The Theory of Equilibrium Fluctuations of Thermodynamic Quantities in Open Systems with a Small Number of Particles

Fluctuations in open subsystems with a small number of particles and fixed volume immersed in an equilibrium system (thermostat) are treated. Within the framework of classical thermodynamics, the distribution functions of fluctuations of the number of particles, momentum, and energy in these subsystems are derived. These results are compared with the corresponding data obtained for hard spheres using the method of molecular dynamics. Their satisfactory agreement is observed. It is demonstrated that the distribution functions of fluctuations of the number of particles and energy are significantly different from both the Gauss and Poisson distribution. The distribution function of fluctuations of momentum of a small subsystem is Gaussian and independent of the density of the thermostat.     

Magnetoresistance of the Two-Dimensional Electron Gas in GaN Quantum Wells in a Parallel Magnetic Field

We present results for the resistivity of the two-dimensional spin-polarized electron gas as realized in GaN quantum wells at zero temperatures. A parallel magnetic field is used to create a spin-polarized electron gas. We discuss the density dependence of the magnetoresistance for impurity scattering and interface-roughness scattering. Finite width effects of the electron gas on the magnetoresistance are described.     

Peculiarities of the Azimuthal Transport of High-Current Electron Beams in a Curvilinear Cavity with a Transverse Magnetic Field

Technical Physics Letters - Peculiarities of the azimuthal transport of high-current electron beams in a curvilinear cavity with a transverse magnetic field have been studied by means of PIC...     

The Phase Diagram and the Structure of the Charge-Density-Wave State in High Magnetic Field

The properties of a charge-density wave (CDW) state in quasi-one-dimensional metals in magnetic field below the transition temperature are studied using the mean-field theory. We calculate the phase diagram and show that the CDW state with shifted wave vector in high magnetic field (CDW_x phase) has at least double-cosine modulation on the most part of the phase diagram. The transition from CDW₀ to CDW_x state below the critical temperature is accompanied by a jump of the CDW order parameter and of the wave vector rather than by their continuous increase. This implies a first order transition between these CDW states and explains the strong hysteresis accompanying this transition in many experiments. We examine how the phase diagram changes in the case of imperfect nesting. The results are applicable to different families of organic metals.     

磁场中Bi-Mn合金中MnBi相定向排列组织的形成规律和机制

研究了Bi-Mn合金从固液两相区开始凝固，合金中MnBi相在磁场中形成定向排列组织的临界条件及其变化规律。实验研究表明，Bi-Mn合金中MnBi相在磁场中的定向排列能够迅速形成；MnBi相形成定向排列组织所需的临界温度随外磁场强度的增大而降低；恒温温度相同时，6％Mn合金形成定向排列组织所需的临界磁场强度高于3％Mn合金。在临界值以上，定向排列因子T随恒温温度的升高和磁场磁感应强度的增强而增大。理论分析结合实验研究表明，合金中MnBi晶体在磁场中取向的机制主要是旋转取向，晶体取向驱动力主要是晶体具有磁各向异性，在磁场中磁化过程中所受的磁力矩，阻力主要是晶体间碰撞阻碍。     

Effect of Surface Roughness on the Magnetic Field Profile in the Meissner State of a Superconductor

It is well known that the London model (valid for a hard type II superconductor) predicts an externally applied magnetic field decays exponentially as a function of depth into the superconductor on a length scale λ. Direct measurements of the field profile using low energy μSR on high-Tc superconductors, such as YBa₂Cu₃ O _x measure deviations from a simple exponential decay. In particular, there is a short length scale δ close to the surface where the magnetic field does not decay. It has been proposed that this is due to surface roughness, which leads to a suppression of the order parameter near the surface. A model of surface roughness has been studied for the case of a sinusoidally modulated surface roughness on an isotropic superconductor showing that in some cases the profiles resulting from surface roughness may be qualitatively similar to the dead layer phenomena in that the field magnitude decay rate may be slowed near the surface relative to a flat interface but that for modest roughness, the quantitative value of the length over which the field decay is slowed is much smaller than the experiments measure. In this paper, we extend this work in two directions: firstly, by using atomic force microscopy data of YBa₂Cu₃ O _x crystals, we predict the expected field profiles within the context of the Isotropic London model of superconductivity given their actual surface geometry; and secondly, we consider how surface roughness could affect experimental values for λ and δ. The main finding is that roughness within an isotropic model does not produce the dead layers found in experiments on YBa₂Cu₃ O _x . However, we suggest that roughness in a highly anisotropic superconductor could account for the observed dead layer.     

The Low-Energy State of an Electron Beam in a Coaxial Diode with a Homogeneous Anode and Inhomogeneous Magnetic Field Profile

Technical Physics Letters - The possible formation of an extended low-energy state of electron beam in a coaxial diode with homogeneous cylindrical anode and moderate magnetic field with...     

Influence of Magnetic Field and LO Phonon Effects on the Spin Polarization State Energy of Strong-Coupling Bipolaron in a Quantum Dot

On the basis of Lee–Low–Pines unitary transformation, the influence of magnetic field and LO phonon effects on the energy of spin polarization states of strong-coupling bipolarons in a quantum dot (QD) is studied by using the variational method of Pekar type. The variations of the ground state energy $E_0$ and the first excited state the energy $E_1$ of bipolarons in a two-dimensional QD with the confinement strength of QDs $\omega _0$ , dielectric constant ratio $\eta $ , electron–phonon coupling strength $\alpha $ and cyclotron resonance frequency of the magnetic field $\omega _{c}$ are derived when the influence of the spin and external magnetic field is taken into account. The results show that both energies of the ground and first excited states ( $E_0$ and $E_1)$ consist of four parts: the single-particle energy of electrons $E_\mathrm{e}$ , Coulomb interaction energy between two electrons $E_\mathrm{c}$ , interaction energy between the electron spin and magnetic field $E_\mathrm{S}$ and interaction energy between the electron and phonon $E_{\mathrm{e-ph}}$ ; the energy level of the first excited state $E_1$ splits into two lines as $E_1^{(1+1)}$ and $E_1^{(1-1)}$ due to the interaction between the single-particle “orbital” motion and magnetic field, and each energy level of the ground and first excited states splits into three “fine structures” caused by the interaction between the electron spin and magnetic field; the value of $E_{\mathrm{e-ph}}$ is always less than zero and its absolute value increases with increasing $\omega _0$ , $\alpha $ and $\omega _c$ ; the effect of the interaction between the electron and phonon is favorable to forming the binding bipolaron, but the existence of the confinement potential and Coulomb repulsive energy between electrons goes against that; the bipolaron with energy $E_1^{(1-1)}$ is easier and more stable in the binding state than that with $E_1^{(1+1)}$ .     

Microwave Oscillations of the Giant Magnetoresistive Element in a Magnetic Field Perpendicular to the Plane

Applied voltage and magnetic field dependence of microwave oscillations have been studied for the giant magnetoresistive (GMR) element. In this experiment, a magnetic field is applied perpendicular to the plane of GMR films. It is found that the power spectral density (PSD) is less dependent on the direction of magnetic field and the resistance gap induced by the magnetic field corresponds to the rapid increase of PSD. Moreover, noise peaks are observed in a high frequency range (about 8 GHz or higher) when a magnetic field of more than 2000 Oe is applied. The peak frequencies shift to higher frequency as the magnitude of magnetic field increases. In contrast to a magnetic field, the peak frequencies and their widths are more susceptible to the direction of current.     

磁性材料在RAM中的应用及其进展

雷达波吸波材料（ＲＡＭ）在军事及民用领域具有广泛的应用前景。本文在讨论磁性ＲＡＭ工作的机理、特点的基础上,阐述磁性ＲＡＭ国内外发展状况,并介绍了本单位近年来部分研究成果,说明磁性材料在ＲＡＭ应用中具有重要价值。     

The Effect of a Longitudinal Magnetic Field on the Radiation of Glow Discharge in Helium

The effect of a longitudinal magnetic field on the radiation of a glow discharge in helium is investigated experimentally. It is demonstrated that, with pressures in the discharge chamber below approximately 140 Pa, the intensity of radiation of spectral lines of helium and of continuous spectrum increases by a factor of 200–300 under the effect of a magnetic field. So strong an increase in the radiation of glow discharge in helium is observed in the region where no electric field of discharge is present. An attempt is made at attributing the obtained result to the decrease in the ambipolar diffusion of electrons to the wall; this brings about an increase in the density and temperature of electrons which start exciting metastable states of helium.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 4, 2005, pp. 516–525.Original Russian Text Copyright © 2005 by I. M. Ulanov and A. Yu. Litvintsev.     

Calculation and Analysis of the Magnetic Force Acting on a Particle in the Magnetic Field of Separator. Analysis of the Equations Used in the Magnetic Methods of Separation

This paper presents the derivation of equation for calculating the magnetic force acting on a paramagnetic ball situated in a region of a nonuniform magnetic field. We suggest the method of its extrapolation to the case of a nonlinear medium-the case of a ferromagnetic ball found under the influence of the magnetic field which is varying in a certain properly defined range. An experimental setting intended for the measurement of the magnetic force is described as well. For the first time a rigorous proof of the isodynamic magnetic field inexistence, that is a field which generates a magnetic force having the same constant direction and the same constant magnitude at each space point, is given. Moreover, using the method of conformal mappings we derive the analytic expressions for the magnetic field strength calculation in the case of a multipole system. With their help, different important features of the magnetic field behavior in this case are elucidated. The latter is done in a wide region of the geometric parameter space of the magnetic system. The interpretations of different issues discussed in the paper are compared to those that seems to be widely accepted in the magnetic methods of separation branch     

Stability of Ferrofluid Flows in a Horizontal Channel Subjected to a Longitudinal Temperature Gradient and an Oblique Magnetic Field

The stability of thermoconvective flows of a ferrofluid in a horizontal channel subjected to a longitudinal temperature gradient and an oblique strong magnetic field is studied. The flows are governed by the mass, momentum, heat balance and Maxwell equations, in the Boussinesq approximation. Strong magnetic fields are characterized by a small parameter measuring the deviation of the magnetization across the layer from the external magnetic field. An approximate solution of the stationary hydrodynamic problem was found in analytical form in Hennenberg et al. (Phys Fluids 18:093602, 2006). The flow depends on the thermal (gravitational) and magnetic Rayleigh numbers. In the present paper the linear stability of that basic flow is studied by the Galerkin method. The analysis shows that for large Prandtl numbers, typical for ferrofluids, and relatively small magnetic Rayleigh numbers, only oscillatory instability can appear. For a given magnetic Rayleigh number, the critical wavenumber does not depend on the inclination of the magnetic field, while the critical thermal Rayleigh number slightly changes. For horizontal and vertical magnetic fields, both critical numbers decrease when increasing the magnetic Rayleigh number.     

Convective and Absolute Instability in the Incompressible Boundary Layer on a Rotating Disk in the Presence of a Uniform Magnetic Field

The stability of a conducting fluid flow over a rotating disk with a uniform magnetic field applied normal to the disk, is investigated. It is assumed that the magnetic field is unaffected by the motion of the fluid. The mean flow and linear stability equations are solved for a range of magnetic field-strength parameters and the absolute/convective nature of the stability is investigated. It is found that increasing the magnetic field parameter is in general stabilizing.     

Effect of a Rotating Magnetic Field on Convection Stability and Crystal Growth in Zero Gravity and on the Ground

A numerical study of the flows arising in a conducting liquid under the action of a rotating magnetic field as well as under its interaction with gravitational and thermocapillary convection has been made. The boundaries of the transition to the oscillating regime of convective flows have been determined. Regions of mixed flows, in which the impurity macrosegregation in crystals grown by the Bridgman and floating-zone methods decrease, have been revealed. It has been shown that regions of flows in which both a smooth increase in the impurity macrosegregation and a change in the form of a clearly defined extremum are observed also exist. The possibilities of mathematical modeling of geophysical problems with the use of a rotating magnetic field are discussed.