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Diffuse X-ray scattering has been an important tool for understanding the atomic structure of binary systems for more than 50 years. The majority of studies have used laboratory-based sources providing 8 keV photons or synchrotron radiation with similar energies. Diffuse scattering is weak, with the scattering volume determined by the X-ray absorption length. In the case of 8 keV photons, this is not significantly different from the typical extinction length for Bragg scattering. If, however, one goes to energies of the order of 100 keV the scattering volume for the diffuse scattering increases up to three orders of magnitude while the extinction length increases by only one order of magnitude. This leads to a gain of two orders of magnitude in the relative intensity of the diffuse scattering compared with the Bragg peaks. This gain, combined with the possibility of recording the intensity from an entire plane in reciprocal space using a two-dimensional X-ray detector, permits time-resolved diffuse scattering studies in many systems. On the other hand, diffraction features that are usually neglected, such as multiple scattering, come into play. Four types of multiple scattering phenomena are discussed, and the manner in which they appear in high-energy diffraction experiments is considered.

The diffuse scattering of body-centered cubic β-Ti–V was measured using high-energy synchrotron X-rays and two-dimensional detectors. The study included in situ experiments of the equilibrium β-phase and room temperature measurements of the quenched metastable state. The kinematical X-ray scattering revealed details in reciprocal space that could not be detected by the electron diffraction employed in previous studies. The signal was analyzed using a statistical thermodynamic approach based on physically motivated parameters. The characteristic features attributed to an ω-like structure or a “diffuse ω-phase” in the past are explained by static lattice distortions due to atomic size mismatch.

Diindenoperylene (DIP) thin films deposited on SiO 2 -surfaces by organic molecular beam deposition in ultra-high vacuum at room temperature have been investigated by atomic-force-microscopy and X-ray scattering as a function of film thickness. We observe the evolution of two distinct molecular orientations; the long axis of the DIP molecule is pointing either along the surface normal (σ-orientation) or along the surface ( λ -orientation). Initially, the films exhibit predominantly σ-orientated islands. Increasing the film thickness, elongated, fibrous λ -orientated islands nucleate on top of the σ-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 °C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the σ-orientation) and growth kinetics (favoring the λ -orientation).

We present a real-time X-ray scattering study of the growth modes in organic molecular-beam epitaxy. We have studied the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Ag(111) and find a temperature-dependent transition from layer-by-layer growth to islanding. The transition smears out for low substrate temperatures, T, implying that the degree of the layer-by-layer growth of the wetting layer decreases with decreasing T. This behavior has been analyzed quantitatively and reproduced by kinetic Monte Carlo simulations. The implications and consequences of our findings for the understanding of the organic molecular-beam epitaxy are discussed.

The recently proposed spin-echo resolved grazing-incidence scattering (SERGIS) uses the well-known neutron spin echo effect for encoding the momentum transfer in reflectometry. By the application of tilted magnetic-field borders, SERGIS measures the scattering angle in grazing incidence experiments in absence of any geometrical beam-defining tool, such as slits. The main difficulty in such set-ups is the realization of geometrically flat field borders. The possibility of the application of neutron resonance spin echo (NRSE) for such a purpose is discussed, where the field borders are defined by current sheets. Prototype SERGIS experiments performed on holographically made optical gratings at a NRSE triple-axis spectrometer are shown.

The analysis of X-ray standing wave (XSW) data for the determination of structural parameters of adsorbates (coherent position, P C h , and coherent fraction, F C h ) is usually done in the dipole approximation. Using carbon and oxygen containing molecular adsorbates on Ag(1 1 1), we show how the non-dipolar contributions can be quantified and that they can have a strong impact on the resulting P C h and F C h . The results are particularly important for XPS detection of XSW intensities from light elements, for which fluorescence detection does not work. Moreover, we discuss how the XSW technique allows determining both the amplitude and the phase from quadrupole contributions in photoelectric scattering.

A surface-sensitive X-ray line shape analysis of the ordering process in Cu3Au(001) has been performed following a rapid temperature quench from the disordered phase into the ordered phase. We study the temporal evolution of ordered domains in the subsurface region. We find distinct and anisotropic antiphase domain patterns in the unstable and metastable growth mode. This leads to rather unusual surface-induced coarsening schemes.

Properties of the free ferrofluid (colloid of fine magnetite particles suspended in heavy water) surface and interface silicon–ferrofluid is studied by means of the neutron reflectivity and off-specular scattering. The measurements were performed in zero magnetic field, as well as in the presence of the field, applied within the surface (interface) plane, either perpendicular or parallel to the incoming beam. Proposed theoretical model allows deriving quantitative information about ferrofluid structure from the data of the interface measurements while consideration of the free surface data requires some additional contemplation. The experimental results collected, mainly in maps initial angle – scattered angle, were treated by means of two-dimensional fit (the least-squares routine) and values of the ferrofluid parameters such as correlation radius, critical angle, scattering length density were obtained in dependence on value and direction of external magnetic field. The capability of polarized neutron reflection experiment for the investigation of the magnetization process in the vicinity of the ferrofluid free surface (interface) is discussed.

In this article we discuss surface and subsurface thermodynamic phenomena as they emerge when the bulk of the system undergoes a discontinuous order–disorder phase transition. We have chosen Cu 3 Au(001) as a model system and demonstrate that pronounced order, disorder and composition profiles are found at the Cu 3 Au(001) surface. Surface-sensitive X-ray scattering techniques that exploit the high brilliance of modern synchrotron radiation sources are ideally suited to unravel this novel thermodynamics between two and three dimensions.

Near surface properties of ferrofluid (FF) are studied by means of the neutron reflectivity and off-specular scattering measured in zero magnetic field, and in a field applied within the surface plane, both perpendicular and parallel to the incoming beam direction. It is found that low magnetic fields substantially perturb the free FF surface which results in dramatic changes in specular reflection and the off-specular scattering. This phenomena may be attributed to the field-induced anisotropy in the particle density correlation function. The anisotropy is found to be much less pronounced at the interface between FF and the silicon wafer.

An X-ray study of a crystalline organic–organic heterostructure, consisting of a film of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on a self-assembled monolayer (SAM) of decanethiol on Au(1 1 1), is presented. The sandwich structure acts as a “molecular interferometer”, where the interference between the Laue function of the PTCDA film and the crystal truncation rod (CTR) of the substrate is governed by the SAM spacer thickness, d 0 . A pronounced destructive interference feature is observed, which allows the determination of d 0 with great sensitivity.

We present a novel surface phenomenon found in the investigation of the ordering kinetics near the surface of Cu{sub 3}Au(001). Monitoring different classes of Fourier components of the ordered L1{sub 2} structure by time-resolved x-ray scattering, we found a giant anisotropy in the relaxation behavior. The layered structure normal to the surface relaxes ultrafast while lateral ordering occurs much more slowly ({tau}{sup -1}{sub {perpendicular}}{gt}20{tau}{sup -1}{sub {parallel}}). We argue that the recently discovered oscillating segregation profile acts as a two-dimensional nucleating sheet for ordering wave vectors normal to the surface. {copyright} {ital 1997} {ital The American Physical Society}

In this work we report X-ray scattering studies of the Cu 3 Au(001) surface for temperatures above the bulk order-disorder transition temperature T 0 =390°C. The temperature dependence of the asymptotic Bragg tails normal to the surface gives direct evidence for an oscillatory, exponentially damped Au segregation profile. It turns out that the depth range (λ) of this surface segregation phenomenon is governed by the bulk correlation length ξ b and, thus, depends sensitively upon the temperature, in particular, we find that λ ∝( T — T sp ) − v upon approaching the ordering spinodal temperature T sp with v =0.49. The inplan of the asymptotic Bragg tails exhibits a broadening which changes characteristically with the normal momentum transfer L An intimate coupling between the terrace structure and the lateral coherence of the surface segregation profiles is found. First experimental results from experiments performed at the ESRF are presented which disclose surface segregation-related diffraction features in higher Brillouin zones normal to the surface. The sensitivity of this X-ray probe to details of the surface segregation profile is discussed and the experimental limits are indicated.

The onset of surface melting at [lcub]00.1[rcub] (“basal”), [lcub]10.0[rcub] and [lcub]11.0[rcub] (“nonbasal”) surfaces of hexagonal ice is studied experimentally by glancing-angle X-ray scattering. The preparation of mirror-like single crystal ice surfaces and a special in situ X-ray chamber is described in detail. The X-ray scattering experiments using synchrotron radiation give clear evidence that all the investigated high-symmetry surfaces exhibit surface melting with onset temperatures T s ≅ − 13.5°C for the basal and T s ≅ − 12.5°C for the nonbasal surfaces. The temperature dependence of the thickness of the quasiliquid is presented and discussed in view of various theoretical predictions. In addition we found thermal faceting at nonbasal surfaces as well as a diffuse (“rough”) solid-quasiliquid interface. The thermal properties of the observed surface disorder implies that the correlation length within the socalled “surface melt” is much larger than the correlation length in the bulk liquid indicating the presence of significant “substrate” interactions.

We demonstrate that crystal truncation rod scattering can ideally be applied to extract detailed information on surface segregation profiles in multicomponent systems. We present an x-ray study of the surface segregation at C${\mathrm{u}}_{3}$Au(001) at various temperatures above the order-disorder transition temperature. It exhibits a pronounced exponentially decaying, oscillatory behavior versus depth. The associated decay length $\ensuremath{\Lambda}$ is found to obey $\ensuremath{\Lambda}\ensuremath{\sim}{t}^{\ensuremath{-}\ensuremath{\nu}}$, with $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.49\ifmmode\pm\else\textpm\fi{}0.04$ and $t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}({T\ensuremath{-}T}_{\mathrm{SP}})/T$, and, thus, is related to the bulk order parameter fluctuations and the bulk spinodal temperature ${T}_{\mathrm{SP}}$.

The characterization of ice 1 h single crystal surfaces has been performed by means of X-ray scattering under grazing incidence conditions using synchrotron radiation. We report exit-angle profiles of evanescent Bragg reflections as observed under various angles of glancing incidence, in-plane rocking curves and specular reflection profiles on prism [h0.0] and [hh.0] vicinal surfaces. The measured intensities are compared with models in the framework of the distorted wave Born approximation.

Neutron reflectometry and off-specular scattering is used for studying the ordering in colloidal solutions of ferromagnetic nanoparticles in the vicinity of a flat monocrystalline silicon wafer. The experiments reveal layering whose morphology and kinetics depend strongly on the colloidal parameters and the applied magnetic field.

We report extraordinary structural order along the surface normal in thin films of the organic semiconductor diindenoperylene (DIP) deposited on silicon–dioxide surfaces. Cross-sectional transmission electron microscopy (TEM), noncontact atomic force microscopy (NC–AFM), as well as specular and diffuse x-ray scattering measurements were performed to characterize thin films of DIP. Individual monolayers of essentially upright-standing DIP molecules could be observed in the TEM images indicative of high structural order. NC–AFM images showed large terraces with monomolecular steps of ≈16.5 A height. Specular DIP Bragg reflections up to high order with Laue oscillations confirmed the high structural order. A semi-kinematic fit to the data allowed a precise determination of the oscillatory DIP electron density ρel.,DIP(z). The mosaicity of the DIP thin films was obtained to be smaller than 0.01°.

An understanding of the structure of ultrathin polymer films on solid substrates has scientific importance in applications as well as in fundamental studies of polymer diffusion or adsorption. We present studies of the organization of dewetted droplets of polymers on a silicon surface using a new neutron scattering technique, spin-echo resolved grazing incidence scattering (SERGIS), that has the potential to address at the same time the droplet-droplet correlations and the chemical configuration inside each droplet. For the seminal experiments, the polarized neutron reflectometer EVA at the Institut Laue-Langevin, Grenoble, France, was equipped with a spin-echo setup, and measurements were taken on surface structures previously characterized by different techniques. The dewetted polymers used in our studies were pure polystyrene, a mixture of polystyrene and polyparamethylstyrene, and a diblock copolymer of the two homopolymers. Even with a provisional setup SERGIS, we were able to determine the correlation between the droplets, providing results in excellent agreement with those obtained by atomic force microscopy and grazing incidence small-angle X-ray and neutron scattering. In addition, it was confirmed that the correlation function for diblock copolymer droplets is more complex than for polymer mixtures, exhibiting partial ordering of the copolymer within each droplet.

In the application of neutron scattering to problems related with surface and interfacial magnetism three different experimental techniques may be envisaged: reflectivity measurements, glancing angle neutron scattering and truncation rod scattering. While neutron reflectivity measurements have become a routine experimental tool nowadays, the two other techniques are virtually unexplored. In this article I focus onto glancing angle neutron Bragg scattering and discuss some present attempts and applications, future prospects and the limitations of this new technique in the field of surface-related magnetism. A special emphasis is thereby put on the performance of the evanescent neutron wave diffractometer EVA at the Institut Laue-Langevin in Grenoble.

We apply the Krivoglaz theory of x-ray scattering to thin epitaxial films containing misfit dislocations and reanalyze the seemingly puzzling x-ray scattering phenomena observed in several heteroepitaxial films. We show that the two-line shape scattering distribution and its dependence upon film thickness and momentum transfer can be understood in natural way and on a quantitative level. Extended diffuse x-ray scattering maps have been obtained from Nb(110)/Al2O3(1120) which are discussed within the framework of this theory disclose a particular dislocation network at the Nb–Al2O3 interface.

We describe the details of a glancing angle X-ray scattering study of the critical behaviour of the order parameter and the order parameter fluctuations at the Fe 3 Al(110) surface when the alloy approaches the bulk critical temperature. The temperature dependence of the X-ray scattering at and around the (111) superlattice reflection has been measured in a surface sensitive scattering geometry and analyzed in view of the predictions of modern renormalization theories which account for the presence of a surface. It is found that the equilibrium surface displays two different regimes: the top three layers are strongly affected by surface segregation phenomena and display a complicated thermodynamic behaviour, the observed critical phenomena in the subsequent layers are governed by the new surface exponents β 1 = 0.75 ± 0.06, γ 11 = −0.33 ± 0.06, η ∥ = 1.52 ± 0.04 and ν = 0.60± 0.15. It is demonstrated by this study that on the one hand the presence of the surface alters the character of the order-disord transition near the surface in a significant manner, and that on the other hand the observed surface critical scattering and, consequently, the surface free energy and the surface-dominated pair correlation function still show scaling behaviour. In particular the scaling laws 2 β 1 = ν (1 + η ∥ ) and γ 11 = ν (1 − η ∥ ) are confirmed by the experiment.

We reconsider the mobility of hydrogen in bcc metals, in particular H and D in Nb, and its influence on the coherent and incoherent quasielastic neutron and x-ray scattering. An approach to describe time-dependent lattice distortions around highly mobile protons is presented, using the frequency-dependent Green's function of the host lattice so that effects of the so-called quasielastic effects and resonances with host lattice phonons are included. We discuss within this formalism how noncubic components of the long-ranged displacement field can fade away in the presence of a high defect mobility and we demonstrate by a detailed calculation that the experimentally observed disappearance of the coherent quasielastic diffuse (x-ray and neutron) scattering in ${\mathrm{NbH}}_{\mathit{x}}$ (${\mathrm{NbD}}_{\mathit{x}}$) from these Fourier components of the displacement field points to an unusual, hitherto neglected rapid local diffusion process between adjacent tetrahedral sites with a short-time constant of ${\mathrm{\ensuremath{\tau}}}_{\mathrm{loc}}$\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}13}$--${10}^{\mathrm{\ensuremath{-}}14}$ s. The consequences of such a diffusion mechanism on the incoherent quasielastic neutron scattering are discussed in detail. We show that the elastic incoherent structure factor of the interstitial proton exhibits, in addition to the usual Debye-Waller factor behavior, distinct oscillations as a function of the neutron momentum transfer.A neutron-scattering study of ${\mathrm{NbH}}_{0.02}$ at various temperatures between room temperature and 573 K is presented. The experimental results of the elastic incoherent structure factor displays, after a proper correction for the proton Debye-Waller factor, a pronounced oscillatory behavior in the [100] and [111] direction as predicted for a rapid local diffusion on neighboring tetrahedral sites. It is shown that this property of the proton form factor is temperature dependent. We present measurements of the quasielastic broadening of the coherent diffuse neutron scattering from the time-dependent long-ranged displacement field (``Huang linewidth'') and discuss further implications of our findings on the interpretation of structural and dynamical properties of H in Nb, as the local modes and the coherent diffuse neutron scattering from the lattice distortions around the local defect neighborhood.

The Fe3Al alloy undergoes a bulk order-disorder transition at a critical temperature of 780 K. The question of possible surface segregation in this temperature range is of interest with respect to investigations of the critical behavior at the surface. Therefore the (110) surface of a 71at%Fe–29at%Al crystal was investigated with low-energy ion scattering. Segregation of Al to the surface was found well below the transition temperature, at 700 K the equilibrium coverage is ∼ 94% Al in the first layer and 100% at higher temperatures. In the second atomic layer an Al concentration below the bulk values is found. From the time dependence of the segregation the activation energy for Al diffusion can be estimated. Above 700 K a rearrangement of the Al atoms is observed, indicating that the distance between Al atoms in the top layer is about twice the bulk value. The segregation behavior does not show extraodinary changes when crossing the bulk transition temperature.

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.

We present an experimental study of the surface effects on the order-disorder phase transition in the binary alloy ${\mathrm{Cu}}_{3}$Au. By x-ray scattering under the condition of total external reflection, we obtained depth profiles of the order parameter at the (001) surface when the transition temperature ${\mathit{T}}_{0}$ is approached. The scattered intensity is analyzed in the framework of the distorted-wave Born approximation, which is briefly outlined. The temperature and depth dependence of the evanescent (100) superlattice intensity is consistent with a wetting transition, which is driven by the first-order bulk transition: Close to ${\mathit{T}}_{0}$ the disordered phase appears at the (001) surface and grows according to L(t)=(13.1 \AA{})ln\ensuremath{\Vert}1/t\ensuremath{\Vert} with reduced temperature t. During these measurements the near-surface Debye-Waller factor was monitored via the (200) evanescent Bragg intensity, which, however, showed no significant surface effects at ${\mathit{T}}_{0}$. In several temperature quench experiments, where the system was rapidly cooled from Tg${\mathit{T}}_{0}$ to ${\mathit{T}}_{0}$-\ensuremath{\Delta}T, we measured time- and depth-resolved evanescent superlattice intensities. We present depth-resolved near-surface relaxation times that exhibit a very distinct depth dependence. A first quantitative analysis of the relaxation processes is indicated and gives results consistent with the assumption of an ordinary ``surface-induced disorder.''

The surface sensitivity was achieved by exploiting the effect of total reflection. The radial distribution of the critical diffuse intensity around a superlattice reflection exhibits a cusplike shape at the critical temperature T c . From this distribution we determined for the first time the critical surface exponent ηη=1.52±0.04 which agrees well with the prediction for the ordinary transition of a semi-infinite Ising system

We present an extended synchrotron x-ray scattering study of the structure of thin manganite films grown on SrTiO3(001) substrates and reveal a new kind of misfit strain relaxation process which exploits twinning to adjust lattice mismatch. We show that this relaxation mechanism emerges in thin films as one-dimensional twinning waves which freeze out into a twin domain pattern as the manganite film continues to grow. A quantitative microscopic model which uses a matrix formalism is able to reproduce all x-ray features and provides a detailed insight into this novel relaxation mechanism. We further demonstrate how this twin angle pattern affects the transport properties in these functional films.

We have performed bulk and surface-sensitive inelastic x-ray scattering experiments on liquid indium with 3 meV energy resolution. The experimental data are well reproduced within a generalized hydrodynamic model including structural and microscopic relaxation processes. We find a longitudinal viscosity of 22 mPa s in the near-surface region compared to 7.4 mPa s in the bulk. The origin of the increase is associated with a slowing down of the collective dynamics in a subsurface region of 4.6 nm.

Exploiting the high brilliance of synchrotron radiation, we performed surface-sensitive and depth-resolved x-ray scattering experiments on thin films of boron nitride grown on Si(001) substrates. In-plane strains of different structural phases, namely turbostratic and cubic, grain sizes and textures were determined. Annealing the films up to temperatures of 1000 °C leads to large strain relaxation of about 70%, while the grain size stays constant at 80 A.

We report a new type of short-range order correlations at the (001) surface of Cu 3 Au which no longer produces the 2k F -splitting characteristic for the bulk short-range order scattering. We present the temperature dependence of this phenomenon and a theoretical interpretation of its origin. We argue that this new surface effect is caused by a drastic change of the strain-induced interactions at the surface.

The conceptual design of a new instrument is presented, which makes use of the spin echo technique to analyze the scattering angle of neutrons impinging upon a rough or corrugated surface at grazing incidence. In a grazing incidence geometry the roughness of the surface and of submersed interfaces give rise to a neutron scattering pattern at angles well resolved from that of specular reflection for corrugation lengths up to several microns, but only in the plane of specular reflection. In contrast, scattering caused by corrugations of comparable length perpendicular to the reflection plane is limited to angles that can be separated only with an extreme tightening of the instrumental resolution. However this scattering can be well resolved by spin-echo methods for polarized neutrons. Results from scattering in and out of the reflection plane provide complementary information on the structure of the roughness and its location in a system with complex layering. This spin-echo technique may also distinguish static from time-dependent roughness.

The scaling exponents alpha, beta, and 1/z in thin films of the organic molecule diindenoperylene deposited on SiO2 under UHV conditions are determined. Atomic-force microscopy, x-ray reflectivity, and diffuse x-ray scattering were employed. The surface width displays power law scaling over more than 2 orders of magnitude in film thickness. We obtained alpha = 0.684+/-0.06, beta = 0.748+/-0.05, and 1/zeta = 0.92+/-0.20. The derived exponents point to an unusually rapid growth of vertical roughness and lateral correlations. We suggest that they could be related to lateral inhomogeneities arising from the formation of grain boundaries between tilt domains in the early stages of growth.

We present a theoretical and experimental study of the effect of lattice distortions on the short-range order and the energetics of ordering binary alloys. Applying a reciprocal space approach which accounts for the elastic response of the lattice, the diffuse scattering of the model system Cu(83)Mn(17) can be explained with only a few physical parameters. The model calculations point to a nonanalytic diffuse intensity at q = 0. X-ray scattering experiments are presented providing clear evidence for this phenomenon which carries detailed information on the strain-induced interaction.

We consider the (001) surface of a 3-dimensional Cu3Au single crystal which undergoes the well-known order-disorder phase transition at T0=663K. The character of such a semi-infinite phase transition is modified in a fundamental manner near the surface. Since the atoms at the surface are less tightly bound than the bulk atoms, we expect that any long range order should slightly be reduced near the surface (“ordinary case”), thus, upon approaching T0, one would expect the disordered phase emerge at the surface prior to the actual bulk phase transition. In contrast to pure 2-dimensional surface phenomena those surface effects are not bound to the outermost layers, but instead, a disordered surface sheet of mesoscopic or even macroscopic thickness may emerge1. In addition the free surface allows surface fields enter the scenario giving rise to new phenomena which have no counterpart in the bulk, one of them being surface segregation, i.e. the enrichment of one component at the surface. Many of the experimental evidence of disordered and segregated surface sheets have been found and corroborated by x-ray scattering in surface-sensitive modes, that is glancing-angle diffraction and asymptotic Bragg scattering which are shortly discussed in the following, before the observed surface phenomena at Cu3Au(001) are presented.

We report the first exit angle resolved magnetic neutron diffraction experiment under grazing incidence. The antiferromagnetic (100) Bragg reflection was excited under grazing incidence on a (001) MnF2 single crystal surface. By a careful control of the exit angle we were able to identify unambiguously the surface scattering. We present the experimental set up and compare some results with those of a CaF2 single crystal.

We present a dedicated experimental spin-echo resolved grazing incidence scattering (SERGIS) setup for the investigation of surfaces and thin films exhibiting large lateral length scales. This technique uses the neutron spin to encode one in-plane component of the wave-vector transfer in a grazing angle scattering experiment. Instead of the scattering angle, the depolarization of the scattered beam is measured. This allows one to achieve a very high in-plane momentum resolution without collimation of the incident neutron beam in the corresponding direction. SERGIS can therefore offer an alternative or complementary method to conventional grazing incidence neutron scattering experiments. We describe the experimental setup installed at the neutron sources ILL (Grenoble) and FRM II (Garching) and present data obtained with this setup on various samples exhibiting characteristic mesoscopic length scales in the range of several hundred nanometers. We also derive general formulas and error margins for the analysis and interpretation of SERGIS data and apply them to the cases of a one-dimensional structure and of an island morphology.

We review two different experiments in order to demonstrate the power of grazing incidence diffraction of x-rays in studying phase transitions near surfaces and to show that it is well suited to get information on structural details even from subsurface layers: We have measured the near surface critical scattering at the continuous order disorder transition of an Fe3Al single crystal which provides information on modifications of critical behavior of a bulk transition near a surface. These modifications are detectable to a considerable depth due to the diverging range of correlations. We determined three different critical surface exponents which allow, for the first time, to confirm scaling laws for near surface critical behavior.- The experiment at a (100) surface of a discontinuous ordering Cu3Au single crystal shows that the surface is wetted by a disordered layer below the transition temperature. By means of the adjustable depth sensitivity of grazing incidence scattering we were able to demonstrate that the thickness of this layer increases logarithmically when approaching the transition temperature.

The authors report a study on the growth of organic heterostructures based on di-indenoperylene (DIP) (p-type conduction) deposited on top of F16CuPc (n-type conduction) by real-time x-ray scattering and atomic force microscopy. Above a threshold temperature of 90°C, the formation of highly crystalline DIP islands takes place via Stranski-Krastanov growth. The temperature dependence of the growth behavior can be exploited to control the resulting morphologies, ranging from well ordered layered heterostructures to the self-organization of highly crystalline DIP islands with tunable size. This self-organization scenario of organic ambipolar nanostructures carries the potential for bottom-up growth of organic architectures for solar cell structures.

A portable UHV molecular beam deposition system has been developed for synthesis, in situ, and real-time x-ray diffraction measurements of organic thin films, multilayers, and superlattices. The system has been optimized for small size, while still incorporating full features necessary to achieve thin film growth under molecular beam epitaxy (MBE) conditions. It can be used independently for thin film growth, or it can be transported and mounted on standard diffractometers. Additionally, it can be docked to a stationary multipurpose MBE growth system for sample transfer, thus permitting more extensive growth and characterization. The design and performance of this system are reported, with emphasis on modifications required to deposit organic materials. To demonstrate the capabilities for real-time x-ray scattering experiments, some preliminary results of a study of epitaxial growth of 3,4,9,10-perylene-tetracarboxylic dianhydride on Ag(111) substrates are given.

The coherent, elastic-diffuse neutron scattering from a ${\mathrm{NbO}}_{0.020}$ and a ${\mathrm{NbO}}_{0.009}$ single crystal has been determined experimentally in absolute units. We demonstrate that the lattice distortions around one single oxygen impurity give rise to an intensity map which has the same qualitative features as observed in ${\mathrm{Zr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$ alloys. Model calculations for the diffuse scattering have been performed within the framework of Kanzaki forces. It is shown that a so-called ``3f '' force field with one free parameter is able to reproduce the absolute values of the entire observed scattering pattern. The complementary interpretation of the experimental data as short-range-order scattering demands defect-induced \ensuremath{\omega}-phase embryos which have a volume of approximately ten unit cells. A real-space interrelation between the Kanzaki force model and the picture of defect-induced, local domains of \ensuremath{\omega} phase could be achieved by demonstrating that the local, defect-induced lattice distortions which are associated with the proposed Kanzaki force field contain static fluctuations of \ensuremath{\omega} phase. It is shown that the attraction of the second-nearest-neighbor Nb shell around the oxygen atom is directly correlated to the structure of the locally condensed phase.

The quasistatic local lattice distortions around individual interstitial deuterium defects in niobium have been investigated by means of coherent quasielastic neutron scattering. The important experimental issues of this study are discussed in detail. We compare the observed absolute scattering cross section with various model calculations in which many different defect locations and the associated Kanzaki forces are tested. The results show that the heretofore anticipated defect models are insufficient to describe the local defect environment. A new model which takes into account the occurrence of diffusion-induced nonequilibrium distortions is able to describe virtually all of our experimental findings. By this analysis we confirm on one hand that D in Nb is preferentially located at tetrahedral sites. We demonstrate on the other hand that a partial triangular site occupancy has to be taken into account, which is associated with a perturbed flight phase of the highly mobile defect. The evidence in our study of local nonequilibrium distortions sheds new light on the cubic symmetry of the force dipole moment and on other distortion-mediated defect properties.

The Bragg scattering excited under the conditions of total external reflection is discussed within a kinematic approach based on the distorted-wave approximation. Due to evanescent absorption at grazing angles the maximum of the interference function of the surface Bragg reflection is displaced into the vacuum by an amount determined by the mean electron density of the medium. X-ray scattering experiments performed on a mosaic single crystal with synchrotron radiation confirm the predictions of the kinematic theory and indicate the potential for obtaining structural information on the first atomic layers.

Diffuse neutron or x-ray scattering close to the Bragg peaks (Huang scattering) and far away from Bragg peaks (Zwischenreflex-scattering) supplies detailed information on point defects and small agglomerates in crystals [1]. The defect structure, i.e. the lattice location, the displacements of the near neighbours and the strength and symmetry of the long ranged displacement field can be determined. In order to demonstrate the power of this technique, we report on recent experimental results. Interstitially dissolved N and O in Nb are located on octahedral sites and create rather large displacements in their vicinity. Their long ranged displacement field shows the symmetry of the defect site. The light interstitials H and D in Nb are located on tetrahedral sites and their long ranged displacement field shows the symmetry of the host lattice. A rather complicated defect model is necessary in order to explain the local defect structure.

Previous neutron scattering studies on nominally pure Nb (Chang et al, 1977, Werner et al) showed unusual diffuse scattering peaks at Q = (4/3 4/3 4/3)2π/a with varying intensities from sample to sample.

Diffuse neutron or X-ray scattering close to the Bragg peaks (Huang scattering) and far away from Bragg peaks (Zwlschenreflex-scatterlng) supplies detailed information on point defects and small agglomerates in crystals [1]. The defect structure, i.e. the lattice location, the displacements of the near neighbours and the strength and symmetry of the long ranged displacement field can be determined. In order to demonstrate the power of this technique, we report on recent experimental results. Interstlllally dissolved N and O in Nb are located on octahedral sites and create rather large displacements in their vicinity. Their long ranged displacement field shows the symmetry of the defect site. The light Interstltlals H and D in Nb are located on tetrahedral sites and their long ranged displacement field shows the symmetry of the host lattice. A rather complicated defect model is necessary in order to explain the local defect structure.

For O in Nb the experimental scattering distribution can be described by a so-called 3-force model. The scattering pattern exhibits close similarity to the one observed for the ω-phase in some bcc alloys. It is shown that the lattice distortion calculated from the proposed 3-force model contains static fluctuation of the ω-phase. The ω-phase is closely related to the minimum in the LA 2/3 (111) phonon dispersion branch. With decreasing temperature the frequency of this phonon decreases, whereas the elastic diffuse scattering at the same position increases. This “softening” of the lattice, together with an increasing “central peak”, gives an equivalent description of the situation.