Characterization of the bulk defects in InP crystal with a high-resolution triple-crystal X-ray diffractometer

Intensity distributions for various reflections from InP single crystal were measured with a triple-crystal X-ray diffractometer. The measured intensity patterns were compared with those calculated for a perfect crystal. This comparison showed a good agreement for the Bragg-case diffraction, while a broadening of the measured intensity profiles was observed for the Laue-case diffraction. This broadening results from the presence of microdefects in the bulk of the material. The critical radius of the microdefects was estimated to be in the range of 1 to 2 m from the power law of scattering for the Laue reflection curves. The microdefects were found to be of interstitial type and the computer simulations suggest that they are formed by aggregation of dislocation loops consistent with {111} 110 model.     

A role of second-order derivatives of amplitudes in X-ray beam dynamical diffraction equations

Dynamical diffraction equations for X-ray beams are considered with retaining the second-order derivatives of amplitudes in the direction perpendicular to the diffraction plane. For the dynamical diffraction problem, the retarded Green function is determined in case of a perfect crystal. Amplitudes of transmitted and diffracted waves in the crystal are represented as convolution over the crystal surface with use of determined Green function. Such representation can be used for solving diffraction problems in Laue and Bragg geometries in perfect crystals with both plane and not plane entrance and exit surfaces.     

On the existence of interior solutions in general relativity

The existence problem of matter sources for given stationary axisymmetric solutions of Einstein's vacuum field equations is investigated. The existence of sources of a differentially or rigidly rotating perfect fluid can be proved at least in the neighborhood of boundary surfaces if these can be chosen suitably (theorem). In particular, there exist such half-local perfect fluid sources for the Kerr metric. Hence the existence of a global regular Kerr-interior solution cannot be excluded by local considerations in an arbitrarily small neighborhood of a possible boundary.Work supported in part by the Deutsche Forschungsgemeinschaft.     

Determination of the Refractive Index of Dielectrics by Means of Light‐Induced Diffraction Gratings

Consideration is given to the possibilities of measuring the refractive index n of dielectric samples by means of spontaneous gratings produced by an spolarized laser beam in a photosensitive thin film applied to a sample in vacuum. With the specific choice of the film thickness, the value of n is determined from diffraction measurements of the period of a spontaneous grating with the error n = ±0.002. The method allows measurement of n isotropic and anisotropic samples within the range 1.32.5 on plane and cylindrical surfaces with the area determined by the section of the laser beam that induces the spontaneous grating. Examples of measurements of n on plane surfaces of glasses, a LiNbO₃ crystal, and on the cylindrical surface of a ruby rod are presented.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

Ultrasound effect on neutron Bragg diffraction in a perfect and deformed silicon single crystal

The features of neutron Bragg diffraction in a perfect and bent (deformed) silicon single crystal have been studied under ultrasound excitation. In contrast to a perfect crystal where an increase in the diffraction intensity has been observed with an increase in the ultrasound wave’s amplitude, in a deformed silicon crystal the intensity sharply decreases (two times) already at small voltages on a piezoelectric transducer. The depth and position of minima of the total intensity depend on the ultrasound wave’s amplitude and the bending radius of a crystal. To explain the observed effects the modified Penning-Polder-Kato model has been successfully used, in which the role of ultrasound is reduced to the formation of resonance transitions of imaging points between different sheets of dispersion surface. Experimental proofs of validity of the used model are presented. Estimations of the probabilities of one- and multiphonon scattering processes obtained from experimental data have a good agreement with theoretical ones.     

Adsorption of Pb on NiAl(1 1 0): energetics and structure

The adsorption of Pb onto a NiAl(1 1 0) single crystal surface at 300 K has been studied by Auger electron spectroscopy (AES), Low energy electron diffraction (LEED), molecular beam/surface scattering and single crystal adsorption calorimetry (SCAC). AES indicates a Stranski-Krastanov growth mode, i.e., Pb initially grows on NiAl(1 1 0) two-dimensionally until the first layer completes at 0.89 ML, where a superstructure is observed by LEED, followed by 3D islanding. Measurements of the Pb gas that does not stick indicate that Pb sticks on NiAl(1 1 0) with an initial probability of 0.99. The initial heat of adsorption of Pb on NiAl(1 1 0) is 249 ± 10 kJ/mol. Due to the repulsive interactions between Pb adatoms, the heat of adsorption decreases within the first layer to a value identical to the heat of sublimation of bulk Pb (195 kJ/mol), where it remains at higher coverages. This first application of adsorption calorimetry on such a thick sample (75 μm versus 0.2-8 μm previously) demonstrates that adsorption calorimetry can be extended to a wider range of surfaces, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning.     

Beam coupling ratios and diffraction efficiencies of a layer-structured liquid crystal device; the effect of injection temperature and layer thickness

Beam coupling ratios and diffraction efficiencies of a photorefractive liquid crystal cell were measured while changing the injection temperature and the layer thickness of the liquid crystal in a layer-structured fabrication. A high beam coupling ratio and first-order Raman–Nath diffraction efficiency were observed at low injection temperature and small cell gap, which showed high order parameters. However, as the cell gap was reduced, the shear stress at the interfaces between the liquid crystals and the photoconducting layers increased, so that the reorientation of the liquid crystals by an internal field could be restricted by the geometry and interrupted by a highly-induced surface energy, reducing photorefractive effects. An optimum exists in the neighborhood of 7.5 m when LCs are injected at 25 °C under our experimental conditions . PACS 42.65.Hw; 42.70.Df     

表面垂直取向对HPDLC光栅特性的影响

为了提高聚合物/液晶（HPDLC）光栅的衍射效率并改善光栅的表面形貌,研究了表面垂直取向处理对HPDLC光栅的影响。首先,研究了表面垂直处理对液晶分子的取向作用,发现垂直取向层对液晶的锚定作用随着盒厚的增加而逐渐减弱,取向层的作用范围大概在3 m ~5 m之间;其次,对相分离程度进行了实验表征,结果表明,随着液晶盒厚度的增加,相分离开始的时间越来越快,并且分离程度也越来越彻底。最后,讨论了表面垂直取向对HPDLC光栅衍射效率的影响,随着盒厚的增加,相分离出来的液晶微滴形成连续的区域,光栅的衍射效率逐渐升高,当盒厚增加到一定程度,其衍射效率和无取向处理的光栅接近。当盒厚过大时,垂直取向处理对HPDLC光栅散射损失并没有太大的改善,只有当盒厚适中（12 m）时,光栅的衍射效率最高,散射损失最小。     

Neutron backscattering on vibrating silicon crystals-experimental results on the neutron backscattering spectrometer IN10

Neutron backscattering on vibrating silicon crystals has been measured in transmission geometry on the backscattering spectrometer IN10 at the Institute Laue-Langevin (ILL). The crystals were excited into longitudinal mechanical resonance vibrations by LiNbO₃ transducers glued to the crystal surface. The transmitted intensity of the 111 Bragg reflection was measured as a function of the deformation amplitude at vibration frequencies from 2.5 MHz to 30 MHz. By vibrating the initially perfect silicon crystal, an increase in E/E of the diffracted profile was achieved, accompanied by an increase of the backreflected intensity. The width of the reflection and the resolution is a linear function of the ultrasonic strain over a wide range of excitation strengths. E/E can be easily tuned in a reproducible way by varying the voltage on the piezoelectric transducer. For the highest excited ultrasonic strain an increase of the backscattered intensity of about a factor of 50 is observed. Sound wave amplitudes can be determined from the measured width in E/E of the transmission curves. The obtained results are relevant for the design and operation of intensity-to-resolution tunable monochromators for high resolution instruments.     

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Influence of surface disorder on RHEED patterns from GaAs(001)−2 × 4 surfaces

Reflection high-energy electron diffraction measurements have been carried out for a nearly perfect and a disordered GaAs(001)−2 × 4 surface in the [ 10] azimuth. The surfaces were prepared in-situ by molecular beam epitaxy and the measurements were made at a substrate temperature of 560 °C under different flux conditions, giving the different degrees of surface ordering. Diffracted intensity distributions along various reciprocal surface lattice rods were measured for different angles of incidence using a primary beam energy of 15 keV. Drastic differences with surface order were found for both the (quasi) elastically diffracted beams (rocking curves) and the diffuse beams. The rocking curves can be interpreted largely by Bragg reflection from the bulk crystal and a number of causes for the intensity changes are given. In the case of the disordered surface the intensity distributions along the reciprocal lattice rods are dominated by broad diffuse peaks which occur at the intersection of reciprocal rods and Kikuchi lines. It is shown that while a kinematical model is unable to explain the experimental observations, a previously published model provides a very satisfactory interpretation.     

Second-harmonic generation from metal surfaces: Beyond jellium

The generation of Second-Harmonic (SH) radiation from simple metals has been the subject of numerous investigations for almost three decades. The use of clean metal surfaces in the experiments and increasingly sophisticated jellium-based theoretical models has led to a convergence on the magnitude and some of the dispersion characteristics of SH generation in free-electron metals like aluminum. However, the sensitivity of the SH-generation process to crystal symmetry, surface morphology, and surface electronic states indicates that nonlinear optical properties are influenced by processes which cannot be explained using a model based on an isotropic electron gas. Here, we review some recent experimental work indicating where deviation from jellium-based models occurs and attempts to understand these.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Adsorption of firefly luciferase at interfaces studied by total internal reflection fluorescence spectroscopy

The adsorption of luciferase onto silica surfaces was studied by total internal reflection fluorescence (TIRF) spectroscopy. Two model surfaces were used: hydrophilic and hydrophobic silica. Luciferase adsorbed differently on these two surfaces. Initial kinetics of luciferase adsorption onto the hydrophilic surface showed that luciferase adsorbs over an adsorption energy barrier of 3 kT The quantum yield of luciferase fluorescence decreased at the hydrophilic silica surface, which indicated that the protein conformation was altered during adsorption. Luciferase adsorption onto the hydrophobic silica surface proceeded with a small adsorption energy barrier and the fluorescence efficiency of adsorbed protein remained unchanged after adsorption. The affinity of luciferase for luciferin was measured using quenching of luciferase fluorescence with luciferin. The binding constant of the adsorbed luciferase-luciferin complex at the hydrophilic silica surface was two orders of magnitude smaller than the respective binding constant in the solution. Adsorbed luciferase showed an absence of ATP-dependent visible luminescence, indicating that the adsorbed enzyme was not active at either of the two silica surfaces.     

Eikonal approximation of dynamical diffraction equations for X-ray beams with two-dimensional curvature of the wave front. I. Basic formulas

Taking into account the second derivatives of amplitudes with respect to the coordinate perpendicular to the diffraction plane, the eikonal approximation of dynamical diffraction equations for X-ray beams with two-dimensional curvature is presented. The diffracted field outside the crystal, in vacuum is described by the corresponding eikonal approximation of the parabolic equation of diffraction. The corresponding eikonal equations and complete integrals are obtained in case of a perfect crystal. A method is described, by which the eikonals and trajectories inside the crystal as well as in vacuum, outside the crystal, satisfying the given boundary conditions, can be obtained. This method allows to describe diffraction using sufficiently general assumptions on the incident wave front and for the non-plane entrance and exit surfaces of the crystal.     

Radiative absorption in an infinitely long hollow cylinder with Fresnel surfaces

Radiation absorption in an infinitely long hollow cylinder with Fresnel surfaces is studied using the ray tracing method. It is found that the inner boundary can be modeled as a total reflective surface for the infinitely long hollow cylinder. Radiative absorption of hollow cylinders with Fresnel surfaces is compared to diffusive surfaces predicted by the finite volume method. Effects of refractive index, optical thickness and hole size on radiative absorption are studied. Abrupt changes in radiative absorption near τ_r/τ_Ro=1/n are observed for hollow cylinders with Fresnel surfaces. It is because the Fresnel relation predicts a critical angle at . This trend is not observed in diffusive surfaces. Refractive index and optical thickness are two competing factors that govern the radiative absorption. Higher refractive index drives higher absorption close to the inner surface, while higher optical thickness yields higher absorption near the outer surface. The results of this study can also serve as benchmark solutions for modeling radiative heat transfer in hollow cylinders with Fresnel surfaces. It is also found that the directional or hemispherical emittance can be calculated without solving the radiative transfer equation in the media when the temperature variation in the media is small.     

Molecular dynamics study on surface structure and surface energy of rutile TiO2 (1 1 0)

The formula for surface energy was modified in accordance with the slab model of molecular dynamics (MDs) simulations, and MD simulations were performed to investigate the relaxed structure and surface energy of perfect and pit rutile TiO₂(1 1 0). Simulation results indicate that the slab with a surface more than four layers away from the fixed layer expresses well the surface characteristics of rutile TiO₂ (1 1 0) surface; and the surface energy of perfect rutile TiO₂ (1 1 0) surface converges to 1.801±0.001 J m⁻². The study on perfect and pit slab models proves the effectiveness of the modified formula for surface energy. Moreover, the surface energy of pit surface is higher than that of perfect surface and exhibits an upper-concave parabolic increase and a step-like increase with increasing the number of units deleted along [0 0 1] and [1 1 0], respectively. Therefore, in order to obtain a higher surface energy, the direction along which atoms are cut out should be chosen in accordance with the pit sizes: [] direction for a small pit size and [0 0 1] direction for a big pit size; or alternatively the odd units of atoms along [1 1 0] direction are removed.     

Field and temperature dependence of magnetization of U3P4

Conclusions We conclude that the occurrence of a maximum in the magnetization versus temperature curve observed under certain circumstances on U₃P₄ is not connected with its intrinsic magnetic structure, but substantially depends on the form in which the material is studied (polycrystalline powder, single crystal, single crystal powders with various dimensions of grains). This phenomenon seems to be affected by quenching of the state from the region of low anisotropy at higher temperature in the small particles with a strong tendency to single domain behaviour and thus it may be a consequence of rotation magnetization process in the fields relatively small for U₃P₄. The assertion about transitions between magnetic ordering, sometimes proposed in the literature on the basis of observed anomalies in the temperature course of magnetization without simultaneous confirmation by neutron diffraction, should be accepted with some reservation, especially in the case of simple uranium compounds. However, another situation arises for pseudobinary alloys of ferro- and antiferromagnetic components (e.g. UP_0·75S_0·25) where such transitions were confirmed both by magnetic methods and by neutron diffraction studies [13].The authors would like to express their thanks to Ing. A.Meovský for careful preparation of samples.