Extending holographic LEED to ordered small unit cell superstructures

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 pres

Holographic Image Reconstruction from Electron Diffraction Intensities of Ordered Superstructures

We report on a novel holographic reconstruction of well resolved atomic images from discrete spot intensities appearing in low-energy electron diffraction (LEED) from crystalline surfaces. This opens holographic LEED to the wide field of ordered systems giving access to rather complex surface structures

Direct reconstruction of three-dimensional atomic adsorption sites by holographic LEED

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. © 1996 The American Physical Society

Influence of the data base and algorithmic parameters on the image quality in holographic diffuse LEED

A detailed examination of the influence of different parameters used in diffuse low-energy-electron-diffraction holography is presented. Effects due to the finite and discrete character of the data that are input to the method's reconstruction algorithm are investigated, as well as the influence of additional parameters introduced both for the removal of experimental Bragg-spots and for the recently proposed correction for the anisotropy of the reference wave. As a test case, we applied the reconstruction algorithm to simulated data of the disordered adsorption system O/Ni(001). Guidelines for the width and resolution of the data base to be used, as well as for the proper selection of algorithmic parameters are deduced, which should hold also for other and unknown systems of disordered atomic adsorption. Using a parameter configuration optimized according to our results, a well-resolved and fully three-dimensional image of the local adsorption geometry is reconstructed from the data

Retrospective analysis and clinical evaluation of mandible reconstruction with free fibula flap

The objective of this investigation was to evaluate the indications for and the functional and cosmetic results of mandible reconstruction using free vascularized fibular flaps. It was also designed to assess the patients' quality of life, daily activities, and self-esteem, with special emphasis on patient satisfaction. We conducted a retrospective chart review of all patients who underwent osseus or osteocutaneous free flap reconstruction of the mandible over a 10-year period (1992-2002). Those in whom a vascularized fibular flap was used were included in the study and invited for a follow-up comprehensive physical examination and x-ray evaluation. A questionnaire was also administered to evaluate patient satisfaction. Twenty-six patients were included in the study. Fourteen patients died after an average of 2years and 9months postoperatively due to their prognosis despite surgical intervention and adjuvant therapy. Of the remaining 12 patients, 6 appeared for the follow-up evaluation, 2 were unavailable for follow-up, and 4 denied participating. Of these 6 patients, all experienced a decrease in pain and were satisfied with their results. At the time of the latest follow-up, they experienced some difficulties swallowing, and in 3 patients, their articulation was impaired. All 6 patients would undergo the procedure again. Reconstruction of the mandible using a vascularized fibular graft produces satisfactory functional and cosmetic results. In benign lesions, the procedure is highly indicated. However, in the case of malignancy, most patients do not survive their primary tumor. Given the patients limited life expectancy, the improvement in their quality of life as a result of the improved appearance and function of the reconstructed mandible needs to be weighed against the potential morbidity of the operative intervention on an individual basi

Novel Reconstruction Mechanism for Dangling-Bond Minimization: Combined Method Surface Structure Determination of SiC(111)-(3×3)

The SiC(111)−(3×3) phase was analyzed by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) holography, density functional theory (DFT), and conventional LEED. A single adatom per unit cell found in STM acts as a beam splitter for the holographic inversion of discrete LEED spot intensities. The resulting 3D image guides the detailed analyses by LEED and DFT which find a Si tetramer on a twisted Si adlayer with cloverlike rings. This twist model with one dangling bond left per unit cell represents a novel (n×n)-reconstruction mechanism of group-IV (111) surfaces

Approaching the low-temperature limit in nucleation and two-dimensional growth of fcc (100) metal films Ag/Ag(100)

We analyze the formation of two-dimensional Ag islands following deposition of about 0.1 ML of Ag on Ag(100) over a temperature regime ranging from classical nucleation and growth behavior to almost immobile adatoms, from 300 to 125 K. Particular emphasis is placed on the post-deposition dynamics at the lower end of the temperature range, where the saturation island density is not reached at the end of the deposition, and nucleation and aggregation processes continue with adatoms from the remaining adatom gas. Our analysis combines VT scanning tunneling microscopy experiments with kinetic Monte Carlo simulation of appropriate atomistic models. The only adjustable parameters in the model are the terrace diffusion barrier and prefactor, which can be determined from island density behavior near room temperature. Other processes such as rapid edge diffusion, and “easy” nucleation and aggregation of diagonally adjacent adatoms, are treated as instantaneous. The model excellently reproduces all aspects of behavior at low temperatures, demonstrating that nucleation and growth processes can be described in one consistent scheme, down to the regime of almost immobile adatoms

LEED Holography applied to a complex superstructure: a direct view of the adatom cluster on SiC(111)-(3x3)

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.Comment: 11 pages RevTex, 6 figures, Phys. Rev. B in pres

Retrospective analysis and clinical evaluation of mandible reconstruction with free fibula flap

The objective of this investigation was to evaluate the indications for and the functional and cosmetic results of mandible reconstruction using free vascularized fibular flaps. It was also designed to assess the patients' quality of life, daily activities, and self-esteem, with special emphasis on patient satisfaction. We conducted a retrospective chart review of all patients who underwent osseus or osteocutaneous free flap reconstruction of the mandible over a 10-year period (1992-2002). Those in whom a vascularized fibular flap was used were included in the study and invited for a follow-up comprehensive physical examination and x-ray evaluation. A questionnaire was also administered to evaluate patient satisfaction. Twenty-six patients were included in the study. Fourteen patients died after an average of 2years and 9months postoperatively due to their prognosis despite surgical intervention and adjuvant therapy. Of the remaining 12 patients, 6 appeared for the follow-up evaluation, 2 were unavailable for follow-up, and 4 denied participating. Of these 6 patients, all experienced a decrease in pain and were satisfied with their results. At the time of the latest follow-up, they experienced some difficulties swallowing, and in 3 patients, their articulation was impaired. All 6 patients would undergo the procedure again. Reconstruction of the mandible using a vascularized fibular graft produces satisfactory functional and cosmetic results. In benign lesions, the procedure is highly indicated. However, in the case of malignancy, most patients do not survive their primary tumor. Given the patients limited life expectancy, the improvement in their quality of life as a result of the improved appearance and function of the reconstructed mandible needs to be weighed against the potential morbidity of the operative intervention on an individual basi