy.     

A new particle method for simulation of incompressible free surface flow problems

A new Lagrangian particle method called the consistent particle method (CPM), which solves the Navier–Stokes equations in a semi‐implicit time stepping scheme, is proposed in this paper. Instead of using kernel function as in some particle methods, partial differential operators are approximated in a way consistent with Taylor series expansion. A boundary particle recognition method is applied to help define the changing liquid domain. The incompressibility condition of free surface particles is enforced by an adjustment scheme. With these improvements, the CPM is shown to be robust and accurate in long time simulation of free surface flow particularly for smooth pressure solution. Two types of free surface flow problems are presented to verify the CPM, that is, two‐dimensional dam break and liquid sloshing in a rectangular tank. In the dam break example, the CPM solutions of pressure and wave elevation are in good agreement with published experimental results. In addition, an experimental study of water sloshing in tank on a shake table was conducted to verify the CPM solutions. Copyright © 2011 John Wiley & Sons, Ltd.     

Kinetic Theory of Spin Coherence of Electrons in Semiconductors

We investigate the spin precession and dephasing of electrons in semiconductors, using the kinetic Bloch equations for a four-spin-band model. Various scatterings, such as carrier–carrier, carrier–phonon, and carrier–impurity scatterings are taken into account. Their contributions to the evolution of carrier distribution, optical coherence, and more importantly, spin coherence in undoped semiconductor quantum wells and in n-type bulk semiconductors are considered. We highlight the importance of the kinetic theory approach by comparing our results with those from Fermi's golden rule. Furthermore, a new spin dephasing mechanism is proposed, that is, spin dephasing due to spin conserving scatterings in the presence of inhomogeneous broadening.     

Thermophysical Property Measurements in Microgravity: Chances and Challenges

The microgravity environment offers considerable advantages for the measurement of thermophysical properties, in particular, for high-temperature metallic melts. The absence of containers and of convection are the two major benefits. This paper reviews past microgravity experiments dealing with thermophysical property measurements and discusses the methods used. An outlook into the space station era is also given with special emphasis on chances and challenges.