共查询到20条相似文献,搜索用时 31 毫秒
1.
A. I. Alekseev 《Journal of Experimental and Theoretical Physics》1997,85(2):248-256
The optically induced magnetic moment of a stationary atom is calculated as a function of time and the resonance detuning
ω-ω
ba to within a constant factor having the dimensions of a magnetic moment based on the symmetry of an atom in the field of a
resonant light pulse and symmetry with respect to time reversal including the initial conditions. The even dependence of the
optically induced magnetic moment on ω-ω
ba for an elliptically polarized pulse with an isotropic initial state of the atom and its odd dependence on ω-ω
ba in the case of a linearly polarized pulsed with an anisotropic initial state in the atom’s alignment are shown to be consequences
of symmetry with respect to time reversal and the initial conditions. This behavior is retained even after passage of the
light pulse, when the resulting relaxation destroys the time reversal symmetry. The optically induced magnetization of an
atomic gas is found to have analogous properties.
Zh. éksp. Teor. Fiz. 112, 453–469 (August 1997) 相似文献
2.
In the present work, we have studied electronic and optical properties of a lens-shaped quantum dot under an external magnetic field. For this goal, we have calculated the energy levels and wave functions using the finite element method(FEM) for different values of magnetic field. We have also studied effect of magnetic field on second harmonic generation(SHG) and third-harmonic generation(THG) in the lens-shaped quantum dot. In this regard, we have obtained an analytic expression for the SHG and THG by a compact density matrix approach and an iterative procedure. According to the obtained results, it is found that the presence of the magnetic field affects the symmetry of the system. The SHG and THG are decreased with increasing the magnetic field. The magnetic field has a great influence on the energy levels, wave functions, the SHG and THG in a lens shaped quantum dot. 相似文献
3.
In the present work, we have studied electronic and optical properties of a lens-shaped quantum dot under an external magnetic field. For this goal, we have calculated the energy levels and wave functions using the finite element method (FEM) for different values of magnetic field. We have also studied effect of magnetic field on second harmonic generation (SHG) and third-harmonic generation (THG) in the lens-shaped quantum dot. In this regard, we have obtained an analytic expression for the SHG and THG by a compact density matrix approach and an iterative procedure. According to the obtained results, it is found that the presence of the magnetic field affects the symmetry of the system. The SHG and THG are decreased with increasing the magnetic field. The magnetic field has a great influence on the energy levels, wave functions, the SHG and THG in a lens shaped quantum dot. 相似文献
4.
The quantum Hall liquid is a novel state of matter with profound emergent properties such as fractional charge and statistics. The existence of the quantum Hall effect requires breaking of the time reversal symmetry caused by an external magnetic field. In this work, we predict a quantized spin Hall effect in the absence of any magnetic field, where the intrinsic spin Hall conductance is quantized in units of 2(e/4pi). The degenerate quantum Landau levels are created by the spin-orbit coupling in conventional semiconductors in the presence of a strain gradient. This new state of matter has many profound correlated properties described by a topological field theory. 相似文献
5.
Mancini F. 《The European Physical Journal B - Condensed Matter and Complex Systems》2005,45(4):497-514
The Ising model, in presence of an external magnetic field, is
isomorphic to a model of localized interacting particles satisfying
the Fermi statistics. By using this isomorphism, we construct a
general solution of the Ising model which holds for any
dimensionality of the system. The Hamiltonian of the model is solved
in terms of a complete finite set of eigenoperators and eigenvalues.
The Green’s function and the correlation functions of the fermionic
model are exactly known and are expressed in terms of a finite small
number of parameters that have to be self-consistently determined.
By using the equation of the motion method, we derive a set of
equations which connect different spin correlation functions. The
scheme that emerges is that it is possible to describe the Ising
model from a unified point of view where all the properties are
connected to a small number of local parameters, and where the
critical behavior is controlled by the energy scales fixed by the
eigenvalues of the Hamiltonian. By using algebra and symmetry
considerations, we calculate the self-consistent parameters for the
one-dimensional case. All the properties of the system are
calculated and obviously agree with the exact results reported in
the literature. 相似文献
6.
The search for Majorana fermions has been concentrated in topological insulators or superconductors. In general, the existence of these modes requires the presence of spin–orbit interactions and of an external magnetic field. The former implies in having systems with broken inversion symmetry, while the latter breaks time reversal invariance. In a recent paper, we have shown that a two-band metal with an attractive inter-band interaction has non-trivial superconducting properties, if the k-dependent hybridization is anti-symmetric in the wave-vector. This is the case, if the crystalline potential mixes states with different parities as for orbitals with angular momentum l and l+1. In this paper we take into account the effect of an external magnetic field, not considered in the previous investigation, in a two-band metal and show how it modifies the topological properties of its superconducting state. We also discuss the conditions for the appearance of Majorana fermions in this system. 相似文献
7.
The distribution of the magnetic field in the unit cell of the Abrikosov vortex lattice near the surface of a uniaxial, anisotropic,
type-II superconductor in an oblique external magnetic field is determined on the basis of the London model for the cases
in which the symmetry axis is perpendicular and parallel to the boundary of the superconductor. The distribution of the local
magnetic field is obtained as a function of the distance from the surface of the superconductor and the inclination angle
of the external field. It is shown for an YBaCuO high-T
c superconductor that the investigation of the distribution function of the local magnetic field as a function of the angle
of the external magnetic field relative to the symmetry axis and to the surface of the superconductor can yield important
information about the anisotropic properties of the superconductor.
Fiz. Tverd. Tela (St. Petersburg) 39, 1935–1939 (November 1997) 相似文献
8.
研究囚禁在环形势中的Rashba自旋轨道耦合玻色-爱因斯坦凝聚体在六极子磁场中的基态特性。在这种情况下,磁场破坏了自旋轨道耦合哈密顿量的旋转对称性,但系统仍具有2π/3的离散对称性。数值结果发现:在弱相互作用情况下,六极子磁场和Rashba自旋轨道耦合使环形囚禁的凝聚体呈类六边形的基态密度分布,当磁场强度超过某一临界值时,凝聚体将崩塌;在强相互作用情况下,半量子涡旋出现在凝聚体中,且被六极子磁场钉在方位角Ф=nπ/3的径向位置,涡旋的旋转方向取决于径向磁场的方向。 相似文献
9.
We have calculated the effect of a magnetic field on the evolution of angular momentum eigenfunctions of a charged particle. An additional harmonic potential is supplemented to trap the wave packet. We find the probability density of the wave function is oscillating in the radial direction with a time period determined by the strength of the effective harmonic potential. When the magnetic field is along the z direction, if the initial wave function is an eigenfunction of ${\hat{L}}_{z}$, the probability density of the particle remains axis-symmetric. While for the case of an eigenfunction of ${\hat{L}}_{x}$, it is anisotropic in the x−y plane and rotates with a time period inverse proportional to the strength of the external magnetic field. We also extend the results in a phenomenological way to the case with an external magnetic field that varies harmonically in time. 相似文献
10.
N. D. Sen Gupta 《Pramana》2003,60(1):11-19
In this paper, we attempt to obtain the nature of the external field such that the Dirac equation with external electro-magnetic
field is invariant. The Poincaré group, which is the maximal symmetry group for field free case, is constrained by the presence
of the external field. Introducing infinitesimal transformation ofx and ψ, we apply Lie’s extended group method to obtain the class of external field which admit of the invariance of the equation.
It is important to note that the constraints for the existence of invariance are explicity on the electric and magnetic field,
though only potentials explicity appears in the equation.
Presented at the Sixth Chittagong Conference on Mathematical Physics, January 2001. 相似文献
11.
A. I. Alekseev 《JETP Letters》1997,65(3):242-247
It is shown that the even dependence of the light-induced magnetic moment on the detuning ω-ω
ba
from resonance in the case of a circularly polarized pulse and an isotropic initial state of the atom and the odd dependence
on ω-ω
ba
in the case of a linearly polarized pulse and an anisotropic initial state in the form of alignment of the atom are consequences
of the symmetry under time reversal t→−t and of the initial conditions at time t=0. In a number of cases, this fundamental law makes it possible to determine the vector properties of a light-induced magnetic
moment and its dependence on the time t and ω-ω
ba
without solving the equation for the density matrix in detail and without calculating the sum over the projections of the
angular momenta in the formula for the magnetization of an atom by light.
Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 231–236 (10 February 1997) 相似文献
12.
A. A. Bukharaev D. V. Ovchinnikov N. I. Nurgazizov E. F. Kukovitskii M. Klaiber R. Wiesendanger 《Physics of the Solid State》1998,40(7):1163-1168
Isolated Ni nanoparticles were studied in situ by atomic and magnetic force microscopy in the presence of an additional external field up to 300 Oe. By comparing topographic
and magnetic images, and also by computer modeling of magnetic images, it was established that particles smaller than 100
nm are single-domain and easily undergo magnetic reversal in the direction of the applied external magnetic field. For large
magnetic particles, the external magnetic field enhances the magnetization uniformity and the direction of total magnetization
of these particles is determined by their shape anisotropy. Characteristics of the magnetic images and magnetic reversal of
particles larger than 150 nm are attributed to the formation of a vortex magnetization structure in these particles.
Fiz. Tverd. Tela (St. Petersburg) 40, 1277–1283 (July 1998) 相似文献
13.
Using the field theoretic renormalization group technique the model of passively advected weak magnetic field by an incompressible isotropic helical turbulent flow is investigated up to the second order of the perturbation theory (two-loop approximation) in the framework of an extended Kazantsev-Kraichnan model of kinematic magnetohydrodynamics. Statistical fluctuations of the velocity field are taken in the form of a Gaussian distribution with zero mean and defined noise with finite correlations in time. The two-loop analysis of all possible scaling regimes is done and the influence of helicity on the stability of scaling regimes is discussed and shown in the plane of exponents ? ? η, where ? characterizes the energy spectrum of the velocity field in the inertial range E ∞ k 1 ? 2ε, and η is related to the correlation time at the wave number k which is scaled as k ?2 + η. It is shown that in non-helical case the scaling regimes of the present vector model are completely identical and have also the same properties as those obtained in the corresponding model of passively advected scalar field. Besides, it is also shown that when the turbulent environment under consideration is helical then the properties of the scaling regimes in models of passively advected scalar and vector (magnetic) fields are essentially different. The results demonstrate the importance of the presence of a symmetry breaking in a given turbulent environment for investigation of the influence of an internal tensor structure of the advected field on the inertial range scaling properties of the model under consideration and will be used in the analysis of the influence of helicity on the anomalous scaling of correlation functions of passively advected magnetic field. 相似文献
14.
Alexandra Imre Gyrgy Csaba Gary H. Bernstein Wolfgang Porod Vitali Metlushko 《Superlattices and Microstructures》2003,34(3-6):513
This paper presents a magnetic force microscopy study of antiferromagnetic ordering along chains of dipole-coupled single-domain permalloy nanomagnets with a variety of shapes. Magnetization reversal processes occur due to antiferromagnetic coupling between the closely spaced dots when an appropriate external magnetic field is applied. The goal of this study was to investigate the switching properties and correlation lengths as a function of nanomagnet geometry. We have found that certain shapes (due to their stronger stray fields) clearly show stronger interaction than others when the chain is demagnetized. In addition we have seen that the performance of the nanomagnets also depends on the method of demagnetization, and this fact must be taken into account when shape engineering is used to design coupled nanomagnet systems for a given application. 相似文献
15.
In this paper, we study the Hubbard model with intersite Coulomb
interaction in the ionic limit (i.e. no kinetic energy). It is shown
that this model is isomorphic to the spin-1 Ising model in presence
of a crystal field and an external magnetic field. We show that for
such models it is possible to find, for any dimension, a finite
complete set of eigenoperators and eigenvalues of the Hamiltonian.
Then, the hierarchy of the equations of motion closes and analytical
expressions for the relevant Green's functions and correlation
functions can be obtained. These expressions are formal because
these functions depend on a finite set of unknown parameters, and
only a set of exact relations among the correlation functions can be
derived. In the one-dimensional case we show that by means of
algebraic constraints it is possible to obtain extra equations which
close the set and allow us to obtain a complete exact solution of
the model. The behavior of the relevant physical properties for the
1D system is reported. 相似文献
16.
We study, theoretically, the ground state spin of a carbon nanotube in the presence of an external potential. We find that when the external potential is applied to a part of the nanotube, its variation changes the single electron spectrum significantly. This, in combination with Coulomb repulsion and the symmetry properties of a finite length armchair nanotube, induces spin flips in the ground state when the external potential is varied. We discuss the possible application of our theory to recent measurements of Coulomb-blocked peaks and their dependence on a weak magnetic field in armchair carbon nanotubes. 相似文献
17.
The orthogonalized plane wave method of energy bandstructure calculation is generalized to the case of a metal under the influence of an external de magnetic field, with the magnetic translational symmetry taken into account fully. The magnetic field-dependent effective lattice potential or pseudopotential derived from it is interpreted as a “magnetic” pseudopotential and shown to depend only weakly on the magnetic field strength so that, to a good approximation, it can be replaced by an ordinary pseudopotential, and treated as a perturbation in the calculation of magnetic energy bands and galvanomagnetic properties in nearly-free-electron metals. Physical properties connected with the phenomenon of magnetic breakdown, in particular the Landau level broadening, which were previously shown by Pippard, Zak and others to be proportional to an unspecified pseudopotential, are reformulated in terms of the magnetic pseudopotential. The convergence of the method is also discussed. 相似文献
18.
G. E. Khodenkov 《JETP Letters》1998,68(11):864-868
As a result of the absence of a definite spatial symmetry in the structure of a 180-degree domain wall in ferromagnets with
negative crystallographic magnetic anisotropy, terms which are even functions of the external driving magnetic field appear
in the velocity of a one-dimensional domain wall, and the wall undergoes drift in an oscillating field.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 823–827 (10 December 1998) 相似文献
19.
P. Schillak 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,84(1):17-24
The problem of an exciton in the cylindrical nanostructure exposed to an external static magnetic field is investigated. The
theoretical model assumes anisotropic masses which are different inside and outside the nanostructure. The confinement potential
has finite value at the boundaries and magnetic field is parallel to the axis of the cylinder. The screened Coulomb interaction
between an electron and a hole is assumed. The consistent mathematical procedure is developed to calculate the magnetoexciton
eigenfunctions and eigenenergies. Our method applies to the systems exhibiting cylindrical symmetry where, due to confinement
effects accompanied by the e-h Coulomb interaction, the separation of relative- and center-of-mass motion is not possible. Numerical calculations have been
performed for the quantum disk, the cylinder and the quantum rod. The magnetic field dependent energy spectrum and corresponding
wave functions, expressed in terms of known one-particle electron and hole eigenfunctions, are calculated. Additionally, we
point out the different role of Coulomb interaction in every case. 相似文献
20.
从永磁体的分子电流观点、退磁场、工艺等出发, 以矩形永磁体为例, 从理论上分析了影响永磁体外部磁场不均匀性的各种因素.研究结果表明, 永磁体外部磁场宏观不均匀性(好场区均匀度和面积相对大小) 和空间距离及永磁体的外形设计密切相关. 退磁场对永磁体外部磁场微观不均匀性有着复杂影响. 永磁体工艺如粉末颗粒、取向度、烧结凝固、机械加工等将影响永磁体外部磁场的不均匀性, 如磁化偏角、对称性、光滑性等.
关键词:
永磁体
外部磁场
不均匀性
退磁场 相似文献