Impact ionization in crossed electric and magnetic fields in polar semiconductors

Using the model of gyrational quasiscattering the impact-ionization rate in InSb at 77K and 300K and in InAs at 300K is calculated, following previous work by Reuter and Hübner [11]. We include the nonparabolicity of the conduction bands and use Conwell's formula for polar-optical scattering in emission and absorption in a modified version taking into account the nonparabolicity. Below the energy where the energy-exchange from gyrational scattering equals the optical phonon-energy the numerically calculated distribution function is continued by a Maxwellian.The resulting ionization rates for InSb are compared with recent measurements. Good agreement is found for both 77K and 300K.     

平行电磁场中里德堡氢原子的自相似结构研究

利用半经典方法研究了平行电磁场中里德堡氢原子的分形自相似现象. 通过研究平行电磁场中里德堡氢原子的逃逸时间和初始出射角间的关系, 发现了逃逸时间图的自相似结构, 并通过研究与图中冰柱对应的逃逸轨道, 得到了自相似结构和逃逸轨道之间的关系, 发现了该类自相似逃逸轨道满足的规律. 进一步研究了标度能量和标度磁场对体系动力学的影响, 表明标度能量和标度磁场均控制体系的分形自相似结构. 当标度能量或标度磁场比较小时, 没有自相似现象, 随着标度能量或标度磁场的增大, 自相似出现, 体系变复杂.     

Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields

We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.     

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.     

Thermodynamics of ionization and dissociation in hydrogen plasmas including fluctuations and magnetic fields

Applying Gibb's geometrical methods to the thermodynamics of H-plasmas we explore the landscape of the free energy as a function of the degrees of ionization and dissociation. Several approximations for the free energy are discussed. We show that in the region of partial ionization/dissociation the quantum Debye-Hückel approximation (QDHA) yields a rather good but still simple representation which allows to include magnetic field and fluctuation effects. By using relations of Onsager-Landau-type the probability of fluctuations and ionization/dissociation processes are described. We show that the degrees of ionization/dissociation are probabilistic quantities which are subject to a relatively large dispersion. Magnetic field effects are studied. Received 10 September 2002 / Received in final form 26 November 2002 Published online 11 February 2003     

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

Above-threshold ionization of hydrogen atom in chirped laser fields

Above-threshold ionization(ATI) of a hydrogen atom exposed to chirped laser fields is investigated theoretically by solving the time-dependent Schr o¨dinger equation. By comparing the energy spectra, the two-dimensional momentum spectra, and the angular distributions of photoelectron for the laser pulses with different chirp rates, we show a very clear chirp dependence both in the multiphoton and tunneling ionization processes but no chirp dependence in the single-photon ionization. We find that the chirp dependence in the multiphoton ionization based ATI can be attributed to the excited bound states. In the single-photon and tunneling ionization regimes, the electron can be removed directly from the ground state and thus the excited states may not be very important. It indicates that the chirp dependence in the tunneling ionization based ATI processes is mainly due to the laser pulses with different chirp rates.     

Deformation of nematic layers in crossed electric and magnetic fields

A parallel nematic layer will show a Fréedericksz-transition in an electric field when the voltage exceeds a threshold value U_c. If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U_c increases with increasing magnetic field field H above a threshold field H_c2. From a plot of U_c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.     

Velocity distribution of polarons in crossed electric and magnetic fields

A procedure is presented, based on analytical methods, to solve the Boltzmann equation for polarons at finite temperature in crossed electric and magnetic fields. Explicit results for the distribution function are obtained, showing the effect of phonon absorption on the population inversion. Preliminary results are reported for comparison with the experimental data of Komiyama, Masumi and Kajita for the Hall current and for the mobility.     

Investigating the hydrogen atom in crossed external fields by the method of moments

Recurrence relations of perturbation theory for the hydrogen ground state are obtained. With the aid of these relations, polarizabilities in constant mutually perpendicular electric and magnetic fields are computed up to the 80th order of perturbation theory. The high-order asymptotic expression is compared with its semiclassical estimate. For the case of an arbitrary relative orientation of external fields, a general sixth-order formula is given. The energy and the width of the ground state are obtained by means of a perturbation-series summation.     

The excitation and ionization of the hydrogen atom in strong laser fields

The probability of excitation and ionization of the hydrogen atom by short strong laser pulses is calculated by solving numerically the time-dependent Schrödinger equation. The probability of excitation to different states is investigated, which helps to identify the important ionization mechanisms. The ionization probability density was also calculated and a good agreement with the other theoretical results was found.     

Experimental investigation of barium spectra in crossed electric and magnetic fields

This paper has studied the barium spectra in external magnetic and electric fields on the recently-built system based on a super-conducting solenoid with high stability of better than 1 Gauss. Firstly, we compared the spectra in a high pure magnetic field of $B=4.00000$\,T for different polarized laser, $\sigma^{+}$ and $\sigma^-$. We prove that after a proper shift, the $\sigma^+$ and $\sigma^-$ spectra are completely identical. We also investigated the dependence of the spectral feature in the high magnetic field on an additionally applied crossing electric field. Along with the increase of the electric field, there is an energy shift for every spectral line. Based on a transformative Hamiltonian, we explained this energy shift well.     

Pattern formation in reaction-diffusion system in crossed electric and magnetic fields

We consider a reaction-diffusion system in crossed electric and magnetic fields lying on the reaction plane. It is shown that a charge separation along the direction normal to the reaction plane resulting in a diffusional flux may cause a differential flow induced chemical instability and stationary pattern formation on a homogeneous steady state. This pattern is generically different from a Turing pattern modified by the crossed fields. The special role of magnetic field is emphasized. Our theoretical analysis is corroborated by numerical simulation on a reaction-diffusion system in three dimensions.     

Semisoft nematic elastomers and nematics in crossed electric and magnetic fields

Nematic elastomers with a locked-in anisotropy direction exhibit semisoft elastic response characterized by a plateau in the stress-strain curve in which stress does not change with strain. We calculate the global phase diagram for a minimal model, which is equivalent to one describing a nematic in crossed electric and magnetic fields, and show that semisoft behavior is associated with a broken symmetry biaxial phase and that it persists well into the supercritical regime. We also consider generalizations beyond the minimal model and find similar results.     

Resonances of a hydrogen atom in strong parallel electric and magnetic fields using B-spline basis sets

The B-spline basis set plus complex scaling method is applied to the numerical calculation of the exact resonance parameters $E_r}$ and $\Ga/2$ of a hydrogen atom in parallel electric and magnetic fields. The method can calculate the ground and higher excited resonances accurately and efficiently. The resonance parameters with accuracies of $10^{-9}-10^{-12}$ for hydrogen atom in parallel fields with different field strengths and symmetries are presented and compared with previous ones. Extension to the calculation of Rydberg atom in crossed electric and magnetic fields and of atomic double excited states in external electric fields is discussed.