Azimuthal dependence of the purely nuclear diffraction for 14.4 keV resonant gamma-rays

An azimuthal dependence of the intensity of the resonant gamma-ray nuclear Bragg reflection from a sodium nitroprusside single crystal has been observed. The dependence reflects the structure of the electric field inside the crystal.     

Resonant diffraction in stishovite near the <Emphasis Type="Italic">K</Emphasis> absorption edge of silicon

The X-ray resonant diffraction in a stishovite crystal near the K absorption edge of silicon (E _K = 1839 eV) is studied theoretically. For such a long wavelength, the only possible Bragg reflection is the 100 reflection, which is forbidden by the space group of the crystal. It can be excited solely due to anisotropy of the X-ray scattering amplitude. The crystal symmetry is used to determine the polarization and azimuthal dependence of the reflection intensity. Since this reflection is single, it can be detected upon diffraction from a powder, which substantially widens the possibilities of investigations. The numerical calculations of the energy dependences of the forbidden reflection intensity and the absorption coefficient demonstrate that the dipole-quadrupole, quadrupole-quadrupole, and dipole-octupole contributions to the resonant diffraction and absorption are small and that the dipole-dipole contribution is the most important one.     

Triple-q(-->) octupolar ordering in NpO2

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.     

Determination of the magnetic cubic anisotropy field in (111) oriented films by FMR

The angular characteristics of the resonant field in films oriented in the (111) plane is analyzed using a resonance relation and a condition for equilibrium orientation of the magnetization. A method for determining a cubic anisotropy field from the azimuthal dependence of the FMR field, which eliminates the need to first find the crystallographic directions in the film by other methods, is proposed on the basis of the results obtained. Zh. Tekh. Fiz. 67, 35–40 (February 1997)     

Experimental Study of a Ka-Band Cylindrical Open Resonator

A study of the electrodynamical properties of a Ka-band gyrotron open resonator was experimentally conducted. Experiments were accomplished to measure resonant frequencies and their respective loaded quality factors for TE modes in the frequency range from 26 to 40 GHz. In particular, a perturbation technique was used to determine the axial, radial and azimuthal electric field profiles, as an identification method of the TE₀₂₁ mode operating around 35 GHz. In any experimental event, good agreement with the values predicted by theory was found.     

Identification of surface optical photoemission (div Ā ≠ 0) from silver: The significance of high index faces

The spectral and crystallographic dependence of the photoyield Y_p for p-polarized light has been measured on a cylindrical silver single crystal. In particular on high index faces, the effect of varying not only the polar but also the azimuthal angle of incidence was used to determine the character of the contributing electron excitation processes. A drastic decrease of the observed azimuthal asymmetry towards energies below the volume plasma frequency has been found. The experimental features support the conclusion that nonlocal electron excitations are dominant in the energy range below 3.8 eV.     

The chromium spin density wave: magnetic X-ray scattering studies with polarisation analysis

We report on X-ray magnetic diffraction studies of the spin density wave antiferromagnetism formed in the conduction electron band of chromium. Non-resonant X-ray magnetic scattering was used to directly determine that chromium has zero orbital magnetisation. Furthermore, the azimuthal dependence of this scattering provides unique evidence that chromium forms a linearly polarised wave. In the vicinity of the K absorption edge, resonant X-ray magnetic scattering was observed. A consistent model of the magnetic scattering has been derived from the resonant and non-resonant magnetic amplitudes. The enhancement of the magnetic intensity arises primarily from dipole transitions from the core 1s level to 4p states. Quadrupole transitions to the magnetic 3d states are essentially non-existent due to their sensitivity to (and the absence of) orbital moment. This effect is predicted from atomic considerations of the 3d⁵ ( = 0) transition metal ions. Received 22 September 2000     

The coupling between the radial and thickness vibration of the circular resonator

The paper studies the resonant frequency dependence of radial vibration on the ratio of resonator dimensions. It was found that Rayleigh's correction does not explain the decrease of resonant frequency. This phenomenon was explained quite successfully by coupling between the radial and thickness vibrations of the plate. The calculated curves correspond very well with the measured results. Ceramic resonators manufactured from ceramics based on PbZrO₃-PbTiO₃ were used for these measurements.     

Coupling between spin and orbital degrees of freedom in KCuF3

We present the results of resonant x-ray scattering experiments on KCuF3. Structurally forbidden reflections, corresponding to magnetic and 3d-orbital long-range order, have been observed. Integrated intensities have been measured as a function of incident energy, polarization, azimuthal angle, and temperature. The results give evidence for a strong coupling between orbital and spin degrees of freedom. The interplay between magnetic and orbital order parameters is revealed by the temperature dependence of the intensity of orbital Bragg peaks.     

Anomalous temperature dependence of the quadrupole coupling frequency of a lattice defect trapped to111In in Pt

The quadrupole coupling frequency of an¹¹¹In-defect bound state in Pt with ω₀(295 K) = 200 Mrad/s was measured over the range 4 – 300 K using perturbed angular correlations. The temperature dependence exhibited positive curvature, unlike the T^3/2 dependence observed in noncubic metals, and was fitted using a model which assumed that the temperature dependence was caused by vibrations of a resonant mode in the probe-defect complex.     

Elastic anomalies in 1T-TaS2

The temperature dependence of the resonant period in 1T-TaS₂ was measured using the resonant flexural vibration technique. From the close study on the effect of thermal cycles, it is suggested that the anomalous regions observed both on cooling down (183–193 K) and on warming up (218–282 K) were associated with the nearly commensurate-commensurate charge density waves (CDW's) phase transition.     

Calculations,in the three-dimensional approximation,for a half-symmetric unstable resonator with internal axicon (HSURIA) in the three-dimensional diffraction approximation with annular shape of the active medium

6. Conclusions 1. It was shown that, compared with other methods, realization of the analytic Prony method increases strongly the effective establishment of the radiation of the mode structure of the radiation field in an empty cavity having o complex configuration, such as the HSURIA resonator. 2. We investigated the dependence of the losses of various azimuthal and radial modes in an empty HSURIA resonator on the value of N_eq. We have shown that in the region N_eq=20−25 and at M=2 the losses of the highest-Q zeroth and first azimuthal modes amount to ≈0.65 and ≈0.70 respectively, which ensures lasing in the single-mode regime in the case of a sufficiently homogeneous active medium. 3. We found the field distributions of various azimuthal and radial modes in the HSURIA resonator in the absence of a medium. 4. We have shown that in the case of lasing on one highest-Q mode the radiation intensity on the tip of a W-axicon is not more than 2–3 times larger than the average over the central region of the W-axicon. Translated from Preprint No. 31 of the Lebedev Physics Institute, Russian Academy of Sciences, Moscow, 1991.     

Second harmonic generation in Ga nanoparticle monolayers

Ga nanoparticle monolayers formed by evaporation-condensation in ultrahigh vacuum and embedded in a transparent SiO_x matrix generate second harmonic (SH) signals in transmission and reflection when illuminated by a 150 fs, 800 nm laser pulses. The observed SH light exhibits a critical dependence on input and output polarizations, angle of incidence and azimuthal orientation of the samples. The results lead to a consistent picture of shape and orientation of the nanoparticles. Linear transmittance spectra in the visible range support these findings and the observed size dependence of the SH signal. Received 29 November 2000     

Magnetic structure determination using polarised resonant X-ray scattering

Full polarisation analysis of resonant X-ray magnetic scattering (RXMS) is shown to advance the determination of magnetic moment directions of complex magnetic structures within single crystals. Key features of this novel method are the variation of the incident beam polarisation through the use of an X-ray phase plate, and the measurement of the scattered beam polarisation in terms of Poincaré-Stokes parameters. Contrary to the established method, no azimuthal rotation is required. Thus, the major sources of systematic error are eliminated, and the method is compatible with exotic and complex sample environments. The technique is demonstrated with the example of TbMn₂O₅. The RXMS theory briefly outlined in this paper was fitted to the data, and the local and delocalized, band-specific moment directions associated with the magnetic order of the resonating species was refined with a high degree of accuracy.     

High pressure photoluminescence and resonant Raman study of GaAs

Photoluminescence and resonant Raman scattering have been studied in GaAs under hydrostatic stress up to 72 kbar. From the pressure dependence of the photoluminescence intensity the pressure coefficient of the X₆ conduction band minimum in GaAs has been determined. The pressure dependence of zone edge phonon energies in GaAs is also reported.     

Frequency-dependent electromagnetic properties of the Pb-doped manganite measured in the transformer-type circuit

The dependence of the transformation coefficient of the four-pole network with the toroid core made of the La_0.6Pb_0.4MnO₃ manganite on dc biasing magnetic field and frequency was experimentally investigated. The factors which influence the value of the transmission coefficient are analyzed. In the megahertz frequency range the frequency dependence of the transformation coefficient has a resonant form with the position of the maximum and amplitude increasing with the dc biasing. The dc magnetic field dependence of the phase shift between the voltages in the primary and secondary windings was investigated. The results are interpreted taking into account the imaginary component of magnetic permeability of the core material.     

Epitaxial growth of al thin films on mica and investigation of film structure

The temperature dependence of the structure of the aluminium thin film was studied. Reflection and transmission electron diffraction (RED and TED, respectively) patterns show that the changes of the polar orientation (texture) and the azimuthal orientation (due to epitaxy) arise in the film as the temperature of substrate surface is increased. The simultaneous presence of spot patterns at RED and arc or ring patterns at TED in a certain temperature range is explained by the preferred orientation of the film grains in the azimuthal direction and the nearly perfect orientation in the polar direction. The particular changes of the azimuthal orientation became at the temperature of the largest desorption of water molecules from mica so that we deduce that the presence of water molecules is necessary for the particular degree of the azimuthal orientation at lower temperature. At a higher temperature (500°C) of the substrate surface the well-oriented films were prepared with the same diffraction patterns as single crystal with orientation (111)_Al// //(001)_M and (110)_Al//(010)_M (double positioning is present in the films).The author expresses his gratitude to Dr. L.Eckertová and Dr. Z.Hájek for valuable discussions and to Dr. M.Rozsíval and Dr. P.Kratochvíl for their aid at the structure analysis.     

Elastic and proton-dissociative \rho^0 photoproduction at HERA

Elastic and proton-dissociative photoproduction (, , respectively, with ) has been studied in interactions at HERA for photon-proton centre-of-mass energies in the range GeV and for GeV, where is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range . For the elastic process, the invariant mass spectrum has been investigated as a function of . As in fixed target experiments, the resonance shape is asymmetric; this asymmetry decreases with increasing , as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant production. The cross section has been studied as a function of ; a fit to the resonant part with the form gives . The resonant part of the cross section is b at GeV. The dependence of the cross section can be described by a function of the type with (stat.) (syst.) GeV and (stat.) (syst.) GeV. The dependence has also been studied as a function of and a value of the slope of the pomeron trajectory GeV has been deduced. The spin density matrix elements , and have been measured and found to be consistent with expectations based on -channel helicity conservation. For proton-dissociative photoproduction in the mass range, the distributions of the two-pion invariant mass, and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The distribution has been fitted to an exponential function with a slope parameter GeV. The ratio of the elastic to proton-dissociative photoproduction cross section is . Received: 3 December 1997 / Online publication: February 26, 1998     

Electron paramagnetic resonance of Fe3+ in orthorhombic KNbO3

EPR spectra of KNbO₃:Fe single crystals are obtained in the - 10–110°C temperature range. The angular dependence of resonant lines is well reproduced by spin Hamiltonian parameters relevant to a Fe³⁺ impurity ion substituted to Nb ion in KNbO₃ crystal. The temperature behaviour of resonant lines is explained by a quadratic dependence of axial parameter D vs polarization P_s of the form D = βP²_s in the orthorhombic phase.