Electromagnetic absorption of acoustic-wave packets in metals

Damping of quasimonochromatic sound-wave packets in metals caused by the electromagnetic interaction of lattice vibrations with resonant current carriers is studied. It is assumed that the acoustic wavelength λ is much shorter than the electron mean free path length l but much longer than the characteristic damping depth σ of an electromagnetic wave in metals under anomalous-skin-effect conditions. It is also assumed that the transit time of a resonant particle through the region of field nonuniformity (∼λ) is shorter than the electronic relaxation time , where is the characteristic velocity of the resonant electrons). The distribution of the potential of the eddy electromagnetic field accompanying an acoustic radiopulse with a prescribed shape is found by solving the kinetic equation and Maxwell’s equations. The force exerted on the lattice by the resonant electrons is investigated, and the equation from the theory of elasticity that describes the evolution of a sound wave is solved. It is shown that a weak damping (of the order of the small parameter a) at the extrema of the deformation on the pulse edges as well as the appearance of high-frequency precursors near the pulse boundaries are characterisic for the evolution of a powerful transverse radiopulse. Such precursors were observed earlier by Fil’ et al., as components of a noise burst arising on the leading edge of a powerful transverse radiopulse propagating in ultrapure gallium. Fiz. Tverd. Tela (St. Petersburg) 40, 966–973 (June 1998)     

Anisotropic resonant X-ray scattering: Beauty of forbidden reflections

Experimental results and their theoretical explanation are reviewed for fundamentals of anisotropic resonant X-ray scattering. Resonant scattering depends on X-ray polarization, i.e. the scattering reflects anisotropic environment of atoms in crystal. The polarization anisotropy in atomic scattering can excite the forbidden Bragg reflections. Studying this type of forbidden reflections we can distinguish electronic orbitals of specific symmetry. This method is very useful for studying local electronic states in crystal. We reveal detailed property of the anisotropic scattering, effect of quadrupole transition, thermal motion, magnetic scattering and so on. Especially successful examples are given in detail: observation of phase of the scattering factor, the hybridization of states with different parity, local chirality of atom in centrosymmetric crystals, thermal-motion-induced resonant reflections, etc.     

Short-pitch helicoidal modes in antiferroelectric liquid crystals and scattering of resonant X rays

The structure of antiferro-and ferroelectric phases in chiral smectic liquid crystals is studied on the basis of the theory of modes with a short pitch. The existence of ferroelectric phases is explained both by the characteristic incommensurability of short-pitch modes with respect to the thickness of the smectic layer and by distortions of the short-pitch antiferroelectric structure of a long-wavelength mode. It is pointed out that the details of the short-pitch order can be observed in resonant x-ray scattering. The fundamental features of the pattern of such scattering in the phases considered are predicted. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 226–231 (10 February 1999)     

Ab initio calculations of the forbidden Bragg reflections energy spectra in wurtzites versus temperature

Thermal-motion induced (TMI) scattering is caused by the influence of atomic displacements on electronic states in crystals and strongly depends on temperature. It corresponds to dipole-dipole resonant x-ray scattering, but is usually accompanied by dipole-quadrupole scattering. The phenomenological theory supposes the dipole-quadrupole term to be temperature independent (TI). As a result, the transformation of the energy spectra with temperature observed experimentally in ZnO and GaN corresponds to the interference between the TMI and TI terms. In the present paper the direct confirmation of this theoretical prediction is given. Ab initio molecular dynamics was used to simulate the sets of atomic sites at various temperatures followed by quantum mechanical calculation of resonant Bragg reflection energy spectra. The results of simulation are in excellent coincidence with experimental energy spectra of forbidden reflections and confirm the earlier phenomenological conjecture about the interference between the TI dipole-quadrupole and TMI dipole-dipole contributions to the resonant atomic factor.     

Magnetic anisotropy of epitaxial iron films on single-crystal MgO(001) and Al_2 O_3 (11\bar 20) substrates

The ferromagnetic resonance and magnetization of single-crystal thin (27–100 Å films grown in the (110) direction are measured in the temperature range 20–400 K. The films are prepared by molecular-beam epitaxy on single-crystal sapphire substrates with a Nb(110)buffer layer. The angular dependence of the parameters of the ferromagnetic resonance spectrum is observed to have a 180° character when the static magnetic field is rotated in the plane of the sample. It is established that this angular dependence can be described on the assumption that the lattice distortions are essentially trigonal. A comparative analysis of previous data for Fe(001) films with the data for Fe(110) films shows that the source of the corrections to the cubic anisotropy constant is the characteristic distribution of the strains along the thickness of the film. Zh. éksp. Teor. Fiz. 115, 689–703 (February 1999)     

Thermal expansion anomalies in TbVO4 due to the cooperative Jahn-Teller effect

The thermal expansion anomalies in TbVO₄ due to the cooperative Jahn-Teller effect were studied experimentally and theoretically. Characteristic magnetoelastic anomalies were observed in the curves of Δa/a and Δc/c of the cell parameters at T<T _c. Calculations of the magnetoelastic contribution to Δa/a and Δc/c from fully symmetric and and low-symmetry ɛ^δ modes were performed using the general crystal-field formalism, and the values of the magnetoelastic coefficient B ^δ were obtained from spectroscopic and spontaneous-deformation data. It is shown that the thermal expansion of TbVO₄ in both the tetragonal and orthorhombic phases can be described well on the basis of a general model based on a single set of interaction parameters. Fiz. Tverd. Tela (St. Petersburg) 40, 1663–1666 (September 1998)     

The electronic structure of a silicon divacancy calculated by the open shell method

The restricted Hartree-Fock-Roothaan method with projection of the electron density matrices for a filled and an open electron shell and the quasimolecular large unit cell model are used to calculate the electronic structure of a silicon divacancy in the charge states 0, ±1 in the fully-symmetric atomic configuration with relaxation and with symmetry-lowering distortions. It is shown that the Jahn-Teller effect is bimodal and that there is a vibronic coupling between the electron orbital doublet and the resonant mode and pairing mode of the distortion. Fiz. Tverd. Tela (St. Petersburg) 41, 404–410 (March 1999)     

Methods for calculating the resonant part of the atomic factor in crystals with partial filling of the crystallographic position

The resonant part of the tensor atomic factor, which is important for incident radiation energies close to the atom absorption edges, is sensitive to the local environment of a scattering atom (near-range order). In this paper, a method for mathematical modeling of the resonant part of the atomic factor is developed by taking into account crystal-lattice relaxation caused by the partial filling of a crystallographic position. It is shown that an additional contribution leading to the appearance of purely resonance “forbidden” reflections can exist in the resonant atomic factor. As examples, crystals of potassium dihydrophosphate (KDP) and rubidium dihydrophosphate (RDP) are used to calculate such a contribution to the atomic factor of potassium and rubidium, and the last is compared to the thermally induced and dipole-quadrupole contributions to the atomic factor.     

The nature of the {\bar p} metastability in helium

A new interpretation of the nature of the metastable states of antiprotons in helium is proposed. The first step is the formation of an ion ( ), which shall be called “ioncule”. After its decay a neutral “atomcule” ( ) is formed which was observed in the experiments on metastable antiprotonic helium. This model explains the experiments in a natural way, in particular the quenching by atomic and molecular admixtures and the pressure dependence of quenching in pure helium. At high helium densities the ioncule creates a vacuum bubble in helium. It is shown that the annihilation of slowed-down can be suppressed at helium pressures of ∼ 10 000 bar. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamic fluctuation phenomena in double membrane films

Dynamics of double membrane films is investigated in the long-wavelength limit qh≪1 (q is the wave vector, and h is the thickness of the film) including the overdamped squeezing mode. We demonstrate that thermal fluctuations essentially modify the character of the mode due to its nonlinear coupling to the transverse shear hydrodynamic mode. The renormalization can be analyzed if the condition g≪1 is satisfied (where g∼T/κ, T is the temperature, and κ is the bending modulus). The corresponding Green’s function acquires as a function of the frequency ω a cut along the imaginary semiaxis. At the effective length of the cut is ∼Tq ³/η (where η is the shear viscosity of the liquid). At the fluctuations lead to an increase in the attenuation of the squeezing mode: it is larger than the ‘bare’ value by a factor . We also present the analysis of the elastic modes. Zh. éksp. Teor. Fiz. 113, 2096–2108 (June 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Influence of the concentration of Ca impurity on the magnetic threshold of the magnetoplastic effect in NaCl crystals

It is found experimentally that the threshold magnetic field B _c for the magnetoplastic effect, i.e., the field at which the depinning of dislocations from paramagnetic impurities in an external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C=(0.5–100) ppm. It is shown that the dependence B _c(C) exhibits a distinct tendency toward saturation. The physical interpretation of the observed dependence rests on the notion that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the impurity atoms changes according to a definite pattern. In particular, the average number of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to the previously observed increase of B _c with increasing temperature. The proposed interpretation is further supported by good correlation of the experimental dependence B _c(C) with the calculated function . Fiz. Tverd. Tela (St. Petersburg) 40, 81–84 (January 1998)     

Effect of mechanical stresses on the magnetoelastic properties of TmVO4

In this paper the influence of mechanical stress on magnetoelastic properties, i.e., magnetostriction and thermal expansion in the neighborhood of a structural phase transition of the Jahn-Teller crystal TmVO₄ is investigated experimentally and theoretically. It is shown that the magnetoelastic properties of TmVO₄ for a magnetic field H∥[001] do not change the domain structure of the sample, which is rather well described when mechanical stresses in the crystal are taken into account using the parameter . Conversely, for magnetic fields along the direction of spontaneous strain [110] the magnetoelastic properties are primarily caused by reorientation of the Jahn-Teller domains and short-range order effects. It is shown that the “true” magnetostriction of a single-domain crystal for H∥[110] diverges at the phase transition point T _c=2.15 K in the absence of mechanical stresses and is strongly decreased by these stresses. Fiz. Tverd. Tela (St. Petersburg) 40, 701–705 (April 1998)     

Event-shape of dileptons plus missing energy at a linear collider as a supersymmetry/Arkani-Hamed-Dimopoulos-Dvali discriminant

An event-shape analysis of the dileptons in the process e ⁺ e ⁻ → ℓ⁺ℓ⁻ , studied in ILC or CLIC, can clearly discriminate between a supersymmetric or a large extra dimensional (ADD) production mechanism.      

On nonquasineutrality and turbulent heat transfer in a tokamak

A new approach is proposed for describing a steady turbulent state formed far from thermodynamic equilibrium in the plasma column of a tokamak as a result of the development of microinstabilities. A fundamental feature of such a highly nonequilibrium plasma is its nonquasineutrality, i.e., the plasma properties are largely determined by electric fields localized on a scale of the order of the Debye radius. It is established that the transverse thermal diffusivity is determined by the expression

Ab initio simulation of resonant forbidden reflections in Ge crystals

Forbidden reflections are observed in the case of diffraction of synchrotron radiation with wave-lengths close to the absorption edges in crystals. A new method for calculating the intensity of thermal-motion-induced (TMI) forbidden reflections is proposed in this paper. It includes two stages: simulation of instantaneous thermal atomic displacements using ab initio molecular dynamics and subsequent quantum-mechanical calculations of the resonance scattering amplitude for various configurations. This procedure is used to calculate the temperature dependence of the 600 reflection intensity for Ge. The proposed method for simulating forbidden TMI reflections is suitable for any crystal structures and can explain many results so far obtained using synchrotron.     

Excitonic light-absorption and amplification bands in the presence of laser radiation

We examine the absorption and amplification bands of a weak probe signal in the presence of Bose-Einstein condensation of excitons that emerges in nonequilibrium conditions in the field of coherent laser radiation with a wave vector k ₀. We assume that the detuning from resonance between the energy ħω _ex (k ₀)+L ₀ of the exciton level, which is shifted because of exciton-exciton interaction, and the laser photon energy ħω _L , is generally nonzero. The elementary excitation spectrum consisting of the quasiexcitonic and quasienergy branches determines the optical properties of the system. When there is real induced Bose-Einstein condensation, at the two branches touch, as they do in spontaneous Bose-Einstein condensation. In virtual induced Bose-Einstein condensation, when , instabilities emerge in the spectrum in certain regions of the k-space. These instabilities are caused by a real transformation of two laser photons into two extracondensate particles. Nonequilibrium extracondensate excitons strongly affect the absorption and amplification of the probe light signal. We show that light absorption is due to the quantum transition from the ground state of the crystal to the quasiexcitonic branch of the spectrum. On the other hand, amplification of the signal is caused by the transition from the quasienergy branch to the ground state of the crystal. The same transition can be explained by a real transformation of two laser photons into a vacuum photon of frequency ħcq and a crystal exciton with a wave vector 2k ₀−q. Finally, we show that the excitonic absorption and light-amplification bands are essentially anisotropic at and depend on the orientation of the vectors q and k ₀. Zh. éksp. Teor. Fiz. 112, 167–179 (July 1997)     

Atomic squeezing in assembly of two two-level atoms interacting with a single mode coherent radiation

Saito and Ueda [Phys. Rev. A 59, 3959 (1999)] studied atomic and radiation squeezing in interaction of a single mode coherent state of radiation with two excited two-level atoms, using the Jaynes Cummings Hamiltonian. They considered α real and studied squeezing of the Dicke operator S_x using the Kitagawa-Ueda criterion for squeezing and coupling times less than or nearly equal to . We obtain results to all orders in coupling time for atoms, which are initially in (i) fully excited, (ii) superradiant or in (iii) ground states and obtain more general results. We use our recently reported criterion for atomic squeezing, of which the Kitagawa-Ueda criterion is a special case, and obtain a much stronger (nearly 95%) atomic squeezing than that (nearly 1.1%) reported by Saito and Ueda.     

A quantitative study of diffracted X-ray intensities from type I natural diamond crystals by high resolution X-ray diffractometry and comparison with nearly perfect silicon single crystals

Results of accurate measurements of peak and integrated intensities of , 111 and 333 reflections of natural diamonds of type I and nearly perfect silicon single crystals are reported. Highly monochromated and collimated MoK _{α 1} exploring beam was used. A quadrupole crystal X-ray diffractometer was employed in (+, −, +) and (+, −, +, −) settings. (111) platelets of diamond and silicon crystals with thicknesses of about 1 mm were selected. High resolution diffraction curves, stationary and traverse topographs were recorded. Diffraction curve half widths of diamond and silicon crystals were in the range: 45–200 arc sec and a few arc sec respectively. The experimental values of integrated intensitiesρ for diamond crystals were found to lie between the theoretical values for ideally perfect and ideally imperfect crystals. Experimental values ofρ for silicon were closer to the “perfect crystal” values. This is consistent with the results of diffractometric and topographic evaluation. The peak intensities of all reflections were higher for diamond crystals in comparison to the silicon crystals. The ratioI _C/I _Si lies in the range 1.3 (111 reflection) to 10.5 and (333) reflections. This is anomalous and cannot be accounted for by considering the degree of perfection, structure factor and difference in absorption coefficient.