Magnetic susceptibility and collisionless spin waves in liquid3He and3He-4He mixtures

The transverse spin susceptibility for a Fermi liquid in a uniform magnetic field is calculated in the collisionless regime using the Landau theory of Fermi liquids. The singularities corresponding to the l=0 and l=1 spin waves are investigated in pure and dilute³He as a function of the wave vector q up to the threshold of the Landau damping.     

Numerical study of the stability of solutions for the half-space Ginzburg–Landau model

Based on a shooting alternative that allows one to numerically solve the one-dimensional system of Ginzburg–Landau in an unbounded domain, a numerical study of the stability of solutions of this system is performed here. This stability notion, from a physical point of view, means that each solution of the system is identified as stable when it minimizes the corresponding Ginzburg–Landau functional. As opposed to a previous paper, the present one is concerned with a more general study since the weak and large regimes of the Ginzburg–Landau parameter are considered and the initial data are no longer subject to the de Gennes condition. Certain conjectures regarding the superheating field are also investigated numerically.     

Quantum‐Hall to Insulator Transition in Ultra‐Low‐Carrier‐Density Topological Insulator Films and a Hidden Phase of the Zeroth Landau Level

A key feature of the topological surface state under a magnetic field is the presence of the zeroth Landau level at the zero energy. Nonetheless, it is challenging to probe the zeroth Landau level due to large electron–hole puddles smearing its energy landscape. Here, by developing ultra‐low‐carrier density topological insulator Sb₂Te₃ films, an extreme quantum limit of the topological surface state is reached and a hidden phase at the zeroth Landau level is uncovered. First, an unexpected quantum‐Hall‐to‐insulator‐transition near the zeroth Landau level is discovered. Then, through a detailed scaling analysis, it is found that this quantum‐Hall‐to‐insulator‐transition belongs to a new universality class, implying that the insulating phase discovered here has a fundamentally different origin from those in nontopological systems.     

Graphene Acoustic Phonon‐Mediated Pseudo‐Landau Levels Tailoring Probed by Scanning Tunneling Spectroscopy

Graphene has attracted great interests in various areas including optoelectronics, spintronics, and nanomechanics due to its unique electronic structure, a linear dispersion with a zero bandgap around the Dirac point. Shifts of Dirac cones in graphene creates pseudo‐magnetic field, which generates an energy gap and brings a zero‐magnetic‐field analogue of the quantum Hall effect. Recent studies have demonstrated that graphene pseudo‐magnetic effects can be generated by vacancy defects, atom adsorption, zigzag or armchair edges, and external strain. Here, a larger than 100 T pseudo‐magnetic field is reported that generated on the step area of graphene; and with the ultrahigh vacuum scanning tunneling microscopy, the observed Landau levels can be effectively tailored by graphene phonons. The zero pseudo‐Landau level is suppressed due to the phonon‐mediated inelastic tunneling, and this is observed by the scanning tunneling spectroscopy spectrum and confirmed by the Vienna ab initio simulation package calculation, where graphene phonons modulate the flow of tunneling electrons and further mediate pseudo‐Landau levels. These observations demonstrate a viable approach for the control of pseudo‐Landau levels, which tailors the electronic structure of graphene, and further ignites applications in graphene valley electronics.     

The Symmetry-Driven Transition in Magnetoluminescence at νe = 2: The Rashba Method in Action

The magnetoluminescence spectra of symmetric quantum wells containing an electron gas show an abrupt transition from Landau level behavior (i.e. a linear shift of energy with field) to quadratic (exciton-like) behavior at the field at which the lowest Landau level is just filled, so that the electron filling factor ν_e = 2. Professor Rashba's insight was that this transition is a natural (although surprising, since the high field limit is by no means reached) consequence of the hidden symmetry which is known to hold in two-dimensional systems at high fields. This interpretation also accounts for the appearance ofmagnetoplasmon, “shake-up” and cyclotron satellites when ν_e > 2.     

High-Field Electronic Properties of Graphene

We have measured the energy gaps in single-layer and bilayer graphene by means of temperature dependent transport experiments in high magnetic fields up to 33 T. They follow the expected Landau level splitting when a finite level width is taken into account. The quantum Hall effect, hitherto only observed up to 30 K, remains visible up to 200 K in bilayers and even up to room temperature in single-layer graphene. Our experiments in single-layer graphene show that the lowest Landau level, shared equally between electrons and holes at zero energy, becomes extremely narrow in high magnetic fields. It is this narrowing, together with the large Landau level splitting in graphene that leads to an extremely robust localization and makes the quantum Hall effect visible up to room temperature. In high magnetic fields (B>20 T) we observe a strongly increasing resistance with decreasing temperature. These results are explained with field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements.     

The propagation of magnetic bubbles on permalloy disks

A magnetic bubble generator consisting of a Permalloy disk and a current conductor loop has been used recently in a mass memory design utilizing magnetic bubble technology. The bias field range in which the disk can hold the seed bubble is measured in this report as a function of of the rotating field frequency. Above a critical frequency fc, the bias field margins begin to decrease. The dependence of fcon disk size is obtained for disks with diameters from 16 μm up to 43 μm at rotating fields of 20 and 30 Oe. The separation between Permalloy disks and the garnet film is kept at 0.8 μm or 1.6 μm. Results show that at a fixed rotating field, a smaller disk is preferable at higher frequency for a magnetic bubble material with a given mobility. The critical frequency fcobtained is in good agreement with a theoretical calculation using the viscous damping model by Rossol et al. For frequencies below fc, the bias field margin on the disk is equal to that of the propagating channel and circuit failure due to the loss of the generator seed bubble can be eliminated.     

A practical magnetic bubble AND-OR gate

The design and operation of a magnetic bubble AND-OR gate are reported. Operation at 100 kHz in a 25 Oe rotating field with 28.2 μm circuit periodicity was achieved with about 50 percent of the free bubble bias field margins. A transfer pulse is used to divert bubbles from a propagation path which delivers the AND output tO one which delivers the OR output. The transfer is defeated by the presence of a bubble in the appropriate cycle of the OR path. The AND bubble is then delayed by one cycle instead of being transferred. This frastrated transfer strategy was devised to circumvent the restrictive bias field limitations in the operation of previous logic circuits.     

Circularly polarized electroluminescence of quantum-size InGaAs/GaAs heterostructures with ferromagnetic metal-GaAs Schottky contacts

Electroluminescence (EL) of InGaAs/GaAs heterostructures with quantum wells and ferromagnetic metal (Co, Ni)/GaAs Schottky contacts has been studied in magnetic fields up to 10 T at a temperature of 1.5 K. The EL line corresponding to the recombination of electrons with injected holes exhibits splitting into components corresponding to the Landau levels in the applied magnetic field and shows circular polarization that significantly exceeds the level typical of such structures with nonmagnetic (Au/GaAs) contacts. The degree of circular polarization (P _EL) exhibits a nonmonotonic dependence on the applied magnetic field and is correlated with the filling of Landau levels. The maximum degree of circular polarization reached in the heterostructures studied is P _EL = 40%.     

Optimum design considerations for dual conductor bubble devices

A design for dual conductor, current-access bubble devices with 8-μm periods has been optimized with a numerical calculation method for bubble motion in a propagating magnetic field, generated around hole patterns in conductor layers. Magnetic bias field distributions are calculated for an oval hole chain in the conductor layers. Bubble motion equations are obtained with analytical field distribution functions approximating the calculated field distributions. Minimum drive current density Jminfor normal bubble propagation is determined by a solution to the equations. The hole shape has been optimized by the minimization of the drive power Pmin, the product of Jminand conductor resistance, which is calculated from current distributions around the hole pattern. Optimum layer thickness have also been obtained for 8-μm period bubble devices. Both registration tolerance between the two conductor layers and bubble skew effects have been studied semiquantitatively on the basis of the equations of motion. The numerical calculation method developed here is found to be a highly effective means to optimize pattern design for smaller period devices.     

Magnetization of a two-dimensional electron gas

The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.     

Transient bubble domain configuration in garnet materials observed using high speed photography

The dynamic domain configurations of bubbles in garnet materials have been studied using a sampling optical microscope capable of single exposure photographs with a 10 nsec exposure time. The microscope is an integral part of a sampling system so that the transient shape of the bubble is recorded at various times after a field pulse or, for bubbles in field access devices, during a clock cycle. A triggerable flowing nitrogen gas laser pumping a low Q Rodamine 6G Dye laser is used as an illumination source giving light pulses of ∼1.5 KW for 10 nsec. This light is sufficient to expose Kodak 4 × 16 mm movie film. Standard pulse generators (HP 214A) are used to make free bubble radial velocity measurements. A modified generator allows free bubble collapse measurement to be made. For bubbles propagating at operating frequency within field access devices, a standard bubble exerciser is used, synchronized to the sampling system. In this case, special samples with an internal mirror and epi-mode illumination are used. Illustrative results of bubble domain shapes in a chevron propagating structure and a 90° chevron expander detector are included.     

Dependence of Quantized Hall Effect Breakdown Voltage on Magnetic Field and Current

When large currents are passed through a high-quality quantized Hall resistance device the voltage drop along the device is observed to assume discrete, quantized states if the voltage is plotted versus the magnetic field. These quantized dissipative voltage states are interpreted as occurring when electrons are excited to higher Landau levels and then return to the original Landau level. The quantization is found to be, in general, both a function of magnetic field and current. Consequently, it can be more difficult to verify and determine dissipative voltage quantization than previously suspected.     

基于Landau二级相变理论的磁热效应分析

将Landau二级相变理论应用于二级磁相变材料的磁热效应,建立了磁熵变与磁场的直接关系表达的理论模型。以La0.7Sr0.3MnO3为例,在居里温度附近利用该理论模型和麦克斯韦关系式计算了磁熵变ΔSM,并进行了对比。结果表明基于Landau理论的计算结果与利用传统方法的计算结果相符合。而根据Landau平均场理论,二级磁相变材料中居里温度TC和磁熵变ΔSM最大的温度Tpeak不一致,但在居里温度附近ΔSM与磁场的相关性ΔSM=kH^n表达的指数为n=2/3。     

Ginzburg–Landau Analysis of the Doll–Näbauer Experiment

The Doll–Näbaucr experiment is reconsidered with respect to its theoretical interpretation in terms of the Ginzburg–Landau theory. We find analytical results for the temperature and magnetic field dependence of the order parameter, the transition temperature in finite fields, and the magnetic flux trapped inside the cylinder. This work is intended to demonstrate once again the predictive power and the conceptual beauty of the Ginzburg–Landau theory and is devoted to Vitalij Ginzburg on the occasion of his 90th birthday.     

熔融聚合物中气泡的运动分析与实验研究

文中求出了CO2气泡注入到熔融聚合物流场后的计算模型,利用熔体流变学理论与两相流理论对气泡的头部、尾部、速度进行分析,计算了气泡在聚合物熔体中的速度表达。结果表明,其头部速度Un,尾部速度Ut等不仅与熔体的性质、流场条件有关,同时还与此气泡在流道中位置、气泡内的压力变化等诸多因素有关。为此引入综合系数C以修正上述理论计算模型,通过一系列实验,得出影响CO2在熔融聚合物中运动的综合系数C,利用其对上述理论运动模型加以有效的实验修正,进一步得到符合实际的气泡在熔融聚合物流场中运动计算模型。     

Non-spherical bubble behavior in vortex flow fields

The boundary element method (BEM) is applied to solve the unsteady behavior of a bubble placed in a vortex flow field. The steady vortex field is given in terms of the viscous core radius and the circulation, both of which may vary along the vortex axis. For this study, 2DynaFS^©, an axisymmetric potential flow code which has been verified successfully for diverse type of fluid dynamic problems, is extended. The modifications to accommodate the ambient vortex flow field and to model the extreme deformations of the bubble are presented. Through the numerical simulations, the time history of the bubble geometry and the corresponding pressure signal at a fixed field point are obtained. A special effort is made to continue the numerical simulation after the bubble splits into two or more sub-bubbles. Indeed, it is found that an elongated bubble sometimes splits into smaller bubbles, which then collapse with the emission of strong pressure signals. The behavior of the axial jets after the split is also studied in more detail. This work was conducted at Dynaflow, Inc. (www.dynaflow-inc.com). The work has been supported by the Office of Naval Research under the contract No. N0014–99-C-0369 monitored by Dr. Ki-Han Kim. This support is greatly appreciated.     

New method for routine measurement of coercivity in magnetic bubble films

We describe a new technique for measuring coercivity in magnetic bubble films which consists of placing the film in a weak field gradient (∼1 Oe./μm) in order to obtain a set of finger-like domains. The unconstrained ends of these domains are caused to move back and forth in response to an oscillatory field, and the coercivity is obtained from an extrapolation of the linear portion of the response vs. drive field curve. We present a comparison between coercivity values in materials with 3μm and 1.7μm stripe-widths obtained using the new technique and bubble translation. Good correlation is observed for both types of material, the values obtained with the new technique being somewhat higher than the bubble translation values. The difference is ascribed to material non-uniformities.     

Three-Dimensional Numerical Simulation of Bubble Dynamics in Microgravity under the Influence of Nonuniform Electric Fields

A three-dimensional VOSET method is used along with the adaptive mesh refinement (AMR) method to simulate the behaviors of a bubble departing from the outside wall of a horizontal square-cross-section tube in microgravity under the influence of nonuniform electric fields. The effects of gravity, electric field intensity, fluid permittivity, and bubble initial position on the bubble detachment and rising are investigated and analyzed. Computational results show that the gravity and electric fields have significant influences on the bubble detachment and rising velocity and rising trajectory. Decrease in gravity results in the decrease in the buoyancy exerted on the bubble, considerably mitigating the rising capability of the bubble and delaying the bubble detachment. Imposing a nonuniform electric field, which exhibits physically the strongest intensity in regions near the tube wall, can supply an additional driving force as a replacement of the buoyancy to accelerate the bubble detachment and rising. It is also shown that a larger electric field intensity or larger ratio of liquid permittivity to gas permittivity leads to a larger deformation, easier detachment, and larger rising velocity, of the bubble. The nonuniformity of the electric fields can also affect the bubble motion trajectory and result in the asymmetric deformation of the bubble.