Shift photocurrent induced by two-quantum transitions

A theory of the shift current induced by direct two-photon and indirect one-photon absorption is developed for noncentrosymmetric crystals. A formula is derived for the microscopic shifts of Bloch electrons induced by two-quantum processes. It is shown that the ratio of the two-photon photocurrent at the photon frequency ω to the photocurrent induced by direct one-photon transitions at the photon frequency 2ω, as compared to the corresponding absorption-rate ratio, contains a large factor {ie152-1}ω/(2{ie152-2}ω − E _g), where E _g is the bandgap; i.e., these photocurrent can be comparable in order of magnitude. For crystals of T _d symmetry, the photocurrents induced by one- and two-photon absorption are compared in terms of polarization dependence.     

Plasma waves near the double resonance layer in the ionosphere

Dispersion properties of plasma waves with frequencies close to the upper hybrid frequency _u and the multiple electron cyclotron frequency n_Be (double resonance) are considered for an inhomogeneous plasma with opposite gradients of the electron density and magnetic field magnitude. We show that a region of possible solutions of the dispersion relation decreases in real space as well as in wave vector space as of the wave frequency approaches double resonance. The results are applied to an interpretation of experiments on ionospheric modification by high-power radio waves with frequencies close to n_Be.Radiophysical Research Institute, Nizhny Novgorod, Russia. Swedish Institute of Space Physics, Uppsala Division, Sweden. Swedish Institute of Space Physics, Kiruna Division, Sweden. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 5, pp. 617–633, May, 1994.     

Excitation of short-wave oscillations in the ionospheric plasma by the field of a high-power radio wave with extraordinary polarization due to induced scattering by ions

We analyze excitation of electron-cyclotron or upper hybrid oscillations of the ionospheric plasma at a frequency that is close to the pump frequency as a result of induced scattering of a high-power radio wave with extraordinary polarization by ions. The excited oscillations have a small wavelength of the order of the Larmor electron radius, which allows them to propagate near and below the reflection level of an extraordinary radio wave. We found the instability increment and threshold field, which results from collisional absorption of plasma waves. It is shown that the threshold field is minimal near the reflection level of an extraordinary radio wave when the radio wave frequency f₀ is between electron harmonics nf_Be with n≥2. In an ionospheric F layer it is of the order of 1 W/m. Such fields are easily obtained in ionospheric heating experiments allowing for radio-wave field swelling in the reflection region. In the vicinity of electron harmonics f_o≅nf_Be, the threshold field is increasing. For f_o<nf_Be with f_o≅nf_Be the instability does not develop because of the absence of plasma oscillations with a frequency that is close to the pump frequency (the latter also refers to the case f_o<2f_Be). The expressions obtained are generalized to the case in which the instability under consideration is excited by the field of an ordinary radio wave in the region of its quasilongitudinal propagation. We discuss the possibility of using the emergence of very short-wave plasma oscillations for explaining the experimentally observed phenomena. Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow Region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 5, pp. 541–560, May, 1997.     

Natural electron oscillations in granular aluminum films deposited on rough surfaces of NaCl and KCl single crystals

Granular aluminum films deposited on rough surfaces of NaCl and KCl single crystals form a two-layer coating. In the lower layer, a low-frequency band of plasma resonance is excited due to the joint action of a light-wave field and a field induced by granules-dipoles. In a small amount of granules of the upper layer isolated from each other, a high-frequency band with a frequency ω₀ of natural electron oscillations in the granules is excited. Using the measured frequencies ω₀ and the known dielectric constants of NaCl and KCl, a plasma frequency of aluminum is calculated that agrees well with the known plasma frequency of solid samples. Kharkov State University, 4 Svoboda Sq., Kharkov, 310077, Ukraine. Translated from Zhurnal Prikladnoid Spektroskopii, Vol. 66, No. 6, pp. 853–856, November–December, 1999.     

Return current instability and its effects on beam-plasma system

The return current induced in a plasma by a relativisitc electron beam generates a new electron-ion two-stream instability (return current instability). Although the effect of these currents on the beam-plasma e-e instability is negligible, there exists a range of wave numbers which is unstable only to return current (RC) instability and not to e-e instability. The electromagnetic waves propagating along the direction of the external magnetic field, in which the plasma is immersed, are stabilized by these currents but the e.m. waves with frequencies,ω ²≪Ω _e ² ≪ω _pe ² (Ω _e andω _pe being cyclotron and plasma frequency for the electrons of the plasma respectively) propagating transverse to the magnetic field get destabilized. Heuristic estimates of plasma heating, due to RC instability and due to decay of ion-acoustic turbulence generated by the return current, are made. The fastest time scale on which the return current delivers energy to the plasma due to the scattering of ion-sound waves by the electrons can be ∼ω _pi ⁻¹ (ω _pi being the plasma frequency for the ions).     

Dispersion properties and field structures of eigenmodes of nonuniform plasma waveguides in a longitudinal magnetic field

We study the field structure and dispersion properties of a hybrid eigenmode guided by a nonuniform magnetized plasma waveguide. It is shown that the rotational and quasi-potential waves contribute to the formation of such a mode in the whistler frequency range. Depending on the plasma density, the rotational component of the hybrid mode is determined by either waves with complex transverse wave numbers or whistler waves, or by true surface waves. In the presence of an axial nonuniformity of the plasma in a channel, the transverse field structure of the propagating mode changes, which is stipulated by changes in both the values of transverse wave numbers and their dependence on the radial coordinate. It is found that the spectrum of axial wave numbers of eigenmodes of a plasma waveguide undergoes a pronounced condensation when smoothing the waveguide walls. The damping of the hybrid mode of a nonuniform waveguide due to electron collisions is found and it is shown that collisional losses determine the damping of waves trapped in the waveguide in the experiments on ionization self-channeling of whistler waves. We have found the effect of “displacing” the strong field from the inner core to the background outer region of the waveguide with increasing plasma density on its axis and broadening background region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 607–617, July 2006.     

Ponderomotive acceleration of electrons by a whistler pulse

A Gaussian whistler pulse is shown to cause ponderomotive acceleration of electrons in a plasma when the peak whistler amplitude exceeds a threshold value and the whistler frequency is greater than half the cyclotron frequency, ω>ω _c /2. The threshold amplitude decreases with the ratio of plasma frequency to electron cyclotron frequency, ω _p /ω _c . However, above the threshold amplitude, the acceleration energy decreases with ω _p /ω _c . The electrons gain velocities about twice the group velocity of the whistler.     

Spectra of high-frequency waves in hot magnetized plasmas

On the basis of numerical solution of the dispersion equation, we obtain the spectra of weakly damped high-frequency waves in a hot magnetized plasma for the case where the electron cyclotron frequency ω_He is below the plasma frequency ω_pe. It is shown that the longitudinal wave propagating at an angle to the magnetic field evolves into the slow extraordinary wave for the refractive index n ≤ 1. For n ≫ 1, the longitudinal-wave frequency increases with the refractive index, and the wave evolves into the wave with anomalous dispersion if the angle θ between the wave vector and the magnetic field is close to 90°. In the same range of θ angles, Bernstein modes appear in the spectrum of plasma eigenmode oscillations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 258–266, March 2006.     

Submillimetric cyclotron resonance linewidths and electron relaxation time in copper

For the first time submillimetric microwaves (λ<1 mm) are used to observe Azbel' Kaner cyclotron resonance in metals. The very high frequency used (typicallyF≅400 GHz) gives a large value ofωτ (typically 500) and therefore very sharp peaks. The fundamental resonance fieldH _c=m ^* cω/e is rather high (typically 200 KG), so subharmonicsH _c/n can be observed at many values ofH in the field region 0–27 KG. If relatively few electrons participate in the resonance and ifω _cτ≧50 (ω _c=eH/m ^* c,τ relaxation time) thenChambers has shown that the line shapes are independent of relaxation time while the fractional linewidthΔH/H varies as l/ωτ. For the belly orbit in pure copper the conditions of Chambers' theory are satisfied forH≧20 KG parallel to [111] axis.m ^* is a minimum andτ=1.8×10⁻¹⁰ s.     

Interaction of two superhigh-frequency fields with the resonance system of electron spins

Within the framework of the concept of a spin temperature in a steady-state regime, the interaction with a resonance medium of two superhigh-frequency fields, one of which is saturating and the other of which is trial, is considered theoretically in the general case without using a high-temperature approximation. The case where the absorption of a resonance medium vanishes at the trial-field frequency is investigated in detail. This occurs with an intensity of the saturating field lower than in the case of a high-temperature approximation. This intensity is estimated in the second and third orders by the parameter ω₀β_z/2, where ω₀ is the resonance frequency of the transition, β_z=ħ/(kT_z), T_z is the Zeeman-subsystem temperature, ħ is the Planck constant, and k is the Boltzmann constant. It is shown that the cooling of the dipole-dipole reservoir is more considerable than in the case of a high-temperature approximation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 492–495, July–August, 2000.     

The effect of an external magnetic field on the <Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript> subshell fluorescence yields and level widths for Ta,W, Tl,Th and U at 59.54 keV photons

The effect of an external magnetic field on the L₃ subshell fluorescence yields (ω₃ ) and level widths ($\Gamma_{L_3}$) for paramagnetic Ta, W, Tl, Th and U have been investigated using the 59.54 keV incident photon energy in the external magnetic field of intensities ±0.60 T. L₃ X-ray fluorescence cross sections ($\sigma_{L_3}^{X}$) have been measured for the same elements. The measured ω ₃, $\Gamma_{L_3} $ and $ \sigma_{L_3}^X$ values for B = 0 are in good agreement with the theoretical values. It was observed that the values of $\sigma_{L_3}^X$ and ω₃ with the applied magnitude of the magnetic field in both directions show a decreasing trend for paramagnetic Ta, W, Tl, Th and U. Furthermore, in the presence of an external magnetic field, the values of $\Gamma_{L_3}$ show an increasing trend for the same elements. The results show that the atomic parameters such as spectral linewidth, radiation rates, photoionization cross section and fluorescence yield can change when the irradiation is conducted in a magnetic field.     

Determination of the spectral parameters of nuclear quadrupole resonance from nutation spectra of powdered samples

An analytical expression I(ω) is obtained for a normalized function of the shape of an idealized nuclear quandrupole resonance nutation line of a powdered sample for spins I=3/2 (η≠0). Calculations are made of the initial moments <ωⁿ> of the nutation spectrum of the powder in the form of functions of ω₀=γB₁ and the asymmetry parameter η of the electric field gradient tensor. A method is proposed for determining the spectral parameters η and eQq_zz from the experimentally measured values of <ω>, <ω²>, <ω⁴>, and ω_1/2 of the nuclear quadrupole resonance nutation spectrum of the powder. State University, Kaliningrad. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–8, July, 1997.     

Effect of a magnetic field on the rate of etching of silicon dioxide in a CF4 + O2 plasma

A near-electrode nonuniform magnetic field crossed with an electric field is found to strongly affect the rate of etching of silicon dioxide on glass substrates in a CF₄ + O₂ plasma when the Larmor frequency (≈10⁹ s⁻¹) is much higher than the frequency of collisions of an electron with surrounding plasma particles (≈10⁶ s⁻¹) and the frequency of the applied rf electric field (≈10⁷ s⁻¹). The confinement of electrons by the magnetic field in the immediate vicinity of the substrate surface to be treated increases the rate of generation of chemically active particles, which increases the etching rate of silicon dioxide.     

Decay of an electronic vortex of a collisionless shock wave as a possible mechanism of a “Coulomb explosion”

A new mechanism of a “Coulomb explosion,” where ions are accelerated by the electric field separating charges at the magnetic Debye radius r _B∼B/4πen _e, is proposed on the basis of a nonquasineutral model of electronic vortices in a magnetic field. It is shown by means of numerical calculations that in the process of acceleration of the ions a collisionless shock wave, whose front has an effective width of the order of δ∼r _B, determined by the breakdown of quasineutrality, is formed in a time of the order of ω _pi ⁻¹ , where ω_pi is the ion plasma frequency. The origin of such explosive dynamics is the formation of “holes” in the electron density at characteristic times of the order of ω _pe ⁻¹ (ω_pe is the electronic plasma frequency) as a result of the generation of electronic vorticity by the Weibel instability of an electromagnetic wave. Calculations for a laser pulse with intensity J∼6×10¹⁸ W/cm² show that the ions expand in the radial direction with velocities up to 3.5×10⁸ cm/s. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 669–674 (25 November 1999)     

Radiation Spectrum of an Electron Cyclotron Maser

We consider generation of electromagnetic radiation at the electron cyclotron harmonics by energetic-electron beams having the mean momentum parallel, transverse, or oblique to the external magnetic field. This process is most efficient if the characteristic transverse momentum is sufficiently large. The radiation spectrum of the beam moving exactly along the magnetic field is closest to the equidistant one. The angle between the direction of the maximum emission and the magnetic field varies from 70^° for the field-aligned beam to 90^° for the beam whose characteristic momentum is transverse to the magnetic field. In fairly strong magnetic fields, i.e., for _Be >_pe, where _Be and _pe are the electron cyclotron and plasma frequencies, respectively, the radiation is maximum at low cyclotron harmonics and the second harmonic dominates. In the weaker fields (_Be < _pe), higher harmonics, up to fifth or sixth, are generated. Both wave modes are generated, but generation of the ordinary waves is far less efficient than that of the extraordinary waves under the same conditions.     

Dielectric parameterization of raman lineshapes for GaP with a plasma of charge carriers

We have studied the Raman lineshapes of several samples of GaP with appreciable carrier concentrations. There is no feature identifiable as a plasma resonance, but there are pronounced effects of interaction with the LO phonon resonance. For analysis we have developed a model along lines laid down by Barker and Loudon, employing Nyquist relations to calculate infrared fluctuations which scatter light. We introduce a response matrix α(ω) withseveral resonances; and we uncover some points which seem to be new, for coupled-mode scattering systems in general. In the GaP-plasma problem the data do not necessitate inclusion of the scattering amplitude from the plasma; we ascribe this to large plasma damping rates (ωτ≲1). This provides an account for the lack of any apparent plasma resonance in the scattering and for the modified appearance of the LO phonon, relative to the pure crystal. We emphasize that the following parameters suffice: Lorentz parameters measured in linear infrared experiments, the nonlinear parameterC from a visible-infrared mixing experiment, and the plasma frequency and damping fit to each sample. Beyond treatment of the plasma problem, the theory bears more generally on the conditions under which an LO Raman lineshape measures locally the shape of 〈E ²〉_ω. Also it bears upon the analysis of polariton linewidths to infer the variation of the phonon damping Γ(ω). This work was supported by the National Science Foundation Grant No. GH 32401. It is based on a major segment of the thesis of D.T. Hon, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Physics) in the Faculty of the Graduate School of the University of Southern California.     

Effect of upflowing field-aligned electron beams on the electron cyclotron waves in the auroral magnetosphere

The role of low density upflowing field-aligned electron beams (FEBs) on the growth rate of the electron cyclotron waves at the frequencies ω _r < Ω_e, propagating downward in the direction of the Earth’s magnetic field, has been analysed in the auroral region at ω _e/Ω_e < 1 where ω _e is the plasma frequency and Ω_e is the gyrofrequency. The FEBs with low to high energy (E _b) but with low temperature (T _‖b) have no effect on these waves. The FEBs with E _b < 1 keV and T _‖b (> 1.5 keV) have been found to have significant effect on the growth rate. Analysis has revealed that it is mainly the T _‖b which inhibits the growth rate (magnitude) and the range of frequency (bandwidth) of the instability mainly in the higher frequency spectrum. The inhibition in the growth rate and bandwidth increases with increase in T _‖b. The FEBs with less E _b (giving drift velocity) reduce growth rate more than the beams with larger E _b. The inhibition of growth rate increases with the increase in the ratio ω _e/Ω_e indicating that the beams are more effective at higher altitudes.      

Kinematics of four-wave decay of high-frequency plasma oscillations into upper hybrid and electron-cyclotron plasma waves under multiple electron gyroresonance conditions

We consider the conditions for four-wave decay of two primary plasmons with wave vector and frequency ₀ close to the multiple gyroresonance frequency n_Be into two secondary plasmons with frequencies ₁ > ₀ and ₂ < ₀. The secondary plasmons belong to the upper hybrid and the electron cyclotron branches. It is shown that the main features of the broad upshifted maximum (BUM) in the SEE spectrum can be explained in the context of the proposed process. The BUM feature appears in the region of frequencies having a positive shift from the high-power radio wave frequency. In particular the broad band nature of the BUM can be a result of the broad spectrum of wave number k₀ of the primary plasma waves. In this case the observed cut-off frequency f_cutoff limiting the BUM spectrum on the lower side can result from the lower bound of k₀ (the increase in ₁ corresponds to decay of shorter wave plasmons). In our approach we assume that the generation of primary plasma oscillations by the high-power radio wave and the conversion of secondary plasma waves into the electromagnetic waves is due to coherent scattering of corresponding waves by small-scale magnetic-field-aligned artificial irregularities or to another nonlinear processes.Published from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 3, pp. 301–317, March, 1996.     

Nonlinear resonance of plasma waves in the thermal irregularities of ionospheric plasma

We study the effect of striction plasma density disturbances on the generation intensity of longitudional cold and plasma oscillations due to polarization of the magnetic field-aligned ionospheric plasma irregularities with δN_o<0 by a powerful radio wave. It is assumed that the plasma density level inside the irregularity intersects the upper-hybrid resonance level, in the vicinity of which the cold oscillations excited directly by a powerful radio wave are transformed to shorter-wave plasma oscillations. We consider the short plasma wave limit to reduce the problem to a system of two coupled equations for the cold wave induction and plasma wave electric field. The first equation is supplemented by a local source equal to the integral of the plasma wave electric field in the resonance region. The second equation involves the cold wave induction at the resonance point and describes the electric field of interacting waves in the resonance vicinity. We use simplifications connected with the small absorption of plasma waves propagating inside the irregularity and weak radiation of these waves outside the irregularity. These conditions correspond to the generation of eigenmodes of plasma oscillations trapped in the irregularity. We have obtained a resonance-type nonlinear equation for the electric field intensity (or energy flux) of eigenmode plasma waves with allowance for striction disturbances of the plasma density profile in the resonance region. It is shown that the striction expulsion of plasma is responsible for the occurrence of coefficients describing the change in the intensity of excitation and radiation of plasma waves at the irregularity boundary. Such an expulsion leads to variations of the efficient generation band of plasma eigenmodes with the total phase increment of the wave in the irregularity. It also leads to a change in the phase shift of the plasma wave reflected from the resonance. These coefficients and the nonlinear phase shift are expressed in terms of real wave functions of the nonlinear Airy equation which describes the electric field of the excited waves in the resonance vicinity when the dissipation is absent. Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 3, pp. 270–297, March, 1998.     

Features of the resonant interaction of moving neutral atoms and clusters with superlattice surface

Features of the interaction of moving neutral atoms, molecules, and clusters with a superlattice field (for example, the system of linear magnetic and electric domains) are considered. It is shown that the character of the particle motion depends on the ratio of the frequency ω₂₁ of the internal electromagnetic resonance to the bounce frequency Ω _s determined by the superlattice period, the velocity of the particle motion, and the possible moments of the particle in the ground d ₁₁ and excited d ₂₂ states. The conditions for regimes of attraction and repulsion of particles by the superlattice are considered. The preconditions for formation of a one-dimensional potential well located far from the superlattice and for stable channeling of neutral and charged particles in this well are also considered. Depending on the ratio of ω₂₁ to Ω _s , particle sorting and beam separation occur during interaction of the multicomponent beam consisting of different particles with the superlattice field.