Anisotropic momentum transfer in low-dimensional electron systems in a magnetic field

In low-dimensional systems with an asymmetric quantizing potential, an asymmetric electron energy spectrum ε(p)≠ε(−p), where p is the electron momentum, arises in the presence of a magnetic field. A consequence of such an energy spectrum is that momentum transfer to the electron system in mutually opposite directions in the presence of an external perturbation is different. Therefore, in the presence of a standing electromagnetic wave momentum is transferred from the wave to the electrons, which gives rise to a new type of electromotive force. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 551–555 (25 October 1997)     

Absolute negative resistance in a nonequilibrium two-dimensional electron system in a strong magnetic field

The effect of microwave electromagnetic radiation on the resistance of the 2D electron gas in a GaAs/AlAs heterostructure in a strong magnetic field is investigated. It is shown that, under the nonequilibrium conditions caused by microwave radiation, the aforementioned 2D system exhibits giant oscillations of its resistance with varying magnetic field. When the measuring current density is small, an increase in the microwave power leads to the appearance of an absolute negative resistance at the main minimum of these oscillations, which lies near the cyclotron resonance. The experimental data are found to be in qualitative agreement with the theory of multiphoton photoinduced impurity scattering [J. Inarrea and G. Platero, Appl. Phys. Lett. 89, 052109 (2006)]. Original Russian Text ? A.A. Bykov, D.R. Islamov, D.V. Nomokonov, A.K. Bakarov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 9, pp. 695–698.     

Effect of a magnetic field on the ionization of atoms

The ionization probability of an atomic s state under the action of static electric and magnetic fields is calculated taking into account the Coulomb interaction between the escaping electron and the atomic core. The structure of the perturbation series for the energy of the level is investigated and the asymptotic behavior of the higher orders of the perturbation theory is found. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 398–402 (25 March 1996)     

Using effective operators in calculating the hyperfine structure of atoms

We propose a method for calculating the hyperfine structure (hfs) of multielectron atoms based on a combination of configuration superposition and many-body perturbation theory. The latter is used to construct an effective Hamiltonian and an effective hfs operator in configurational space. The method can be applied in calculations of the matrix elements of any one-electron operators. By way of an example we calculate the magnetic hfs constant A for several lowest levels of neutral thallium. We show that the method achieves a calculation accuracy of about 1%, which earlier was possible only for atoms with a single valence electron. Zh. éksp. Teor. Fiz. 114, 1636–1645 (November 1998)     

Selective reflection of resonance radiation from excited media

According to quantum electrodynamics, the cross section for resonant scattering of radiation on an aggregate of excited atoms can be written as a sum of positive definite terms. This type of structure is not consistent with the Fresnel formulas for the reflection coefficient of radiation from thermally excited media. The difference shows up on a macroscopic level and indicates that semiclassical radiation theory cannot be used. A study of the correlation between elastic scattering and stimulated emission processes clarifies the reason for the discrepancies. The resulting singularities require summing of Feynman diagrams which appear beginning in the sixth order of perturbation theory. A lower bound estimate for the reflection coefficient from a plane layer is given, including processes which violate the statistics of radiation. The contribution of stimulated emission processes caused by the initially scattered photon are examined specifically. An experiment is proposed which would settle the choice of theories. Zh. éksp. Teor. Fiz. 113, 521–538 (February 1998)     

Spectrum of plasma oscillations in structures with a periodically inhomogeneous two-dimensional electron plasma

We develop a rigorous electrodynamic theory of plasma oscillations in a periodically inhomogeneous two-dimensional electron system with a rectangular profile of the spatial modulation of the equilibrium electron concentration. We calculate the frequencies and radiative damping of the main plasma-oscillation types with a zero reduced wave vector. We show that the frequency splitting and the radiative damping of the oscillations are nonmonotonic functions of the modulation percentage and the ratio of the widths of the bands of two-dimensional plasma with low and high electron concentrations. The results of calculations are compared with experimental data and with the results of a perturbation theory developed in earlier work of other researchers. We discuss the physical mechanism for the emergence of radiative damping of plasma oscillations. Zh. éksp. Teor. Fiz. 113, 988–999 (March 1998)     

Electron transport anomaly in a plasma in an intense radiation field

It is shown that in an intense radiation field the electron friction force resulting from electron-ion collisions becomes an electron accelerating force on account of absorption of radiation by electrons. It is pointed out that the thermal conductivity increases. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 19–22 (10 July 1996)     

Spontaneous and induced Cherenkov radiation generated by electrons in cylindrical dielectrics

A theory of induced Cherenkov radiation in cylindrically symmetrical dielectrics in the case when an electron beam is moving close to the dielectric surface is presented. The spectrum of excited radiation modes has been investigated, and analytical expressions for the gain at the frequencies of various modes have been derived. Zh. éksp. Teor. Fiz. 111, 847–861 (March 1997)     

Static and dynamic properties of an isolated strip domain in a thin ferromagnetic film

A perturbation theory is developed for the integrodifferential Landau-Lifshits equation that describes the state of 2π-domain walls in ferromagnetic films. The static and dynamic parameters of a 2π-domain wall are determined, taking into account its micromagnetic structure. The limits of applicability of geometric domain wall models are indicated. Fiz. Tverd. Tela (St. Petersburg) 40, 269–273 (February 1998)     

Generation of soliton packets in a two-level laser

A variant of perturbation theory is constructed for a system of nearly integrable equations. Perturbations of a special type are considered, which makes it possible to represent the system in the form of compatibility condition for “deformed” linear systems. The corresponding deformation of the Whitham equations is found. The mathematical apparatus is used to theoretically examine the generation of a sequence of solitons in a two-level laser. The generation process is described by a system of Maxwell-Bloch equations with pumping of the upper level and with allowance for some relaxation effects. The dynamics of the transformation of the initial perturbation into a sequence of solitons under pumping is studied. Finally, the various generation regimes are analyzed and compared with the experimental data. Zh. éksp. Teor. Fiz. 112, 2237–2251 (December 1997)     

Electron kinetics in a discharge plasma produced by a focused microwave beam in free space

A study is made of the electron kinetics in a discharge plasma produced by a high-power beam of electromagnetic radiation in the centimeter-wave region under conditions approaching free space, when the dimensions of the chamber are much greater than the wavelength of the microwave radiation. Two regimes of discharge production are investigated: the regime of short microsecond pulses at a repetition rate of 200 Hz, and a single millisecond pulse regime. It is shown that at threshold values of the microwave energy flux density the electron density in the initial stages of discharge formation reaches the critical value, and that the average energy of the electrons is of the order of 1.5–3 eV. Zh. Tekh. Fiz. 67, 10–14 (June 1997)     

Nuclear Spin-Flip Satellites in Electron Paramagnetic Resonance Spectroscopy

Under certain circumstances, the primary hyperfine electron paramagnetic resonance spectrum is accompanied by additional lines which are located at nuclear magnetic resonance spacings, almost independent of the hyperfine spin-Hamiltonian matrix A. This situation allows instant identification of the nuclide responsible. The theory behind this phenomenon is available, tractable via perturbation theory, and yields line positions and relative intensities, but is none too translucent. The present work attempts to portray the predictions of the theory using simple examples, utilizing computer-based analysis and spectral simulations.     

Theory of group synchronism in free-electron waveguide lasers fed a sequence of short electron pulses

A theory of free-electron lasers fed a sequence of short electron pulses is developed. It is assumed that the group velocity of the electromagnetic pulse that develops in the cavity is the same as the translational velocity of the particles, and the repetition period of the electron pulses equals the transit time of the electromagnetic radiation in the cavity. Under these conditions of group synchronism, the principal factors governing the feasibility of establishing a stationary pulsed lasing regime are found to be the dispersive spread of the electromagnetic pulse and the channeling properties of an electron bunch. The conditions for self-excitation are found, and the characteristics of the stationary lasing regimes are determined assuming that the cavity has a high Q and using a parabolic equation for the evolution of the electromagnetic pulse shape. Zh. Tekh. Fiz. 69, 78–83 (February 1999)     

Quark and gluon condensates in a magnetic field

The two-loop expression for the vacuum energy density in a constant magnetic field is obtained on the basis of the chiral perturbation theory. The dependence of the quark and gluon condensates on the field intensity H is found. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 711–717 (10 December 1999)     

Convergence of rayleigh-schrödinger perturbation theory in calculations of multiphoton processes

Abstract

We have calculated the high-order ac Stark Shift, multiphoton ionization rates, and nonlinear susceptibilities for high harmonic generation for the hydrogen atom in a radiation field. The calculations are done in the framework of Rayleigh-Schrödinger perturbation theory applied to a complex-rotated Hamiltonian. Our intention is to investigate the limitations of perturbation theory in calculations of multiphoton processes. Comparisons are made with results from nonperturbative calculations. For some frequencies the results of lowest-order perturbation theory are found to disagree with nonperturbative calculations even at moderate to low intensities (I ～ 10¹¹ W cm^?2) and in the absence of resonances. We find that the high-order perturbation expansion theory is not a reliable predictor of the behavior of hydrogen atoms in radiation fields with intensities greater than ～ 10¹² W cm^?2.     

Polar magneto-optic Kerr effect in the near-light field of a small nonmagnetic particle

A theory of elastic light scattering by a small resonantly polarizing particle located near a flat surface of a magnetic medium has been developed. The effective polarizability of the particle was calculated self-consistently taking account of the dynamic “image forces” in all orders of perturbation theory in the interaction of the particle with the demagnetized ferromagnet, and the magneto-optic perturbation was determined to first order in the magnetization. In the case of a ferromagnet magnetized perpendicular to the surface, the light conversion factors and the magneto-optic corrections to the transverse cross sections of all processes in which the scattering of light by a particle and the polar magneto-optic Kerr effect are elementary events, have been calculated. The results, including an analysis of the near-field contribution to light scattering, comprise the physical foundation for constructing a theory of near-field magneto-optic spectroscopy of ferromagnets and magnetic structures. Fiz. Tverd. Tela (St. Petersburg) 39, 560–567 (March 1997)     

Filamentation and stimulated Brillouin scattering in a turbulent plasma

This paper presents a theory of filamentation and stimulated Brillouin scattering (SBS) of high-frequency electromagnetic radiation in a weakly collisional plasma with ion-acoustic turbulence. When the square of the wavelength of the plasma perturbations is less than the product of the two mean free path lengths of an electron with respect to its collisions with turbulent fluctuations and with electrons, the influence of cold highly collisional electrons on the parametric instabilities becomes apparent. It is shown that the plasma turbulence lowers the filamentation threshold, and the SBS threshold can be either lowered or raised. The dependence of the SBS and filamentation thresholds on the electron mean free path length in the turbulent plasma and on the anisotropy of the plasma turbulence is determined. The corresponding dependence of the spatial scale of the most efficiently growing filaments and their spatial growth rate are found. Zh. éksp. Teor. Fiz. 113, 629–645 (February 1998)     

Physical characteristics of electron-induced dichroism in vitreous arsenic trisulfide

The dependence of the electron-induced dichroism effect in vitreous As₂S₃ on the photon energy of the probe radiation and on the temperature of subsequent thermal anneals is investigated. The effect is observed to be completely suppressed after prolonged storage of the samples (10–15 days) following electron irradiation. It is shown by induced-reflection Fourier-transform IR spectroscopy that the observed effect is associated with processes involving the formation of oriented defects in the form of undercoordinated atomic pairs in the glass structural matrix. Fiz. Tverd. Tela (St. Petersburg) 40, 52–56 (January 1998)     

Radiation from relativistic electrons in homogeneous condensed matter at large angles (≫γ−1)

The radiation spectra from 900 and 400 MeV electrons in thin Ta, Cu, and Sn foils are measured at an angle of 19° with respect to the direction of motion of the beam. The radiation yield and its dependence on the electron energy agree satisfactorily with the theory of polarization bremsstrahlung. This result represents the first direct observation of polarization bremsstrahlung from ultrarelativistic electrons in homogeneous condensed matter. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 145–149 (10 February 1996)