Calculation of line width in channeling radiation by optical potential method

Abstract

The line width problem in channeling radiation is considered in terms of the optical potential method. It is shown that the plasmon excitations do not contribute to the line width.     

EPR and NMR line shapes in disordered ferroelectric crystals

A theory is developed for the magnetic resonance line shape in disordered ferroelectric crystals. In a study of the random fields responsible for inhomogeneous line broadening, nonlinear and spatial correlation effects are taken into account. It is shown that homogeneous broadening depends on the temperature and on the magnitude and orientation of the external magnetic field, as well as on the nonlinearity parameters of the ferroelectric material. The resonance line shape is calculated as the envelope of homogeneously broadened Lorentzian spin packets. Analytic expressions for I ₂(ω) and I ₃(ω) are introduced to account for nonlinearity and correlation effects of second and third order, respectively. Calculations are done for centrally symmetric crystals, as well as for those without a center of symmetry. It is shown that homogeneous broadening is important near the line peak, while the wings are determined mainly by inhomogeneous broadening. Manifestations of the predicted effects in the spectrum lines of disordered ferroelectric crystals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 340–347 (February 1998)     

Free and localized positively charged excitons in the emission spectrum of GaAs/AlGaAs quantum wells

The recombination emission spectra of an excitonic complex (A ⁰ X) localized on a neutral acceptor, which have previously been attributed to a positively charged exciton (X ⁺), are investigated. Satellites arising around the main luminescence line as a result of recoil processes during recombination of the complex which leave the surviving hole in an excited state are observed and investigated. It is shown in a computational model based on the Luttinger Hamiltonian that the energy splittings between the main line and the satellites correspond to an in-barrier impurity center located a definite distance from the well. It is shown that as the magnetic field increases, a transition is observed from the singlet ground state of the complex to a multiplet state. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 223–228 (10 August 1998)     

Cooperative population dynamics of an ensemble of Λ atoms in a bichromatic field

Equations are derived which describe the dynamics of three-level atoms with a Λ level scheme, interacting with two coherent resonance fields under conditions such that cooperative relaxation predominates over incoherent spontaneous emission. A numerical calculation of the temporal dynamics of the values of the atomic populations is performed. It is shown that coherent population trapping in the presence of cooperative decay is possible. The quantities characterizing this phenomenon are calculated—the width of the black line and the transition time to coherent trapping in this scheme. Zh. Tekh. Fiz. 68, 19–24 (June 1998)     

Properties of the phase space and bifurcations in the complex Lorenz model

The geometrical structure of the phase space and bifurcations in the complex Lorenz model are investigated. It is shown that the hierarchy of bifurcations in a single-mode laser with detuning of the resonator frequency from the frequency of a spectral line is similar to the hierarchy of bifurcations of the logistic map. Zh. Tekh. Fiz. 68, 1–9 (August 1998)     

Martensitic transformation and soft modes in KAlF4

Abstract

The structural phase transition observed at ?13°C in KAlF₄ is shown to be martensitic. The results of an investigation of the phonon spectrum by inelastic neutron scattering are reported. It is shown that the transition is preceded by the softening of a flat phonon branch. A model is proposed to explain how such a softening is related to the martensitic transition.     

Electron Spectrum Topology and Giant Singularities of the Electron Density of States in Cubic Lattices

The topology of energy surfaces in reciprocal space is studied in detail for simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) lattices in the tight-binding approximation, taking into account hopping integrals t and t′ between the nearest and next-nearest neighbor sites, respectively. It is shown that lines and surfaces formed by van Hove k points can arise at values τ = t′/t = τ* corresponding to a change in the surface topology. At a small deviation of τ from these special values, the spectrum near the van Hove line (surface) only slightly depends on k. This corresponds to a giant effective mass proportional to |τ - τ*|⁻¹ near several van Hove points. Singular contributions to the density of states near these special t values are analyzed and explicit expressions are obtained for the density of states in terms of elliptic integrals. It is shown that, in some cases, the maximum density of states is achieved at energies corresponding to k points in high-symmetry directions inside the Brillouin zone rather than at its edges. The corresponding contributions to electronic and magnetic characteristics are discussed, in particular, in application to itinerant weak magnets.

     

Conversion from constitutive parameters to dispersive transmission line parameters for multi-band metamaterials

Abstract

In this study, we explain an approach including conversion from constitutive parameters to dispersive transmission line parameters using the double-band DNG (double-negative) properties of the circular type fishnet metamaterials. After designing the metamaterial structure, the numerical calculations and the composite right/left-handed (CRLH) modeling of circular-type metamaterials are realized in free space. Detailed dispersion characteristics give us the opportunity to explain the true behavior of the inclusions during the analysis stage. By combining the results coming from the standard retrieval procedure with the conventional CRLH theory, we calculate the actual values of the transmission line parameters for all frequency regimes. The constitutive parameters of an equivalent CRLH transmission line are derived and shown to be negative values. It is shown that the constitutive parameters present the same behavior for all negative refractive index regimes. The double-negative properties and the phase advance/lag behavior of metamaterials are observed based on the dispersive transmission line parameters.     

Manifestation of asymmetry in the spin-phonon coupling line in nonresonant paramagnetic absorption

A theory of nonresonant paramagnetic absorption in concentrated paramagnets in which the spin-phonon coupling (SPC) line is asymmetric relative to the center of the magnetic-resonance line is developed on the basis of a two-temperature Provotorov model. It is shown that the strong asymmetry of the SPC line results in a second absorption peak at low frequencies, together with the peak characteristic for the Debye curve. It is established that this effect is due to the appearance of a second “cross” term, as a result of the asymmetry of the SPC line, in the relaxational evolution of the Zeeman and non-Zeeman low-frequency temperatures. Fiz. Tverd. Tela (St. Petersburg) 39, 522–524 (March 1997)     

Experimental Investigation on Transmission Control Protocol Throughput Behavior in Optical Fiber Access Networks

ABSTRACT

This article describes an experimental investigation on the behavior of transmission control protocol in throughput measurements to be used in the verification of the service-level agreement between the Internet service provider and user in terms of line capacity for ultra-broadband access networks typical of fiber-to-the-x architectures. It is experimentally shown different conditions in high bandwidth-delay product links where the estimation of the line capacity based on a single transmission control protocol session results are unreliable. Simple equations reported in this work, and experimentally verified, point out the conditions in terms of packet loss, time delay, and line capacity, that allow consideration of the reliability of the measurement carried out with a single transmission control protocol session test by adopting a suitable measurement time duration.     

Periodic Traveling Water Waves with Isobaric Streamlines

Abstract

It is shown that in water of finite depth, there are no periodic traveling waves with the property that the pressure in the underlying fluid flow is constant along streamlines. In the case of infinite depth, there is only one such solution, which is due to Gerstner.     

Nonadiabatic effects in the phonon spectra of superconductors

The nonadiabatic corrections to the self-energy part Σ_s(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σ_s(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σ_s(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σ_s(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π ⁻¹ Im D _s (q, ω), where D _s (q, ω) is the phonon Green’s function, at temperatures T<T _c . Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999)     

Nonholonomic constraint between the polarization state of light and the twisted angle of a single-mode optical fiber with linear birefringence

The relation between the polarization state of light propagating in a twisted single-mode optical fiber with linear birefringence and the twist angle of the fiber is discussed for the case where the fiber is stretched in a straight line. It is shown that this relation is of the nature of a nonholonomic constraint. Zh. Tekh. Fiz. 68, 128–129 (November 1998)     

Capabilities of a muon method for studying the diamagnetic domains accompanying the de Haas-van Alphen effect

This paper analyzes the capabilities of a muon (μSR) method for studying the de Haas-van Alphen effect and the diamagnetic domain structure accompanying it. It is shown that, unlike the NMR method, the μSR method makes it possible to observe the formation of a diamagnetic domain structure in all metals. It is not currently known what type of domain structure accompanies the de Haas-van Alphen effect: one-dimensional (laminar) or two-dimensional. It is shown that the line shape of the Fourier spectrum of the signal makes it possible to determine both the character of the domain structure (two-dimensional or laminar) and the magnetic field distribution in the domains. Zh. éksp. Teor. Fiz. 111, 250–261 (January 1997)     

Shape of the magnetic resonance line in a thin film on the surface of an anisotropic superconductor

The shape of the EPR line in a thin (=λ/2, where λ is the London penetration depth of the magnetic field in the superconductor) paramagnetic film deposited on the surface of an anisotropic superconductor is calculated in an oblique magnetic field with allowance for the inhomogeneity of the local magnetic field of the Abrikosov vortex lattice. It is shown that, as the tilt angle of the external magnetic field is varied, the shape of the EPR line changes noticeably. This fact can give additional information about the superconductor parameters (the symmetry type of the vortex lattice and the anisotropy parameter of the superconductor). Fiz. Tverd. Tela (St. Petersburg) 41, 386–388 (March 1999)     

Excitons and excitonic complexes in GaAs/AlGaAs quantum wells with a low-density quasi-two-dimensional electron and hole channel

The recombination spectra of excitons and excitonic complexes in un-doped GaAs/AlGaAs single quantum wells are investigated. It is shown on the basis of a study of the magnetic-field dependence of the emission spectra and the degree of optical orientation in zero magnetic field and on the basis of electrooptic measurements that not only the density but also the sign of the charge carriers in a well depend strongly on the photoexcitation energy. It is shown on the basis of a comparative analysis of the spin splitting of the recombination lines of free and bound excitons that the recombination line which was attributed earlier to a positively charged exciton corresponds to the recombination of an exciton bound on a neutral acceptor. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 707–713 (10 May 1998)     

Textures and physical properties of marbles deformed at 20 – 250°c

Abstract

The experimental equipment at beam line 7A of the IBR-2 pulsed reactor at JINR, which includes a texture diffractometer and a high-pressure device, is used to characterize textural and structural changes of marbles in dependence upon directed stress, temperature and time. It was shown, that texture changes as a consequence of heating are not valid, in contrast, long-acting load even at moderate stresses led to significant texture modification. The neutron diffraction data are also used to demonstrate its usefulness for the calculation of the thermal expansion coefficient a of calcite, and to calculate the lattice stresses induced by temperature and mechanical stresses.     

On the nature of the oscillations of cyclotron absorption in InAs/GaSb quantum wells

The mechanism of oscillations of the half-width and intensity of the cyclotron resonance (CR) line of electrons in a semimetal quantum well based on an InAs/AlSb/GaSb heterostructure is investigated experimentally and theoretically. It is shown that the oscillations of the CR spectrum are due to mixing of states of the spatially separated two-dimensional electrons and holes. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 753–758 (25 November 1998)     

The shapes of Auger decay lines in photoelectron satellite spectra

The theory of the shapes of Auger decay lines of satellite two-hole-one particle states accompanying photoionization based on the Green's function method is developed. The lineshapes of Auger decay of satellite states [2 s 2 p ]( ^1,3 P )3 s ( ² P ), [2 s 2 p ]( ¹ P )4 s ( ² P ) and [3 s 3 p ]( ³ P )4 s ( ² P ) in valence p-photoelectron spectra of Ne and Ar atoms are calculated (hole states are indicated by square brackets throughout). It is shown that in some cases the Auger lineshapes reproduce the shape of the photoelectron satellite line, but in other cases Auger line may be narrower then the photoelectron line and may have opposite direction of asymmetry. The theoretical results are in agreement with experimental low-energy Auger spectra. Received: 25 May 1998 / Accepted: 2 October 1998     

X-ray spectra of multiply-charged hollow ions in the emission from a femtosecond laser plasma

A new type of quasicontinuous emission spectrum from a femtosecond laser plasma near the Ly_α line of H-like ions is obtained and interpreted. It is shown that these spectra are emitted by multiply charged hollow ions, which are present in the plasma on account of its ultrahigh density, which, in turn, is due to the use of high-contrast laser pulses for producing the plasma. A new spectroscopic approach based on analysis of the composition of spectral complexes of hollow ions is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 467–472 (10 April 1998)