Scattering polarization in the presence of magnetic and electric fields

The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q/I profiles and rotation of the plane of polarization in the U/I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers.     

Effective Hamiltonian of silicene in the presence of electric and magnetic fields

An effective Hamiltonian of silicene in the neighborhood of Dirac points in the presence of electric and magnetic fields perpendicular to the plane of the film is constructed on the basis of symmetry analysis. Numerical coefficients of various terms in the Hamiltonian are obtained by the tight binding method in the basis sp ³ d ⁵ s* with regard to the interaction with one nearest neighbor. This method was developed in the previous paper [1] in the case of a sublattice displacement of 0.44 Å, which corresponds to the theoretical value of displacement obtained from first principles for a free film of silicene. The effect of the displacement of sublattices on the orientation of spin and pseudospin in silicene is analyzed. The Hamiltonian obtained allows one to consider spin and electron transport for charge carriers with energy less than 0.5 eV. The orbital motion of electrons in an external magnetic field perpendicular to the film is analyzed in detail.     

Rydberg hydrogen atom in the presence of uniform magnetic and quadrupolar electric fields

Laser cooling and trapping offers the possibility of confining a sample of radioactive atoms in free space. Here, we address the question of how best to take advantage of cold atom properties to perform the observation of as highly forbidden a line as the 6S-7S Cs transition for achieving, in the longer term, atomic parity violation (APV) measurements in radioactive alkali isotopes. Another point at issue is whether one might do better with stable, cold atoms than with thermal atoms. To compensate for the large drawback of the small number of atoms available in a trap, one must take advantage of their low velocity. To lengthen the time of interaction with the excitation laser, we suggest choosing a geometry where the laser beam exciting the transition is colinear to a slow, cold atomic beam, either extracted from a trap or prepared by Zeeman slowing. We also suggest a new observable physical quantity manifesting APV, which presents several advantages: specificity, efficiency of detection, possibility of direct calibration by a parity conserving quantity of a similar nature. It is well adapted to a configuration where the cold atomic beam passes through two regions of transverse, crossed electric fields, leading both to differential measurements and to strong reduction of the contributions from the M₁-Stark interference signals, potential sources of systematics in APV measurements. Our evaluation of signal-to-noise ratios shows that with available techniques, measurements of transition amplitudes, important as required tests of atomic theory, should be possible in ¹³³Cs with a statistical precision of 10^-3 and probably also in Fr isotopes for production rates of Fr atoms s^-1. For APV measurements to become realistic, some practical realization of the collimation of the atomic beam as well as multiple passages of the excitation beam matching the atomic beam looks essential.Received: 5 March 2003, Published online: 17 July 2003PACS: 32.80.Ys Weak-interaction effects in atoms - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.80.Pj Optical cooling of atoms; trapping - 39.90.+d Other instrumentation and techniques for atomic and molecular physicsS. Sanguinetti: Also at E. Fermi Physics Dept., Pisa Univ., Pisa, Italy.     

The electrical breakdown of gases in the presence of crossed electric and magnetic fields

The breakdown characteristics of a gas in the presence of crossed electric and magnetic fields are discussed in terms of the Townsend ionization coefficients. The “equivalent pressure” concept is used to assess the effect of a transverse magnetic field on the first Townsend coefficient and the objections which have been raised to the application of this approach to breakdown potentials are shown to be removed by a consideration of the dependence of the second Townsend coefficient upon electric and magnetic field strengths.     

Theoretical studies on the optical absorption coefficients and refractive index changes in parabolic quantum dots in the presence of electric and magnetic fields

The optical absorption coefficients and the changes in the refractive index in GaAs/AlGaAs parabolic quantum dots(QDs) with applied electric and magnetic fields are studied in detail. Analytical expressions for the linear and nonlinear intersubband absorption coefficients and refractive index changes are obtained by using a compact density matrix approach and an iterative procedure. Finally, the calculated results show the incident optical intensity, the frequencies of the confined potential of the QDs and the applied electric and magnetic fields have a great influence on the optical absorption coefficients and refractive index changes in this system.     

Experiments on the viscosity of some symmetric top molecules in the presence of magnetic and electric fields

In order to obtain information on the scalar structure of non-equilibrium polarizations a comparison between the magnetic and the electric field effect on the viscosity of some symmetric top molecules has been carried out. It is shown that the polarization produced in viscous flow is more complicated than assumed so far.     

Nonlinear optical rectification in parabolic quantum dots in the presence of electric and magnetic fields

The optical rectification (OR) coefficient in a parabolic quantum dots (QDs) subject to applied electric and magnetic fields is theoretically investigated in the framework of the compact-density-matrix approach and an iterative method. The confined wave functions and energies of electrons in the QDs are calculated in the effective-mass approximation. Numerical results are presented for typical GaAs/AlGaAs parabolic QDs. These results show that the OR coefficient strongly depends on the radius of QDs and the magnitude of electric and magnetic fields. And the peak shifts to the aspect of high energy when considering the influence of electric and magnetic fields.     

An exciton in a quantum well in the presence of crossed electric and magnetic fields

An analytical approach to the problem of the Wannier–Mott exciton in a semiconductor quantum well (QW) in the presence of external magnetic and electric fields is developed. The magnetic field is taken to lie in the heteroplanes while the electric field is directed perpendicular to the heteroplanes. Explicit dependencies of the energy levels and wave-functions of the exciton on the magnitudes of the fields for a wide range of the width of the QW are obtained. For the narrow QW, the results are valid for arbitrary electron and hole effective masses. In the case of intermediate and wide QWs, the adiabatic approximation implying the extreme difference of the electron and hole masses is used. In the intermediate QW, the states of the relative motion are the standard Coulomb states affected by the external fields while the states of the centre of mass are the size-quantized states in the QW. We focus particularly on the delocalized states caused by the external electric field and the motion of the excitons centre of mass in the magnetic field. These states are localized far away from the Coulomb centre. A strong influence of the boundaries of the wide QW on the delocalized exciton states is found to occur. Estimates of the expected values are made using typical parameters associated with GaAs QW.     

The second-harmonic generation in parabolic quantum dots in the presence of electric and magnetic fields

The second-harmonic generation (SHG) coefficient for parabolic quantum dots (QDs) subject to applied electric and magnetic fields is theoretically investigated, within the framework of the compact-density-matrix approach and an iterative method. Numerical results are presented for typical GaAs/AlGaAs parabolic QDs. These results show that the radius of QD and the magnitude of electric and magnetic fields have a great influence on the SHG coefficient. And the peak shifts to the aspect of high energy when considering the influence of electric and magnetic fields. Moreover, the SHG coefficient also depends sensitively on the relaxation rate of the spherical QD system.     

Polarization of line radiation in the presence of external electric quadrupole and uniform magnetic fields

The polarization of emission lines formed in a medium immersed in external electric and magnetic fields is studied. The electric field is assumed to be quadrupolar in nature, while the magnetic field is uniform. We show that the quadrupole electric field produces line splitting which is characteristically different from the Zeeman effect. While the line components emitted along the quantization axis are circularly polarized in Zeeman effect, they are, in contrast, linearly polarized in the case of a pure quadrupole electric field. The emission perpendicular to the quantization axis produces three linearly polarized components in Zeeman effect, whereas only two linearly polarized components are observed in the case of quadrupole electric fields. Lack of azimuthal symmetry in the quadrupole electric field leads to polarized line components which appear quite differently for different azimuthal angles of the line of sight.     

The magnetic properties of GaAs parabolic quantum dot in the presence of donor impurity,magnetic and electric fields

The magnetization and the magnetic susceptibility quantities of a single electron moving in a two-dimensional (2D) parabolic quantum dot are studied under the influence of external uniform electric and magnetic fields, in the presence of a donor impurity. The Hamiltonian was solved using shifted 1/N expansion method within the effective mass approximation. The results have been displayed as a function of physical parameters: confinement strength ω₀, magnetic field strength ω_c, temperature T and electric field strength F.     

On the effective electron mass in ternary semiconductors in the presence of crossed electric and magnetic fields

The effective electron mass in ternary semiconductors under cross field configuration depends on the magnetic quantum number, besides the usual energy dependence of the same mass in non-parabolic bands. It is found, taking nHg_1−xCd_xTe as an example, that the index dependent masses are in agreement with the experimental observation.     

Features of electron distribution functions in a plasma in the presence of crossed electric and magnetic fields

We treat the formation features of distribution functions of magnetized electrons over velocities and energies in a plasma located in crossed electric and magnetic fields. It is shown that the variation of electron velocity and energy between collisions, related to the motion in external fields, can substantially affect the shape of the distribution function. It is established that for strong anisotropy of the velocity distribution and for electron scattering by slightly mobile heavy particles the electron distribution function over energy has two maxima. This given feature of the electron distribution is not related to the presence of two electron groups in the plasma, but is a consequence of the transformation of distribution functions in crossed electric and magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 88–94, August, 1991.The author is grateful to A. A. Titov for carrying out the numerical calculations, and to A. S. Dolgov for a number of useful comments.     

Semiconductor electrons in electric and magnetic fields

Optical properties of semiconductors in the simultaneous presence of electric and magnetic fields are reviewed, with particular emphasis on the possibilities of modulation techniques. First, the problem of an electron in crossed and parallel fields is solved in the one-level effective mass approximation (EMA), and the results are used to interpret the experimental interband transitions in Ge, with due account of the degenerate character of the valence band in this material. The limitations of the one-level EMA are discussed, and the two-level model is introduced, which correctly describes the experimentally observed transition from a magnetic type to an electric type of motion in increasing transverse electric field. Possibilities to observe electric field effects in cyclotron resonance transitions are discussed in this approximation. Finally, the three-level model is used to describe properly both orbital and spin properties of conduction electrons. It is demonstrated that in a small-gap semiconductor with large spin-orbit interaction a sufficiently strong transverse electric field destroys the Landau orbital quantization but not the Pauli spin quantization. Possible experimental consequences of this situation are discussed. Influence of finite dimensions of the sample on the character of the electron motion in crossed and parallel fields is examined. A possibility to achieve the semiconductor-semimetal transition in a symmetryinduced zero-gap semiconductor in crossed field configuration is predicted and described, taking into account the Luttinger effects in the magnetic level structure.     

Hofstadter spectrum in electric and magnetic fields

The problem of Bloch electrons in two dimensions subjected to magnetic and intense electric fields is investigated. Magnetic translations, electric evolution, and energy translation operators are used to specify the solutions of the Schrödinger equation. For rational values of the magnetic flux quanta per unit cell and commensurate orientations of the electric field relative to the original lattice, an extended superlattice can be defined and a complete set of mutually commuting space-time symmetry operators is obtained. Dynamics of the system is governed by a finite difference equation that exactly includes the effects of: an arbitrary periodic potential, an electric field orientated in a commensurable direction of the lattice, and coupling between Landau levels. A weak periodic potential broadens each Landau level in a series of minibands, separated by the corresponding minigaps. The addition of the electric field induces a series of avoided and exact crossing of the quasienergies, for sufficiently strong electric field the spectrum evolves into equally spaced discreet levels, in this “magnetic Stark ladder” the energy separation is an integer multiple of hE/aB, with a the lattice parameter.