Spin susceptibilities and the theory of itinerant-electron antiferromagnetism

This article investigates the equilibrium states of antiferromagnetic itinerant-electron systems in the Hartree-Fock approximation. As a result, the spin susceptibilities are determined in the random phase approximation. The lowlying collective excitations are then obtained by finding the poles of these susceptibilities.

We start by giving a brief review of the Hartree-Fock procedure and by indicating how the susceptibilities are obtained. The density matrix approach, where the ground state is interpreted as that minimizing the energy, is used throughout. Using an effective Coulomb interaction of the Hubbard type we consider two distinct systems: a one-band system with an incommensurate spin density wave in its ground state, and a many-band simply commensurate model for f.c.c. manganese.

The first of these is such that the band structure and resulting susceptibilities can be obtained explicitly. The spin-wave energies and wave-vectors are found by a careful, small energy and momentum transfer, expansion of these susceptibilities for the case of a parabolic band. The spin-wave damping, which is shown to arise from spin-wave decay into quasiparticle quasihole pairs, is also obtained for this band structure.

For the case of f.c.c. manganese the antiferromagnetic bands are obtained from a realistic 9-band paramagnetic model by using a many-band generalization of the Hubbard interaction. The enhanced spin susceptibilities are calculated, using the tetrahedral Brillouin zone integration method, and are presented along with their associated collective excitations. The results obtained are discussed with particular reference to the many-band effects. These effects are shown to be very much dependent on the particular form of interaction used.     

Composite-fermionization of the mixture composed of Tonks gas and Fermi gas

This paper investigates the ground-state properties of the mixture composed of the strongly interacting Tonks-Girardeau gas and spin polarized Fermi gas confined in one-dimensional harmonic traps, where the interaction between the Bose atoms and Fermi atoms is tunable. With a generalized Bose-Fermi transformation the mixture is mapped into a two-component Fermi gas. The homogeneous Fermi gas is exactly solvable by the Bethe-ansatz method and the ground state energy density can be obtained. Combining the ground-state energy function of the homogeneous system with local density approximation it obtains the ground-state density distributions of inhomogeneous mixture. It is shown that with the increase in boson-fermion interaction, the system exhibits composite-fermionization crossover.     

激光干涉技术在超声速气体喷嘴特性研究中的应用

在激光与物质相互作用的实验中,气体靶通常由超声速喷嘴在高背压下向真空中高速喷射气体产生。激光与气体靶相互作用时确定打靶条件对整个实验有着十分重要的意义。为了得到不同实验条件下气体靶密度的分布特性，采用马赫-曾德尔干涉法测量了气体靶密度分布，获取了干涉图样。使用基于傅里叶变换的条纹处理方法测得的干涉图样，得到不同实验条件下气体分子密度的全空间分布。实验表明：用M-Z干涉仪测量超声速气体喷嘴产生的气体靶密度分布十分有效。基于傅里叶变换的条纹处理方法具有精度高、实时性好的优点，为打靶时气体靶密度的实时测量提供了可能。     

Kinetic description of the Coulomb explosion of a spherically symmetric cluster

The particle distribution function is calculated for the Coulomb explosion of a spherically symmetric charged cluster formed through the interaction of intense ultrashort laser pulses with a cluster gas. The particle density and mean velocity distributions as well as the energy spectra of the accelerated particles are obtained. These characteristics are analyzed in detail for a cold cluster plasma, where the kinetic effects determine the physics of multiple flows emerging after the turnover of the cluster particle velocity profile. We find the boundaries of the multiple-flow regions and study the characteristics of an exploding cluster as a function of its initial density profile. The energy spectra of the accelerated ions are obtained for a cluster plasma with a specified cluster size distribution.     

人工等离子体云的时间演化

分析了高空中人工等离子体云的时间演化。发现人工等离子体云中电子密度的空间分布存在着指数衰减区和平缓衰减区。等离子体发生器的粒子流密度仅影响指数衰减区,对平缓衰减区没有什么影响。平缓衰减区中的平均电子密度约为中性分子密度的十万分之一。并且可以覆盖较大范围。这些结果对于电磁波与人工等离子体相互作用的研究、导弹和空间飞行器的等离子体隐身研究及人工等离子体发生器的设计有一定的参考价值。     

Density dependence of the transition temperature in a homogeneous bose-einstein condensate

Transition temperature data obtained as a function of particle density in the 4He-Vycor system are compared with recent theoretical calculations for 3D Bose-condensed systems. In the low density dilute Bose gas regime we find, in agreement with theory, a positive shift in the transition temperature of the form DeltaT/T0 = gamma(na(3))(1/3). At higher densities a maximum is found in the ratio of T(c)/T0 for a value of the interaction parameter, na(3), that is in agreement with path-integral Monte Carlo calculations.     

Fine structure of the soliton signal in the magnetic chain in an external magnetic field near a critical value

As is known, there is a critical magnetic field which separates two principally different zones for the soliton signal propagation in magnetic chains, the sine-Gordon zone and the Heisenberg zone. We investigate the fine structure of these signals in a neighborhood of the critical field with nonzero soliton velocity. Explicit formulas both for the azimuthal kink and meridional soliton are obtained. These formulas take into account the nonlinear interaction of soliton structures. Dedicated to the memory of Vladimir Borovikov     

Parameter space map of an electron cyclotron resonance plasma in acompact chamber

Ion current density measurements were made in an electron cyclotron resonance (ECR) plasma reactor for both argon and oxygen discharges. Spatial changes in the ion current density were also recorded across the reactor diameter for changes in pressure and power. These measurements revealed a minimum in the ion current density on the reactor axis. This observation has been explained as a consequence of the shape of the ECR region, which, in turn, is dependent on the mode of coupling. Current density measurements were made as a function of reactor pressure and microwave power for two different axial locations in the system. A Langmuir probe was also used at these two locations to measure the electron temperature as a function of these process conditions. It was observed that the ion current density and/or plasma density measured downstream from the ECR zone, increased significantly in the low-pressure/high-microwave power region. Results from this region of the operating parameter space have not previously been reported. Further existing models do not predict this observed increase in plasma density or ion current density. It has been proposed that a rarefication of the gas in the ECR region, as a result of gas heating, has acted to increase the outward diffusion of electrons from the ECR zone and, thus, has increased the ambipolar diffusion of ions to the downstream location. This proposal has been partially validated by experimental results in which the ion energy was measured as a function of reactor pressure and gas flow rate. The shape of the oxygen parameter space map differs significantly from that for Ar. The principal reasons for these changes are a number of different inelastic electron scattering mechanisms which effect the transport electrons out of the ECR zone and through ambipolar diffusion also the transport of ions. The second factor is the production of negative ionic species which varies with reactor pressure and, thus, T_e     

Interaction of intense laser pulses with atomic clusters at different gas densities

The interaction of intense femtosecond laser pulses with rare gas clusters was studied experimentally,the time-of-flight spectra of ions from exploding clusters at different gas densities have been measured. It is found that while the relative components of ions inlow and high energy of the ion energy spectrum decrease with the increase of the gas density,the average ion energies are the same for different gas densities,which indicates that the effect of gas density on laser-cluster interaction is not important under our experimental conditions.     

Unified low-parametrical equation used to calculate the viscosity coefficient of argon

Using the previously obtained dependence of excess viscosity on internal energy density and low-parametric unified equation of state for calculation of thermodynamic properties of liquid, gas, and fluid, the equation for the excess viscosity of argon in the range of the “mixed” mechanism of momentum transfer in the shear flow was derived. Different versions of approximation of excess viscosity dependence on the density of interaction energy were compared, and the optimal version of this dependence was determined. A simple unified low-parametric equation was obtained for describing the coefficient of argon viscosity in a wide range of state parameters. It is shown that the proposed low-parametric equation for calculating the viscosity coefficient of liquid and gas allows reliable extrapolation beyond the studied region.     

Spray flow-network flow transition of binary Lennard-Jones particle system

We simulate gas-liquid flows caused by rapid depressurization using a molecular dynamics model. The model consists of two types of Lennard-Jones particles, which we call liquid particles and gas particles. These two types of particles are distinguished by their mass and strength of interaction: a liquid particle has heavier mass and stronger interaction than a gas particle. By simulations with various initial number densities of these particles, we found that there is a transition from a spray flow to a network flow with an increase of the number density of the liquid particles. At the transition point, the size of the liquid droplets follows a power-law distribution, while it follows an exponential distribution when the number density of the liquid particles is lower than the critical value. The comparison between the transition of the model and that of models of percolation is discussed. The change of the average droplet size with the initial number density of the gas particles is also presented.     

Momentum distribution and condensate fraction of a fermion gas in the BCS-BEC crossover

By using the diffusion Monte Carlo method we calculate the one- and two-body density matrix of an interacting Fermi gas at T = 0 in the BCS to Bose-Einstein condensate (BEC) crossover. Results for the momentum distribution of the atoms, as obtained from the Fourier transform of the one-body density matrix, are reported as a function of the interaction strength. Off-diagonal long-range order in the system is investigated through the asymptotic behavior of the two-body density matrix. The condensate fraction of pairs is calculated in the unitary limit and on both sides of the BCS-BEC crossover.     

Nuclear Level Density at High Spin and Excitation Energy

The intensive studies of equilibrium processes in heavy-ion reaction have produced a need for information on nuclear level densities at high energies and spins. The Fermi gas level density is often used in investigation of heavy-ion reaction studies. Some papers have claimed that nuclear level densities might deviate substantially from the Fermi gas predications at excitations related to heavy-ion reactions. The formulae of calculation of the nuclear level density based on the theory of superconductivity are presented, special attention is paid to the dependence of the level density on the angular momentum. The spin-dependent nuclear level density is evaluated using the pairing interaction. The resulting level density for an average spin of 52 ħ is evaluated for ¹⁵⁵Er and compared with experimental data. Excellent agreement between experiment and theory is obtained.     

Determination of the dynamic characteristics of aerogels in the energy-release zone of a high-power electron beam

The dynamics of the interaction of a high-current electron beam with SiO₂ aerogels of different density and porosity are investigated by optical methods. A model for describing highly porous materials is developed on the basis of the information obtained regarding the unloading of aerogels in the energy-release zone of an electron beam, as well as the measured energy-release profiles. A corresponding nonlinear self-consistent equation of state is obtained which reflects the fractal properties of aerogels and permits determination of the thermodynamic characteristics of aerogels as the porosity varies tens of times. The influence of electric space charge on the energy-absorption profile of a high-power electron beam in aerogels of different density is discussed. Zh. Tekh. Fiz. 68, 112–120 (October 1998)     

Mean field effects in a lattice gas model of ionic space charge

Lattice gas models of the diffuse space charge layer in liquids and in ionic solids with Schottky or Frenkel disorder are considered with and without mean-field charge-interaction corrections. Even without explicit interaction corrections, the lattice gas model, by taking some account of charge carrier size in the liquid case and the actual lattice structure of the material in the case of single crystal solids, predicts a saturation in local charge density at high potentials, unlike the physically less realistic conventional Gouy-Chapman ideal gas, or independent particle model. Both unmodified and modified mean field corrections to the lattice gas basis are discussed and Coulombic interaction terms are considered in detail. The Monte-Carlo results obtained by other workers show that, as expected, the actual mean-field interaction terms required for fitting must be much smaller than those predicted by the unscreened Coulombic interaction. It is suggested that the mean field approach of Gurevich and Kharkats, who introduce an attractive mean field interaction between charges of like sign, is unlikely to be applicable over the entire space charge region of solid electrolyte systems and thus may not provide an explanation for the conductivity instability observed in α-AgSbS₂. Some possible future directions of investigation for lattice gas models are also briefly discussed.     

关于两个量子统计模型中铁磁态平均场理论的讨论

利用平均场理论讨论了零温下具有长程库仑势的电子气体和短程相互作用的费米原子气体的铁磁相变问题,比较了两个模型中铁磁转变的不同性质.     

强流电子束碰撞电离背景气体研究

利用数值计算与粒子模拟两种方法,结合实际的实验数据,对高功率微波二极管中相对论电子束与背景气体相互作用碰撞产生的等离子体密度进行了研究.研究结果表明：碰撞产生的等离子体密度数值计算结果与粒子模拟结果基本一致,背景气压在0.01 Pa—0.05 Pa时,碰撞产生的等离子体密度在4—12×10⁹cm^-3,即便在考虑电子离子复合的情况下,数值计算结果与粒子模拟结果依然符合得很好.另外,粒子模拟结果表明：随着气压的增加,等离子体密度呈现先增大再减小然后又逐渐增大的过程, 关键词： 相对论电子束 等离子体 数值计算 粒子模拟     

弱相互作用费米气体的不稳定性判据

根据由赝势法得到的弱相互作用费米气体的自由能，利用热力学方法研究了无外场时弱相互作用费米气体的稳定性.结果表明，无外场情况下理想费米气体与存在弱排斥相互作用的费米气体是稳定的；而具有弱吸引相互作用的费米气体在一定条件下可出现不稳定性.给出了不稳定性的粒子数密度判据和温度判据，就不同逸度情况下临界粒子数密度的具体表达结果以及温度、粒子质量和吸引相互作用对临界粒子数密度的影响进行了讨论. 关键词： 费米气体 相互作用 不稳定性判据     

Normal current density effect in RF-discharge according to resultsof two-dimensional numerical modeling

For the first time, the normal density effect is demonstrated by modeling a two-dimensional cylindrical column of a moderate pressure RF-discharge operating in the α-regime. When the current increases, the discharge column is expanded, with the current density and voltage on the electrodes being unchanged. It is clarified why the boundary between currentless and current (filled with plasma) regions is stabilized. Outside the plasma zone, the gas temperature and the gas density at constant pressure are lower (no Joule's energy release) than those in the plasma zone, therefore the normal voltage is not sufficient to maintain plasma there. It is shown that simple one-dimensional models give quite reasonable results for parameters of the normal discharge     

Quantum information transmission between two qubits through an intermediary photon gas

The theory of the quantum information transmission between two semiconductor two-level quantum dots as two qubits through an intermediary photon gas in a cavity is presented. The reduced density matrix of each two-level quantum dot is the quantum information encoded into this qubit. The quantum information exchange between two distant qubits imbedded in the photon gas is performed in the form of the mutual dependence of their reduced density matrices due to the interaction between the electrons in the qubits and the photon gas. The system of rate equations for the reduced density matrix of the two-qubit system is derived. From the solution of this system of equations it follows the mutual dependence of the reduced density matrices of two distant qubits.