Optical Theorem and the Inversion of Cross Section Data for Atom Scattering from Defects on Surfaces

The information content and properties of the cross section for atom scattering from a defect on a flat surface are investigated. Using the Sudden approximation, a simple expression is obtained that relates the cross section to the underlying atom/defect interaction potential. An approximate inversion formula is given, that determines the shape function of the defect from the scattering data. Another inversion formula approximately determines the potential due to a weak corrugation in the case of substitutional disorder. An Optical Theorem, derived in the framework of the Sudden approximation, plays a central role in deriving the equations that conveniently relate the interaction potential to the cross section. Also essential for the result is the equivalence of the operational definition for the cross section for scattering by a defect, given by Poelsema and Comsa, and the formal definition from quantum scattering theory. This equivalence is established here. The inversion result is applied to determine the shape function of an Ag atom on Pt(111) from scattering data.Comment: 29 pages, 9 Postscript figures, more info available at http://www.fh.huji.ac.il/~dan

Pregnancy-associated breast cancer - Special features in diagnosis and treatment

For obvious psychological reasons it is difficult to associate pregnancy - a life-giving period of our existence with life-threatening malignancies. Symptoms pointing to malignancy are often ignored by both patients and physicians, and this, together with the greater difficulty of diagnostic imaging, probably results in the proven delay in the detection of breast cancers during pregnancy. The diagnosis and treatment of breast cancer are becoming more and more important, as the fulfillment of the desire to have children is increasingly postponed until a later age associated with a higher risk of carcinoma, and improved cure rates of solid tumors no longer exclude subsequent pregnancies. The following article summarizes the special features of the diagnosis and primary therapy of pregnancy-associated breast cancer with particular consideration of cytostatic therapy

He Scattering from Random Adsorbates, Disordered Compact Islands and Fractal Submonolayers: Intensity Manifestations of Surface Disorder

A theoretical study is made on He scattering from three fundamental classes of disordered ad-layers: (a) Translationally random adsorbates, (b) disordered compact islands and (c) fractal submonolayers. The implications of the results to experimental studies of He scattering from disordered surfaces are discussed, and a combined experimental-theoretical study is made for Ag submonolayers on Pt(111). Some of the main theoretical findings are: (1) Structural aspects of the calculated intensities from translationally random clusters were found to be strongly correlated with those of individual clusters. (2) Low intensity Bragg interference peaks appear even for scattering from very small ad-islands, and contain information on the ad-island local electron structure. (3) For fractal islands, just as for islands with a different structure, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal power law as previously proposed in the literature. In the experimental-theoretical study of Ag on Pt(111), we use first experimental He scattering data from low-coverage (single adsorbate) systems to determine an empirical He/Ag-Pt potential of good quality. Then, we carry out He scattering calculations for high coverage and compare with experiments. The conclusions are that the actual experimental phase corresponds to small compact Ag clusters of narrow size distribution, translationally disordered on the surface.Comment: 36 double-spaced pages, 10 figures; accepted by J. Chem. Phys., scheduled to appear March 8. More info available at http://www.fh.huji.ac.il/~dani

Transverse "resistance overshoot" in a Si/SiGe two-dimensional electron gas in the quantum Hall effect regime

We investigate the peculiarities of the "overshoot" phenomena in the transverse Hall resistance R_{xy} in Si/SiGe. Near the low magnetic field end of the quantum Hall effect plateaus, when the filling factor \nu approaches an integer i, R_{xy} overshoots the normal plateau value h/ie^2. However, if magnetic field B increases further, R_{xy} decreases to its normal value. It is shown that in the investigated sample n-Si/Si_{0.7}Ge_{0.3}, overshoots exist for almost all \nu. Existence of overshoot in R_{xy} observed in different materials and for different \nu, where splitting of the adjacent Landau bands has different character, hints at the common origin of this effect. Comparison of the experimental curves R_{xy}(\nu) for \nu = 3 and \nu = 5 with and without overshoot showed that this effect exist in the whole interval between plateaus, not only in the region where R_{xy} exceeds the normal plateau value.Comment: 3 pages, 5 EPS figure

Investigation of dual polarization laser modulation

Dual polarization lasers for wideband optical communication