Chemical contrast in STM imaging of transition metal aluminides

The present manuscript reviews recent scanning tunnelling microscopy (STM) studies of transition metal (TM) aluminide surfaces. It provides a general perspective on the contrast between Al atoms and TM atoms in STM imaging. A general trend is the much stronger bias dependence of TM atoms, or TM-rich regions of the surface. This dependence can be attenuated by the local chemical arrangements and environments. Al atoms can show a stronger bias dependence when their chemical environment, such as their immediate subsurface, is populated with TM. All this is well explained in light of combined results of STM and both theoretical and experimental electronic and crystallographic structure determinations. Since STM probes the Fermi surface, the electronic structure in the vicinity of the Fermi level (EF) is essential forunderstanding contrast and bias dependence. Hence, partial density of states provides information about the TM d band position and width, s–p–d hybridization or interactions, or charge transfer between constituent elements. In addition, recent developments in STM image simulations are very interesting for elucidating chemical contrast at Al–TM alloy surfaces, and allow direct atomic identification, when the surface does not show too much disorder. Overall, we show that chemically-specific imaging is often possible at these surfaces

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

Beiträge zur französischen Stilistik

von Wilhelm KottckeProgr. Nr. 15

Global search algorithms in surface structure determination using photoelectron diffraction

Three different algorithms to effect global searches of the variable-parameter hyperspace are compared for application to the determination of surface structure using the technique of scanned-energy mode photoelectron diffraction (PhD). Specifically, a new method not previously used in any surface science methods, the swarm-intelligence-based particle swarm optimisation (PSO) method, is presented and its results compared with implementations of fast simulated annealing (FSA) and a genetic algorithm (GA). These three techniques have been applied to experimental data from three adsorption structures that had previously been solved by standard trial-and-error methods, namely H2O on TiO2(110), SO2 on Ni(111) and CN on Cu(111). The performance of the three algorithms is compared to the results of a purely random sampling of the structural parameter hyperspace. For all three adsorbate systems, the PSO out-performs the other techniques as a fitting routine, although for two of the three systems studied the advantage relative to the GA and random sampling approaches is modest. The implementation of FSA failed to achieve acceptable fits in these tests

Carcinomes canalaires in situ du sein (influence de l'âge sur les critères diagnostiques, thérapeutiques et pronostiques)

Introduction : Les patientes de moins de 40 ans au diagnostic d'un carcinome canalaire in situ semblent avoir un pronostic plus sombre que les patientes plus âgées. Cependant l'impact de l'âge sur le devenir reste controversé. Matériel et méthodes : A partir d'une série rétrospective de 912 patientes traitées à l'Institut Bergonié entre 1971 et 2001, nous avons étudié l'influence de l'âge sur les critères diagnostiques, histologiques, thérapeutiques et pronostiques des carcinomes canalaires in situ. Résultats : Les CCIS ont plus fréquemment une expression clinique chez les femmes de moins de 40 ans que chez les patientes plus âgées. Parallèlement, chez les femmes jeunes, les lésions semblent être de meilleur pronostic d'un point de vue histologique sans que la différence soit significative. Les facteurs pronostiques retrouvés dans notre étude, en analyse uni comme multivariée, sont l'âge de moins de 40 ans, le type de traitement et l'état des berges d'exérèse. Le jeune âge (RR =1,82 IC [1,025 - 3,24] p = 0,041) et le traitement conservateur (RR = 6,057 IC [2,68 - 13,64] p < 10 puis -3 pour les marges non saines) sont les seuls de risque de récidive dans notre série. Conclusion : L'âge de moins de 40 ans apparaît comme un facteur de risque de récidive indépendamment des caractéristiques cliniques et hostologiques. L'autre facteur pronostique retrouvé dans notre étude est le traitement chirurgical initial, surtout un cas de berges envahies. Une chirurgie initiale optimale avec des marges de résection particulièrement larges, apparaît indispensable chez les femmes jeunes qui ont un risque de récidive augmenté.FORT-DE-FRANCE-CHRU-BU (972332102) / SudocBORDEAUX2-BU Santé (330632101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF