感生电场与动生电场的等效性探究

本文认为在感生电场的情况下,磁场的强弱变化可以引起磁场自身的横向运动,使得线圈中电子相对于磁场发生运动,从而等效为一个动生电场,受到洛伦兹力的作用.借助磁感线模拟磁场的运动方式,得到圆形回路中任意一点与磁场相对运动速度的表达式,进而推得该＂等效动生电场＂中的洛伦兹力.以螺线管为例,验证该方法可以解释感生电场所满足的规律.将感生电场与动生电场的产生原因统一为导体中电子与磁场的相对运动,相应电动势的非静电力统一为洛伦兹力.     

Radiative corrections in a vector-tensor model

In a recently proposed model in which a vector non-Abelian gauge field interacts with an antisymmetric tensor field, it has been shown that the tensor field possesses no physical degrees of freedom. This formal demonstration is tested by computing the one-loop contributions of the tensor field to the self-energy of the vector field. It is shown that despite the large number of Feynman diagrams in which the tensor field contributes, the sum of these diagrams vanishes, confirming that it is not physical. Furthermore, if the tensor field were to couple with a spinor field, it is shown at one-loop order that the spinor self-energy is not renormalizable, and hence this coupling must be excluded. In principle though, this tensor field does couple to the gravitational field.     

An ampere-like law for displacement vector field and near-field optical microscopy

We make up a novel and simple theory for near field and near-field optical microscopy (NOM). Our theory is composed of two parts. The first is a formulation to calculate the scattered near field of light by a small dielectric. We state that a wavenumber-vector-independent picture appears in the theory of the near field, and we find that this is expressed by an Ampere-like law for the displacement vector field. The second part is a formulation of field intensity for far field observation and near field observation from a unified point of view. We suggest a theoretical formula for the field intensity corresponding to the NOM image and demonstrate how to understand the relation between the near field and this image.     

Ultrafast field dynamics in large-aperture photoconductors

We investigated field screening mechanisms in large-aperture GaAs photoconductors, using an ultrafast pump-probe terahertz technique. After photoexcitation the bias field decreases to an intensity-dependent value as a result of near-field screening of the bias field. For longer delays the field exhibits an intensity-dependent decrease that results from a space-charge field caused by transport-induced charge separation. These measurements support recent theoretical results that the dominant saturation mechanism that limits terahertz output from large-aperture photoconductors is near-field screening of the bias field because the space-charge field develops on a much longer time scale than that of the terahertz pulse.     

Covariant Field Equations in Supergravity

Covariance is a useful property for handling supergravity theories. In this paper, we prove a covariance property of supergravity field equations: under reasonable conditions, field equations of supergravity are covariant modulo other field equations. We prove that for any supergravity there exist such covariant equations of motion, other than the regular equations of motion, that are equivalent to the latter. The relations that we find between field equations and their covariant form can be used to obtain multiplets of field equations. In practice, the covariant field equations are easily found by simply covariantizing the ordinary field equations.     

Mechanoelectrical transformations upon the elastic impact excitation of composite dielectric materials

The elastic impact excitation of dielectric materials leads to an ac electromagnetic field that is a superposition of the following several fields: the field appearing when a striker approaches a surface with an excess surface charge, a field induced by electrification processes when a striker is in contact with a material surface, the field created by a shift in the acoustic wave of charges at internal inhomogeneities, the field appearing upon acoustic surface-charge oscillations, and the field that is induced in piezoactive heterogeneous materials by the piezoelectric effect.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity.We also find that the stationary quantum discord can be increased by applying a classical driving field.Furthermore,we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence.Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity. We also find that the stationary quantum discord can be increased by applying a classical driving field. Furthermore, we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence. Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

随机横场与晶场作用混合自旋系统的热力学性质

利用有效场理论和切断近似，在伊辛模型的框架内考虑随机横场与晶场作用的混合自旋1/2 和自旋1系统的热力学性质。重点研究了晶场、横场和随机浓度对混合自旋系统相变的影响 ，研究发现在随机横场条件下，较小的晶场存在并不能改变临界横场阈值；取较大横场值时 在某些随机浓度的范围内出现重入相变现象，而取较小横场值时则没有出现重入相变现象。 给出了有关相图并进行了讨论。 关键词： 混合自旋系统 伊辛模型 热力学性质 随机横场     

Electron-Positron Pair Production in a Strong Laser Field Enhanced by an Assisted High Frequency Weak Field

Electron-positron pair production in a strong laser field enhanced by an assisted high frequency weak field is investigated by solving the quantum Vlasov equation.The average and residual pair number densities are obtained for sinusoid electric field and it is found that the high frequency assisted weak field will enhance pair production significantly.There exists an optimal frequency of assisted field that makes the pair production number density get a maximum one,which is a few orders of higher than that without assisted field.We also discuss the other possible assisted fields.     

平面横电磁波模的初值问题

经典理论所描述的电场和磁场处处同相的平面电磁波模并不存在.Maxwell方程组的解依赖于 电磁场的初始值或边界条件,根据不同的初始条件解得的平面电磁波模是不同的.结果表明 ,平面电磁波是横波,在不同的位置,由变化的电场激发的磁场或由变化的磁场激发的电场 振幅不同,电场与磁场的相位差也不同. 关键词： Maxwell方程组 最优微分方程 初始条件 平面电磁波     

Fluctuation polarimetry

We examine the relationship between the strength of the intensity fluctuations and the polarimetric properties of a random electromagnetic field composed of a Gaussian, random field, and nonrandom field, and we present a method for determining the state of polarization of the Gaussian random field. The approach relies on incoherently mixing a Gaussian random field with a controllable reference field and measuring the intensity fluctuations of their superposition. We demonstrate that by controlling the reference field, the full polarimetric information about the Gaussian random field can be uniquely determined.     

外加静电场的聚焦激光脉冲真空加速电子方案

采用五阶修正的聚焦激光光场方程模拟研究了由Singh提出的在电子和激光脉冲作用尾部阶段施加外场的加速方案,将Singh方案中采用的外加磁场改成了外加电场,并且考虑了光束的纵向电场和光束衍射效应.模拟结果显示,电子可以从加速相位阶段被外场导入下一个加速相位阶段而不进入减速相位阶段,因此电子能获得比不加外场方案更高的净能增益. 关键词： 强激光 激光加速     

Electric field and the charge distribution on the surface of an insulator in a vacuum

The free charge steady-state distribution over the insulator surface that arises in a strong electric field in a vacuum can be found by solving the boundary-value problem for the electrostatic field strength if the angle between the field vector and vacuum-insulator interface is given. A general solution to this boundary-value problem is derived for the case of an in-plane field and rectilinear interfaces. Laws of charge and field formation that follow from the solution obtained are considered. Formulas for the electric field strength and charge density in terms of elementary functions are obtained for a number of particular cases. Power-type expressions for the electric field and a critical angle between the electrode and insulator surface that describe the field behavior and charge distribution near the vacuum-insulator-electrode contact are derived.     

四能级原子系统中电磁感应吸收的相位控制

在以三个电偶极跃迁构成简并N型四能级系统中,利用密度矩阵方程计算了介质对探测场的吸收,研究了激光场拉比相位对吸收的影响.结果表明:介质对探测场的吸收和放大取决于控制场和信号场的拉比相位,且吸收和放大随控制场、信号场的拉比相位改变而作周期性变化,周期为2π;而探测场的拉比相位变化对吸收没有影响.同时,控制场、信号场拉比相位对吸收的影响是相同的,而且拉比相位主要影响原子相干,对原子布居影响不大.     

Analytical Ground-State of an Exciton Trapped in a Circular Parabolic Quantum Dot in the Presence of a Magnetic Field

The quasi-exact properties of an exciton are investigated theoretically in the presence of an external magnetic field using the effective-mass approach in GaAs parabolic quantum dot. The energy spectrum is obtained analytically as a function of the dot radius, interaction strength and magnetic field. It is established that, a steady bound state of an exciton in the ground state exists under the effect of a strong magnetic field; also I noticed that the exciton binding energy decreases by increasing both the radius of the dot and the magnetic field strength and the reduction becomes pronounced for larger dots. As expected, it has been found that the exciton total energy decreases with increasing the size of the dot and it enhances by increasing the magnetic field. It appears that the exciton total energy strongly depends on the magnetic field for dots with big size. The magnetic field effect on the exciton size also has been studied. It is shown that the increase in the magnetic field leads to a reduction in the exciton size; due to magnetic field confinement, while the size of an exciton reach its bulk limit as the dot size increases. Moreover, it is shown that, if the dot radius is sufficiently large the oscillator strength saturates and it becomes insensitive to the magnetic field while the increase in the magnetic field gradually weakened the oscillator strength. I have calculated the ground-state distribution for both the electron and the hole. It is found that the localization of the electron/hole increases in the presence of a magnetic field. Moreover, the ground-state optical-absorption intensity is investigated. Finally, the dependence of the lowest five states of an exciton on both the dot radius and the magnetic field are discussed.

     

Quantum-electrodynamic processes in a strong nonclassical electromagnetic field

Quantum-electrodynamic processes that take place in the presence of a squeezed electromagnetic field are discussed. Integral formulas are derived that make it possible to express the probability of any process in a squeezed electromagnetic field in terms of the probability of the same process in a classical electromagnetic field. The main laws that govern processes of first order in the fine-structure constant as functions of the number of photons involved and the quantum fluctuations are examined. Tunneling formulas for the probability of a photon producing an electron-positron pair in a strong squeezed field are derived. Also, resonant electron-electron scattering is examined and the scattering cross section as a function of the statistical properties of the field (the way the field has been squeezed) is investigated. It is found that the quantum fluctuations of the squeezed electromagnetic field give rise to an increase in the scattering cross section, with the probability of the process in a phase-squeezed electromagnetic field always being higher than the probability of the process in an amplitude-squeezed electromagnetic field. Zh. éksp. Teor. Fiz. 112, 1543–1556 (November 1997)     

Hamiltonian Anomalies from Extended Field Theories

We develop a proposal by Freed to see anomalous field theories as relative field theories, namely field theories taking value in a field theory in one dimension higher, the anomaly field theory. We show that when the anomaly field theory is extended down to codimension 2, familiar facts about Hamiltonian anomalies can be naturally recovered, such as the fact that the anomalous symmetry group admits only a projective representation on the Hilbert space, or that the latter is really an abelian bundle gerbe over the moduli space. We include in the discussion the case of non-invertible anomaly field theories, which is relevant to six-dimensional (2, 0) superconformal theories. In this case, we show that the Hamiltonian anomaly is characterized by a degree 2 non-abelian group cohomology class, associated to the non-abelian gerbe playing the role of the state space of the anomalous theory. We construct Dai-Freed theories, governing the anomalies of chiral fermionic theories, and Wess-Zumino theories, governing the anomalies of Wess-Zumino terms and self-dual field theories, as extended field theories down to codimension 2.     

原子与频率随时间变化场相互作用系统中场熵的演化

利用多光子Jaynes-Cummings模型,考虑场频率随时间以正弦函数形式作小量变化,在旋波近似下,研究了二能级原子通过多光子跃迁与单模辐射场相互作用系统中场熵的演化规律.利用数值计算方法给出场熵随时间的演化曲线.研究结果表明:场熵的演化受场频率随时间正弦函数形式变化的调制,场频率振荡的幅值u越大调制作用越强,在场频率振荡的幅值u大于一定值后,场熵将按场频率周期性的变化规律作周期性振荡.     

Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.