Calculation of diffuse thermal x-ray scattering intensity in polycrystalline HCP metals

In this work we perform a calculation of the intensity of thermal diffuse scattering from polycrystalline samples with an hcp structure, an interesting physical problem, by calculating the dynamical matrix and its powers. This is a new approach for polycrystalline hep metals. M. V. Lomonosov State University, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp. 3–6, December, 1998.     

High-pressure study of x-ray diffuse scattering in ferroelectric perovskites

We present a high-pressure x-ray diffuse scattering study of the ABO3 ferroelectric perovskites BaTiO3 and KNbO3. The well-known diffuse lines are observed in all the phases studied. In KNbO3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO3 linearly decreases to zero at approximately 11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.     

On the technique of absolutization of diffuse scattering intensity measurements based on thermal diffuse scattering measurements

Features of microdefect (MD) formation in GaAs(Si) single crystals grown by horizontally oriented crystallization were studied by X-ray diffuse scattering (XRDS). Measurements were performed at room temperature (∼298 K) and near the liquid nitrogen evaporation temperature (∼85 K) using an open-flow cooling nitrogen cryostat. A practical technique for measuring XRDS using a triple-axis X-ray diffractometer was developed and applied to separate scattering on defects and thermal diffuse scattering. For a crystal with n = 2.0 × 10¹⁸ cm⁻³, the radius of detected nonspherical MDs was determined as ∼0.2 μm; thermal diffuse scattering (TDS) was experimentally separated. For a crystal with n = 3.9 × 10¹⁸ cm⁻³, nonspherical MDs ∼0.5 μm in radius were detected; TDS was found to be a negligible fraction of total XRDS. At the same time, in the case of coinciding crystal orientations and identical experimental conditions, TDS measurement data for one crystal can be used for other GaAs(Si) crystals with the same orientation.     

Anomalous dispersion of optical phonons at the neutral-ionic transition: evidence from diffuse x-ray scattering

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.     

Diffuse x-ray scattering near Bragg peaks from point defects in a general lattice

Using the continuum theory of linear elasticity, diffuse x-ray scattering has been calculated in the immediate neighbourhood of Bragg peaks from point defects in a lattice containing more than one atom in the unit cell. General expressions are obtained for the Debye-Waller factor, Huang diffuse scattering and the asymmetric scattering due to the defect. For lattices with one atom per unit cell, these expressions reduce to the well-known formulae of diffuse scattering.     

Mobility of 2D-electrons in scattering on a correlated distribution of impurity ions in doped thin layers

The spatial correlations of impurity ions in doped thin layers have been considered. A model of hard spheres on the plane has been developed for describing the correlations. In this model, an analytical expression has been obtained for the structure factor of 2D-electrons. The concentration dependences of the mobility of 2D-electrons in heterostructures with separate doping have been investigated using Al _x Ga_{1 − x} As/GaAs as an example.     

Investigations of two-level tunneling systems in n-irradiated quartz by diffuse x-ray scattering

Abstract

Recent observation of the acoustic saturation and of phonon echoes in crystalline quartz after light irradiation with fast neutrons indicates that there are the same low-energy atomic tunnelling systems as in Si0₂ glass.

We report measurements of the diffuse X-ray scattering and of the lattice parameter change from neutron irradiated quartz single crystals studying the strength and symmetry of strain defects and their local correlations. For dose 3 × 10¹⁸ n/cm² the diffuse scattering close to the Bragg peaks together with the lattice parameter change shows that the neutrons create defective regions with a volume change of 29 mean atomic volumes. The distortion field of these regions shows no high symmetry. The radius of the heavily strained region is ～ 10 Å. For higher doses the regions grow a little bit but mainly new defective regions are built.

According to the various volume fractions of defective regions at various doses the first strong amorphous halo at about K≈ 1.5 A^?1 was detected. Since this amorphous scattering is isotropic and during annealing the regions disappear mainly without changing their size we are led to the conclusion that the defective regions are glass-like in nature. So the size of the tunnelling system has to be smaller than 20 Å.     

Elastic and inelastic scattering components in the angular intensity distribution of He scattered from graphite

The angular intensity distribution of thermal energy He beam scattered from highly oriented pyrolytic graphite (HOPG) surface has been measured by means of supersonic molecular beam scattering technique in the wide surface temperature range. To separate the elastic and inelastic scattering components, simple analysis method has been developed by applying the classical binary collision theory of the hard cube model (HCM). From the extracted elastic scattering component in the scattering distribution, the Debye temperature of the HOPG surface has been derived as 590 ± 30 K. On the basis of the HCM analysis for the extracted inelastic scattering components of He beam, the effective mass for the HOPG surface has been found to be 72 u (six carbon atoms).     

开口/封闭薄壳体声辐射和散射的统一边界积分方程解法

推导了在二维和三维空间下开口和封闭薄壳体在任意阻抗边界条件下声辐射和散射的统一边界积分方程.相对于以前的求解方法,该方程求解声辐射和散射问题具有相同的影响矩阵,能够同时求解薄壳体气动和振动噪声的辐射和散射现象,以及分析壳体声阻抗对声波传播的影响.推导的方程可以应用于叶轮机械、管道等噪声和消声器消声性能的预测等方面.在此方程基础上,可以进一步考虑运动边界和运动介质对声辐射和散射的影响. 关键词： 薄壳体 声阻抗 积分方程 边界元方法     

Effect of crystal surface defects on the scattering intensity of X rays

The effect of surface defects on the intensity of scattered x rays is investigated. A method is proposed for the determination of the thickness of deformed surface layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 68–71, January, 1984.     

Mechanisms of dislocation generation in Si structures with strained layers: Intrinsic point defects in dislocation nucleation

The peculiarities of dislocation production in silicon compositions with elastically strained layers fabricated by the molecular-beam epitaxy technique (SiGe/Si heterostructures) and by direct bonding of Si(110)/Si(100) wafers are studied with transmission electron microscopy. The role of intrinsic point defects during the process of nucleation of misfit dislocations is explained. The surplus concentration of these defects in heterostructures was produced via low-temperature epitaxial growth of buffer layers or with ion implantation of elastically strained heterostructures. The model of “optimal” and “inverse” intrinsic point defects providing an explanation for the relaxation of misfit strains in heterostructures is proposed.     

Application of Brillouin scattering to the local anisotropy and birefringence measurements of thin layers

Brillouin scattering is an efficient nondestructive and noncontact measurement method to obtain the wave properties of thin layers at hypersonic frequencies. The reflection induced ThetaA (RIThetaA) scattering geometry enables the simultaneous measurement of the phonons, which propagate in each direction of wave vectors of q(Theta A) (propagation in the film plane) and q(180) (back scattering). Using this scattering geometry, we could observe the refractive indices and birefringence of the piezoelectric poly-vinylidene fluoride (PVDF) film as a function of temperature. By introducing the microscopic technique, the elastic anisotropy and refractive index measurements in the minute area of polycrystalline ZnO films were also performed.     

Measurement of two solvation regimes in water-ethanol mixtures using x-ray compton scattering

Water-ethanol mixtures exhibit interesting anomalies in their macroscopic properties. Despite a lot of research, the origin of the anomalies and the microscopic structure itself is still far from completely known. We have utilized the synchrotron x-ray Compton scattering technique to elucidate the structure of aqueous ethanol from a new experimental perspective. The technique is uniquely sensitive to the local molecular geometries at the angstrom and subangstrom scales. The experiments reveal two distinct mixing regimes in terms of geometry: the dilute 5 mol % and the concentrated >15 mol % regimes. By comparing with pure liquids, the former regime is characterized by an intramolecular and the latter by an intermolecular change. The findings bring new light to evaluating the hypothesis of formation of clathratelike structures at the dilute concentrations.