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Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.

The multilayer relaxation of Pb(110) was investigated by LEED. Data were taken at 130 and 300 K using a computer controlled video based technique. They were analyzed by full dynamical calculations. A quantitative comparison between experiment and theory via the Pendry R -factor confirms the strong contraction of the first layer spacing found in earlier work, as well as the oscillatory relaxation of deeper spacings. The results for the relaxations of the first three layer spacings are −17.1 ± 2.9%, + 3.4 ± 2.9%, −6.8 ± 4.6% for 130 K and −19.4 ± 2.9%, + 4.6 ± 4.6%, -6.8 ± 2.9% for 300 K. This means that within the limits of error there is no relaxational change between 130 and 300 K in agreement with earlier ion scattering results. The agreement between experimental and computed spectra is only mediocre though a number of attempts were made to improve the fit.

The multilayer relaxation of the Rh(311) surface was investigated by means of LEED structure determination both for vertical and surface parallel (registry) relaxations. Excellent agreement between experimental and calculated spectra could be achieved mirrored by a minimum Pendry R-factor R = 0.174. The first three layer spacings are oscillatorily relaxed by Δd12d0 = −14.5 ± 1.8%, Δd23d0 = +4.9 ±2.0% and Δd34d0 = −1.0 ±2.0%. There seems to be a coherent registry shift of the fir Δs = 0.03 ± 0.07 A which, however, is within the error limits of the structure determination. Moreover, an energy dependent inner potential is detected. The results are discussed in comparison to equivalent surfaces of other materials as well as for the less open surfaces of rhodium.

The adsorption of hydrogen on the Fe(211) surface was investigated by LEED and thermal desorption mass spectroscopy (TDS) in the temperature range 40–300 K. For T ⩽ 200 K commensurate phases c(2 × 6)-8H, (2 × 1)p1g1-2H, c(2 × 6)-16H, 1 × 2–3H, 1 × 1–2H develop corresponding to coverages θH of 2 3 , 1, 4 3 , 3 2 and 2 . However, these phases are only metastable T > 200 K transform irreversibly into a reconstructed and streaked 1 × 2 phase (“1 × 2S”), which is only removed by complete desorption of hydrogen from the surface. The formation of the 1 × 2S-phase is a thermally activated process which is also controlled by the substrate hydrogen coverage. For coverages below θH = 1 heating to T > 200 K makes the surface start to reconstruct into the 1 × 2S phase prior to any hydrogen removal. For θH > 1 the 1 × 2S phase develops under partial desorption of hydrogen. The two reaction channels are mirrored by the binding states α and β appearing in the TD spectra associated with hydrogen desorbing from unreconstructed (α) and reconstructed (β) areas, respectively. For both the commensurate and reconstructed phases we propose real space models and compare our findings with recent data obtained for the similar adsorption systems H/Co(1010), H/Ni(110) and H/Pd(110).

With adsorption of sulfur on Ni(100) a streaky p(2 × 2) LEED pattern appears at coverage θ = 0.25. In spite of the corresponding existence of some disorder in the adlayer a LEED structure analysis of satisfying quality could be performed (Pendry R-factor R = 0.21). Sulfur adsorbs in the hollow site with an adsorption height of 1.25 ± 0.03 A. The topmost substrate layer spacing is changed from 1% contraction as derived for the clean surface to 0.5 ± 1% expansion. The nickel atoms below the adsorbed sulfur are shifted down by 0.07 ± 0.05 A whereas the other atoms of the second substrate layer are not affected. The results are discussed to be in line with those obtained recently for the c(2 × 2)S structure and the similar superstructures observed with adsorption of oxygen. The observation of some unavoidable disorder in the p(2 × 2)S layer is discussed and contrasted to the similar but well-ordered phase of p(2 × 2)O/Ni(100) investigated earlier. The differences between the two systems are interpreted to be due to the different strength of interaction of the adsorbates with the substrate, in agreement with the details of the structural results.

We present LEED structure determinations of c(2 × 2)S Ni(100) both by a conventional R -factor analysis and a recently developed direct method. The adsorption height of sulfur in the hollow site results to be 1.30 ± 0.02 A. The adsorbate is found to induce almost no substrate reconstruction and only a slight 2 ± 1 % expansion of the first substrate layer distance. The results are compared to earlier structure determinations, to theoretical predictions and to the structure recently obtained for the similar system c(2 × 2)O Ni(100) .

In low energy electron diffraction, diffuse intensity distributions show up when atoms or molecules adsorb disorderedly on a crystalline substrate (DLEED). It is shown that current techniques allow the reliable measurement of the DLEED signal and theory is developed to extract the local adsorption structure by a full dynamical analysis. The potential of DLEED is demonstrated for both atomic and molecular disordered adsorption including adsorbate-induced local substrate reconstruction.

We consider a rectangular lattice gas model with repulsive and attractive interaction between nearest-neighbour sites in the [1 0] and [0 1] direction, respectively. At low temperatures this model exhibits the formation of a two-fold degenerate (2 × 1) superstructure. Monte Carlo simulation with Kawasaki dynamics is used to study the effects of randomly distributed, immobile impurities as well as of surface steps on the growth kinetics of ordered domains which develop after a quench of the initially disordered system to low temperatures. In the presence of impurities a temperature-dependent domain pinning effect can be observed, while surface steps act either as effective boundaries of domain growth (steps in [1 0] direction) or as domain nucleation centers (steps in [0 1] direction).

The adsorption system of p(2 × 2)O/Ni(100) was reinvestigated by a detailed LEED analysis, whereby the purity of the p(2 × 2) phase was controlled by HREELS. Full dynamical calculations were performed allowing for both substrate reconstruction and relaxation which both prove to be essential to yield a satisfying theory-experiment fit. Oxygen resides in the fourfold Symmetrie hollow site at 0.80 A height above the first substrate layer whose distance from the second layer is expanded by 2.5% compared to the bulk distance. The second substrate layer is considerably buckled by as much as 0.10 A. The determined adsorption height as well as the surface relaxation are in agreement with the results of semi-empirical total energy calculations and are consistent with a recent structure determination of the c(2 × 2) phase.

The adsorption of hydrogen on a clean rhodium(110) surface develops—with increasing coverage—a number of commensurate phases (1 × 3-H, 1 × 2-H, 1 × 3-2H, 1 × 2-3H, 1 × 1-2H). Low Energy Electron Diffraction (LEED) was used to investigate both the phase diagram and the position of adsorbate and substrate atoms. Hydrogen adsorption not only weakens the multilayer relaxation but also causes a slight reconstruction of the rhodium substrate which is reduced with increasing coverage. At low coverages hydrogen atoms occupy threefold co-ordinates sites with a Rh-H bond length of 1.84 A ± 0.1 A and a hydrogen radius of 0.54 A ± 0.1 A. At higher coverages repulsive interactions between adjacent hydrogen atoms make adatoms move up to allow for maximum horizontal distances. Coverage dependent transition temperatures and angular beam profile analyses allow the determination of the phase diagram. The critical temperatures for the primitive phases 1 × 3-H and 1 × 2-H are TC = 132 ± 5 K and TC = 150 ± 5 K, respectively. The 1 × 3-H phase transforms to lattice gas disorder through a continuous phase transition, whereas in the case of the 1 × 2-H phase, a reversible transition to an incommensurate phase is observed. For the non-primitive phases TC is near or above the desorption boundary and thermal desorption is a competing process to the order-disorder transition. Order-order transitions are of first order as observed for the transitions of the primitive phases 1 × 3-H to 1 × 2-H as well as for the non-primitive phases 1 × 3-2H to 1 × 2-3H.

We report on the interface between graphene and $4\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(0001)$ as investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). It is characterized by the so-called $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction, whose structural properties are still controversially discussed but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We discuss the role of three observed phases with periodicities $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$, $(6\ifmmode\times\else\texttimes\fi{}6)$, and $(5\ifmmode\times\else\texttimes\fi{}5)$. Their LEED intensity levels and spectra strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. The latter have the potential for an easy and practicable determination of the number of graphene layers by means of LEED.

The epitaxial growth of Fe monolayers on the unreconstructed Ir 001 surface was investigated experimentally by determining film structure, morphology, stress, and magnetism. Our combined quantitative low energy electron diffraction LEED and cantilever stress results indicate pseudomorphic Fe growth with a constant in-plane film strain in an almost perfect layer-by-layer growth mode from 0–10 layers. The combination of the structural analysis by LEED with stress measurements suggests that the first two layers of Fe grow as a face-centered tetragonally fct distorted precursor. Further deposition of Fe leads to the growth of a bodycentered tetragonally distorted phase on top of the fct precursor, which retains its structure. The magnetooptical Kerr effect reveals that the Fe film is ferromagnetic with an in-plane easy magnetization direction along Fe 100 .

Knowledge of pollen movement and frequency of interspecific hybridization in fragmented populations of rare species is a prerequisite for the implementation of conservation measures. In a large-scale study area (14,000 hectares) we analysed 297 Malus sylvestris trees with nine nuclear microsatellite markers. After open pollination of 564 offspring from 51 mother trees located in seven harvesting sites were investigated and genetic paternity analysis was performed. The paternal parent was identified for 213 offspring and the pollen dispersal distances between mother and pollen source were calculated. A large proportion of detected pollination events (42.4%) were observed within a radius of 50 m of the mother tree. The comparison of different tree densities indicated that with decreasing density the pollen dispersal distances increase. We observed pollination over long distances with a maximum of 10.7 km which is probably one of the reasons for a low spatial genetic structure within the M. sylvestris population and a stable genetic diversity in the offspring. Incorporating microsatellite data of 21 apple cultivars, a hybridization frequency of nearly 8% was determined. With decreasing tree density the number of hybridization events increased. Based on the results of our study an enhancement of the density of existing M. sylvestris populations is recommend to reduce the likelihood of hybridization. The production of young plants originated from seeds collected after open pollination is not advisable. Instead of that the seedlings for further reintroduction measures should be produced by controlled crossings in seed orchards to ensure ‘true type’ M. sylvestris individuals.

We report on the formation of ultrathin Co oxide films by oxidation of correspondingly thin Co films epitaxially grown on the unreconstructed---i.e., metastable---$(1\ifmmode\times\else\texttimes\fi{}1)$ phase of Ir(100), on both the clean surface and the surface precovered with half a monolayer of oxygen. As investigated by low-energy electron diffraction patterns and intensities and by atomically resolved images from scanning tunneling microscopy (STM) the oxide forms mainly in its fcc-type rocksalt structure CoO. This is with exception of the submonolayer range and the surface precovered with oxygen for which the ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ spinel-structured phase develops. Rather unusual, the CoO films appear in the polar (111) orientation and so, though the hexagons are slightly distorted, should become increasingly instable with growing thickness. However, STM images recorded under special conditions indicate that there are oxygen vacancies which lead to charge compensation and so may contribute to stabilize the (111) orientation. Also, STM retrieves a considerable buckling of cobalt layers which is inevitable when fcc(111) layers are accommodated on an fcc(100) substrate.

A structural study by quantitative low-energy electron diffraction (LEED) and density functional theory (DFT) has been performed on the Ir(100)-(5x1)-hex surface with 0.6 ML of hydrogen adsorbed at low temperature (

The adsorption of hydrogen on the stable state of the Ir(100) surface, the quasihexagonally reconstructed phase $\mathrm{Ir}(100)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}1)$-hex, was investigated by density functional theory (DFT) for different coverages and zero temperature. It appears that the adsorbate induces significant structural substrate modifications which are typical for the adsorption site(s) and are due to a complex interaction between adsorbate and substrate. The system's energetics, as well as the structure of the adsorbate and substrate, are provided for different coverages at which the mirror symmetry of the clean surface is either broken or saved. The total energy per H atom decreases with coverage. So, there is no island formation but always a homogeneous phase accessible by experiment. The structure of the substrate---which can be determined with high precision by experimental methods as low-energy electron diffraction---can be used by DFT as an identifier for the actual adsorbate coverage, and the corresponding adsorption sites can be determined by the usual energy minimization procedure.

The adsorption of hydrogen on the metastable, unreconstructed Ir(100)-$(1\ifmmode\times\else\texttimes\fi{}1)$ surface is investigated by density functional theory (DFT), quantitative low-energy diffraction (LEED), and thermal desorption spectrometry (TDS) complemented by scanning tunneling microscopy (STM). The bridge site is unequivocally identified as the adsorption site, rather unusual for metallic fcc(100) surfaces. There is excellent quantitative agreement between calculated and experimentally determined structural parameters both for the clean surface and the adsorbate covered surface. Given the uncertainty of DFT to reproduce absolute energies there is also good agreement with the measured adsorption energy ($460\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$/atom). Additionally, theoretical vibrational and electronic properties are provided without, however, related experiments being available.

The geometrical and chemical structure of the CoAl(111) surface is investigated by quantitative low-energy electron diffraction and calculations applying density functional theory. The stacking sequence of the top four atomic planes is Al-Co-Co-Co, followed below by the usual alternating B2 stacking. The topmost layers thus form a unit cell of the well-known bcc-based D0{sub 3} crystal structure [the A{sub 3}B superlattice of bcc(111) atomic planes], although the bulk phase diagram of CoAl shows no D0{sub 3} phase. Its occurrence and stability at the surface is due to a slight Co excess of the nominally stoichiometric sample, equivalent to the presence of Co antisite defects in the bulk. These defects are enriched in undercoordinated near-surface sites of the Al sublattice, which lowers the total energy because more Al atoms can then reside in fully coordinated bulk Al sites. However, all three topmost layers are undercoordinated, and the segregation of Co antisite defects competes with a general trend towards a termination of the surface by Al. In the balance, the third layer is the preferred plane for Co antisite defects.

A reversible switching of the easy axis of magnetization for Ni on Cu(001) from in plane to out of plane is found by changing the partial pressure of hydrogen in the gas phase around the sample, allowing even for oscillations of the magnetization direction. A quantitative low-energy electron diffraction study of the diffracted intensity versus electron energy [$I(E)$] shows that the hydrogen-induced spin reorientation transition is accompanied by changes of the tetragonal distortion of the topmost Ni layer upon hydrogen adsorption. Surprisingly, the orientation switch to perpendicular to the surface comes with a relaxation, i.e., reduction of the film's tetragonal distortion rather than its amplification.

The use of trenchless methods for pipeline water crossings in the Province of Alberta has grown significantly during the past decade, primarily as a result of increasingly strict water protection regulations. Because the feasibility of using a trenchless method is dependent on subsurface conditions, a geotechnical assessment should be performed during the planning stages. The results of this assessment are used in the selection of the most appropriate crossing method, and to assess the risk of encountering difficulties during construction. A discussion of the current state of practice for geotechnical investigations of trenchless projects in Alberta is initially presented. The importance of a good understanding of the engineering geological framework and associated geological-geotechnical complexities is highlighted. Recommended stages of a geotechnical assessment are described, namely the desk study, the site reconnaissance, and the field exploration, which involves drilling and sampling and geophysical surveys where appropriate. It is described how each stage interacts with, and is dependent on the others. It is noted that because the cost of a field drilling and sampling can be expensive when compared to the construction costs, obtaining as much information as possible during the desk study and field reconnaissance stages is considered to be a cost effective strategy. Drilling and sampling techniques currently used in Alberta are briefly reviewed, including continuous flight and hollow stem augering, wet rotary, and cased down-hole air percussion methods. Emphasis is placed on describing the advantages and limitations of each method. Some of the concepts presented are illustrated through evaluation of a recent case study of a recent major river crossing in Central Alberta.Copyright © 2004 by ASME

The combination of density-functional theory (DFT) calculations of geometrically fully relaxed binary alloy surfaces with concepts from statistical physics is applied to construct a DFT-based phase diagram for a binary alloy surface. As a first example, we studied the appearance of Co antisite atoms at CoAl(100) surfaces. The structural parameters as multilayer relaxations, surface buckling, lateral order, and segregation profile of the predicted stable surface phases are in excellent agreement with experimental structure determinations applying low-energy electron diffraction.

We show that the Ir(100) surface forms a new nanostructure in a self-organized way when its reconstructed equilibrium surface is exposed to hydrogen. Scanning tunneling microscopy and quantitative low-energy electron diffraction retrieve that a long-range ordered superlattice of defect-free Ir chains with average lateral spacing of 1.36 nm and micrometer lengths develops. This can be used as a template for the formation of other nanostructures as is demonstrated.

The early stages of iron silicide formation on Si(111) were studied by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. While the initial iron interaction with Si(111)in the submonolayer regime gives rise to inhomogeneous island nucleation, deposition of 1.5 monolayers (ML) iron at room temperature and subsequent annealing at 550-600 °C leads to a flat and homogeneous film with c(8×4) surface periodicity. This c(8×4) surface reconstruction is linked to a definite film thickness and thus seems to be stabilized directly through the interface. The film is terminated by a layer of adatoms whose lateral positions form a (2×2) periodic lattice. At negative tip bias voltages, STM images show an alternating arrangement of darker and brighter adatoms corresponding to the c(8×4) supercell. While the (2×2)-periodic adatom arrangement develops in a wide temperature regime (450-600°C) and also for thicker films, the long range ordered c(8×4) structure can be observed only for 1-2 ML Fe coverage and after high temperature annealing at about 600°C. Then single c(8×4) domains can extend to diameters of several hundred nanometers. The atomic structure of the new phase can be derived from a CsCl (B2) structure, and a number of structural details are elucidated on the course towards the development of a complete structural model.

The early stages of iron film growth on the quasihexagonally reconstructed $\mathrm{Ir}(100)\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}1)$ surface were investigated for deposition temperatures 90--450 K by means of scanning tunnelling microscopy and quantitative low-energy electron diffraction (LEED). For low coverage, single iron atoms diffuse within the one-dimensional troughs formed through the top layer reconstruction. On large step-free terraces a metastable iron phase develops consisting of linear diatomic Fe chains which reside within the troughs and are homogeneously spread over the terrace. With increasing coverage and controlled by temperature, the hexagonal reconstruction is lifted. This surface restructuring proceeds by expelling the 20% extra Ir atoms accommodated in the quasihexagonal layer, whereby a new $(5\ifmmode\times\else\texttimes\fi{}1)$ superstructure consisting of equally spaced Ir rows is formed at the very surface. With further deposition, iron fills the space between these rows, eventually leading to an ordered ${\mathrm{Fe}}_{4}\mathrm{Ir}$ surface alloy with the completion of the top layer. A quantitative LEED analysis corroborates this model and reveals significant atomic corrugations down to the third layer. It is observed that surface defects of any kind act as nucleation centers for both the film growth and the restructuring process.

We develop a technique for the direct determination, by low-energy electron diffraction ~LEED! ,o f the atomic structure of surface layers on a known substrate. The key is the division of multiple-scattering paths into those that scatter solely from the known bulk of the sample and those that include scattering by a surface atom. In a holographic analogy, the sum of the contributions from the former may be identified with a~known! reference wave, and those from the surface with an ~unknown! object wave. The latter may be written as a linear combination of elementary object waves, each of which may be regarded as a renormalized dynamical structure factor of a test two-dimensional ~2D! superlattice of average atoms within a preselected 3D surface slab overlying the bulk crystal. The coefficients of this linear expansion, which may be determined by a maximum entropy algorithm, represent the 3D distribution of atoms within the surface slab. Examples are given of applications of the method for the determination of structures of adsorbates on known substrates from both simulated and experimental LEED data.

Several hardware architectures to implement fuzzy processors have been proposed to satisfy real-time requirements, but very few of these are suitable for automatic synthesis. This paper presents bit scalable architecture that allows the automatic synthesis of fuzzy processors in different bit wide resolution. The synthesis is made from a VHDL description. The size of the internal units is defined from a small number of parameters in the highest level entity.

For the example of the B2 CoAl(100) surface, we demonstrate that even slight deviations from an ordered alloy's ideal stoichiometry in a subsurface region or in the bulk can drastically affect its surface composition. By experimental surface analysis and first-principles calculations, we show that Co antisite atoms segregate to the very surface, driven by the same strong interactions which enforce order in the bulk. Our findings are consistent with the lack of antisite segregation we found earlier for the much weaker ordering FeAl(100), and resolve contradictory reports for NiAl(100).

This paper discusses the various system interactions that can affect the efficiency and dynamic performance of an indirect methanol fuel cell system. The characterization of the load following IMFC system is done using the simulation model developed by the Fuel Cell Vehicle Modeling Program at the University of California-Davis. The first part of the paper briefly describes the components within the UCD-IMFC system model. The second part of the paper gives a qualitative look at the system interactions and their impact on the efficiency and dynamics of the system. There are two primary interactions within the IMFC system that are of interest. These two interactions are the fuel processor/stack interaction and the air supply/stack interaction. With respect to the fuel processor/stack anode interaction, we find a trade-off between going to a higher anode hydrogen utilization to maximize efficiency and going to a lower utilization to avoid starving the stack of hydrogen during heavy dynamic loads on the system. On the air supply/stack cathode interaction, we find that dynamics are not as much of an issue and the best operating method is one where the net power output of the stack/compressor combined is greatest for a given partial pressure of oxygen. This gives the highest efficiency for a given cathode/air supply combination. Finally, when considering the water and thermal management (WTM) of the system, we look at methods to reduce the total parasitic load on the satisfying the thermal requirements and maintaining water self-sufficiency.

We present a detailed investigation of the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{Br}/\mathrm{Pt}(110)$ adlayer structure supplemented by the analysis of the $(1\ifmmode\times\else\texttimes\fi{}2)$ missing-row (MR) structure of the clean Pt(110) surface. Quantitative low energy electron diffraction analyses and first-principles calculations are in impressive agreement in both cases. The clean surface reconstruction is determined with unprecedented accuracy. For the adsorbate, the analysis retrieves a simple Br-adlayer structure with the Br atoms residing in every second short bridge position on the close-packed Pt rows with the MR reconstruction lifted. The Br-Pt bond length $L=2.47 \mathrm{\AA{}}$ is almost equal to the sum of the atomic radii. The substrate below the adsorbate exhibits a contraction of the first layer spacing which amounts to half of that calculated for an unreconstructed clean surface.

A reversible switching of the easy axis of magnetization for Ni on Cu(001) from in plane to out of plane is found by changing the partial pressure of hydrogen in the gas phase around the sample, allowing even for oscillations of the magnetization direction. A quantitative low-energy electron diffraction study of the diffracted intensity versus electron energy [I(E)] shows that the hydrogen-induced spin reorientation transition is accompanied by changes of the tetragonal distortion of the topmost Ni layer upon hydrogen adsorption. Surprisingly, the orientation switch to perpendicular to the surface comes with a relaxation, i.e., reduction of the film's tetragonal distortion rather than its amplification.

We present a solution to the multiple-scattering inverse problem for low-energy electron diffraction that enables the determination of the three-dimensional atomic structure of an entire surface unit cell directly from measured data. The solution requires a knowledge of the structure of the underlying bulk crystal and is implemented by a maximum entropy algorithm.

The initial stages of iron silicide growth on Si(111) were investigated using LEED, AES and STM experiments together with DFT calculations. In 1:1 stoichiometry, a cubic FeSi develops showing a (1×1) surface periodicity which is stable up to 300°C. The epitaxial strain energy stabilizes this bulk-unstable B2 (CsCl-type) phase for up to 250 Å thickness. The surface is Si terminated, the interface coordination of the so-called B8 type. After annealing at ≈ 600°C, a (2×2)-FeSi2 phase of higher Si content is observed which possesses cubic crystal structure. It is stable below 10 monolayers (ML) initial Fe coverage and grows in an island-like morphology. In the initial growth stage, however, a c(8×4) phase forms that completely covers the surface at 1.5 ML Fe content and appears as potential template for further growth of homogeneous films. The interpretation of atomically resolved STM images suggests that the film contains three Si and two Fe layers in B2 structure with vacancies on Fe positions. The vacancy arrangement seems to be responsible for the c(8×4) periodicity displayed by the Si adatoms in T4-position.

A theoretical model of ballistic-electron-emission microscopy ~BEEM! based on linear combination of atomic orbitals Hamiltonians and Keldysh Green’s functions is applied to analyze experimental data obtained for CoSi2 /Si(111) contacts. Hot electrons injected from a scanning tunneling microscope tip into the silicide film form a highly focused beam, which even after propagation through films of moderate thickness is narrow enough to allow the observed atomic resolution of interfacial point defects. On (2 31) reconstructed domains a certain fraction of the initial current is injected into localized surface states, leading to the reported contrast in BEEM images, reflecting the topography at the surface. These results confirm that band-structure effects, both in the bulk and at the surface of the metallic overlayer, intricately influence the interface-related information contained in BEEM data. It is found that for a careful analysis of experimental results, a theoretical model going beyond the ballistic hypotesis is required.

The structures of ultrathin Fe films on Cu(100) prepared by pulsed laser deposition (PLD) and conventional thermal deposition (TD) are compared by means of low-energy electron diffraction (LEED). Below 5 ML coverage, PLD films exhibit up to four times higher integer-order LEED spot intensities, i.e., improved film quality, and show small but significant structural changes as compared to TD films. A quantitative tensor LEED analysis of the 4-ML PLD film reveals a similar $5\ifmmode\times\else\texttimes\fi{}1$ superstructure to that found previously for TD films, but with an enhanced surface buckling and a flat Cu bulklike iron interface layer. The latter is attributed to the incorporation of Cu atoms in the Fe interface caused by sputter effects in PLD. Above 6-ML coverage, both deposition methods yield comparable LEED $I(E)$ spectra, revealing similar surface structures. The proposed structural models allow a consistent interpretation of the strongly altered magnetic properties observed for PLD-grown iron films.

The relaxation of the first seven atomic layers of Cu(117) was determined by quantitative low-energy electron diffraction (LEED). Intensity versus energy spectra, $I(E),$ were measured for the primary beam at near-normal incidence to the (001) terraces. The data collected cover a cumulative energy range of 3200 eV. Computation of $I(E)$ spectra was performed in the angular momentum representation by considering the surface as a single atomic slab. For the variation of parameters, tensor LEED was applied. The experimental spectra are well reproduced over the entire energy range even for regions of low intensity. The first four layer spacings relax in a way to smooth the surface corrugation, whereby the relative changes $\ensuremath{\Delta}{d/d}_{0}$ amount (from top) to $\ensuremath{-}13%,$ $\ensuremath{-}2.0%,$ $\ensuremath{-}10%,$ and $+7%.$ This leads to a reduction of the vertical distance between step and corner atoms by 0.13 \AA{}. Comparison with experimental results for Cu(115) shows that this modification of the step shape is rather independent of the terrace width. Comparison to theoretical results exhibits, however, some discrepancies with respect to both the expansion/contraction sequence and the amplitudes of the layer relaxations.

The surface of SrTiO 3 (100) was analysed by low energy electron diffraction. Intensity spectra were taken using a tv camera whose signal is evaluated under computer control. Model intensities were calculated fully dynamically for energies above 200 eV where details of the scattering potential are of minor influence. The best theory-experiment fit (Pendry R-factor = 0.53) shows the surface to be terminated by both Sr-O and O-Ti-O layers. The top layers are buckled by oxygen ions displaced out of the surface by 0.08 and 0.16 A in the Ti and Sr terminated domains, respectively. The buckling, which creates a static dipole moment in the surface layer, is accompanied by a multilayer relaxation which for the first two layers is d 12 (Ti)/d 0 = +2%, d 23 (Ti)/d 0 = −2% for the Ti termination and d 12 (Sr)/d 0 = −10%, d 23 (Sr)/d 0 = +4% for the Sr terminated domains.

Following on the success of the recent application of holographic LEED to the determination of the 3D atomic geometry of Si adatoms on a SiC(111) p(3x3) surface, which enabled that structure to be solved, we show in this paper that a similar technique allows the direct recovery of the local geometry of adsorbates forming superstructures as small as p(2x2), even in the presence of a local substrate reconstruction., Comment: 10 pages, 5 figures postscript included, revtex, Phys. Rev. B in press

For the example of the SiC(111)-(3x3) reconstruction we show that a holographic interpretation of discrete Low Energy Electron Diffraction (LEED) spot intensities arising from ordered, large unit cell superstructures can give direct access to the local geometry of a cluster around an elevated atom, provided there is only one such prominent atom per surface unit cell. By comparing the holographic images obtained from experimental and calculated data we illuminate validity, current limits and possible shortcomings of the method. In particular, we show that periodic vacancies such as cornerholes may inhibit the correct detection of the atomic positions. By contrast, the extra diffraction intensity due to slight substrate reconstructions, as for example buckling, seems to have negligible influence on the images. Due to the spatial information depth of the method the stacking of the cluster can be imaged down to the fourth layer. Finally, it is demonstrated how this structural knowledge of the adcluster geometry can be used to guide the dynamical intensity analysis subsequent to the holographic reconstruction and necessary to retrieve the full unit cell structure., 11 pages RevTex, 6 figures, Phys. Rev. B in press

In this contribution the cosegregation-induced epitaxial growth of two- and three-dimensional chromium nitrides on ferritic Fe-15%Cr-N(100) (CN= 30 wt-ppm) single crystal surfaces will be discussed. The two-dimensional CrN surface compound is stable between 600 and 720°C. From the (1 × 1) LEED pattern it is inferred that the surface compound is epitaxial to the bcc(100) alloy surface. XPD and LEED-IV investigations have revealed that this surface compound consists of a single CrN compound layer plus an additional subsurface chromium layer with a huge interlayer expansion between both layers. The CrN surface precipitate formed at temperaturesT< 600°C is also epitaxially arranged on the bcc(100) substrate surface. Its structure corresponds to the rocksalt structure, i.e. the structure of the well-known bulk CrN. Starting from a sputter cleaned alloy surface the growth of the epitaxial CrN surface precipitate proceeds via the two-dimensional CrN surface nitride. After completion of this two-dimensional CrN layer the nucleation and growth of the three-dimensional CrN surface precipitate takes place.

Es wurden 89 pseudophake Augen mittels Scheimpflug-Photographie untersucht. Dabei zeigt sich eine zuverlassige Darstellung der Hinterkammerlinsen und der Kapselstrukturen. Die Lagebeziehungen sind gut zu beurteilen und Veranderungen an der Kunstlinse erkennbar. Die Biometrie erweist sich im Vergleich zu tatsachlichen Werten (Kunstlinsendicke) und im Vergleich zur Ultraschallbiometrie als auserst exakt.

Epitaxial cobalt oxide films on Ir(100) exhibit a rich scenario of different structural phases which are reviewed in this paper. The great majority of phases could be, as a rare case, crystallographically described by the joint application of atomically resolved STM and quantitative LEED, whereby structural surprises were more the rule than the exception. So, the oxide grows in the polar (111) orientation for both the Co3O4 and CoO stoichiometry on the bare Ir substrate in spite of the latter's square symmetry. Moreover, the film orientation can be tuned to non-polar (100) growth when one or several pseudomorphic Co layers are introduced as an interface between oxide and Ir substrate. By using the nanostructured Ir(100)-(5 × 1)-H phase as a template a nanostructured Co film can be formed whose oxidation leads to a nanostructured oxide. The nominally polar films circumvent the polarity problem by appropriate surface terminations. That of CoO(111) is, again as a surprise, realized by a switch from rocksalt-type to wurtzite-type stacking near the surface, by which the latter becomes metallic. The stepwise oxidation of a pseudomorphic Co layer on the bare Ir substrate leads to the sequential formation of rocksalt-type tetrahedral Co-O building blocks (with intermediate BN-type blocks) whereby the Co species more and more assume positions determined by the inner-oxidic binding.

We report on the application to measured data of an algorithm for holographic low-energy electron diffraction (LEED), which overcomes the two most important limitations of the technique to date: the ``searchlight'' effect, which tends to highlight only atoms forward scattered by the adsorbates, and the distorting effects on diffuse LEED intensities due to possible long-range order among the adsorbates. The only experimental input required is a set of the most reliably measured diffuse LEED patterns from normally incident electrons. The algorithm is applied to a set of 11 measured diffraction patterns from a K/Ni(001) surface. A fully three-dimensional image is reconstructed from these data by compensating for the anisotropy of the reference wave by an appropriate scattered-wave kernel. \textcopyright{} 1996 The American Physical Society.

Upon sputtering and subsequent annealing, the subsurface region of the (100) surface of FeAl undergoes a transition to a practically stable and ordered Fe{sub 3}Al slab. It is capped by an Al top layer forming a rather complex interface to the bulk FeAl. This shows that routine surface preparation procedures with sputtering involved can lead to substantial restructuring extending deep into the surface. {copyright} {ital 1996 The American Physical Society.}