14,435 research outputs found
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
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
Structure Determination of Disordered Metallic Sub-Monolayers by Helium Scattering: A Theoretical and Experimental Study
An approach based on He scattering is used to develop an atomic-level
structural model for an epitaxially grown disordered sub-monolayer of Ag on
Pt(111) at 38K. Quantum scattering calculations are used to fit structural
models to the measured angular intensity distribution of He atoms scattered
from this system. The structure obtained corresponds to narrowly size-dispersed
compact clusters with modest translational disorder, and not to fractals which
might be expected due to the low surface temperature. The clusters have up to
two layers in height, the lower one having few defects only. The relations
between specific features of the angular scattering distribution, and
properties such as the cluster sizes and shapes, the inter-cluster distance
distribution etc., are discussed. The results demonstrate the usefulness of He
scattering as a tool for unraveling new complex surface phases.Comment: 5 pages, 3 figures, to appear in Surf. Sci. Lett. Related papers
available at http://neon.cchem.berkeley.edu/~dani/He-papers.htm
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
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
Influence of Multiple Traumatic Event Types on Mental Health Outcomes: Does Count Matter?
The experience of potentially traumatizing events (PTEs) may be associated with conflicting outcomes: individuals may experience greater psychological distress (dose-response theory), or individuals may become more resilient against repeated PTEs (stress-inoculation theory). With limited empirical data comparing these theories, we examined the relationships between the count of lifetime PTE types and psychological outcomes [posttraumatic stress disorder (PTSD), depression, impaired distress tolerance] using linear and quadratic regressions. A linear relationship would support the dose-response theory, and a quadratic relationship would support the stress-inoculation theory. We also explored whether there was a threshold number of PTE types fostering resiliency before an increase of distressing outcomes. The sample included 123 (68.30% female) treatment-seeking patients at a community mental health center participating in a larger study (Contractor et al. in Psychiatry Research, 252, 252215–252222, 2017). Linear regression results indicated number of PTE types significantly predicted increasing PTSD and depression severity and distress tolerance difficulties. Quadratic regression model results were not significant. ROC analyses indicated exposure to at least 3.5 PTE types predicted PTSD with moderate accuracy. In conclusion, the dose-response theory was supported, with results indicating there may be a threshold count of lifetime PTE types (\u3e 3) influencing traumatic stress outcomes
Universal temperature dependence of the conductivity of a strongly disordered granular metal
A disordered array of metal grains with large and random intergrain
conductances is studied within the one-loop accuracy renormalization group
approach. While at low level of disorder the dependence of conductivity on log
T is nonuniversal (it depends on details of the array's geometry), for strong
disorder this dependence is described by a universal nonlinear function, which
depends only on the array's dimensionality. In two dimensions this function is
found numerically. The dimensional crossover in granular films is discussed.Comment: 6 pages, 6 figures, submitted to JETP Letter
Inversion of Randomly Corrugated Surfaces Structure from Atom Scattering Data
The Sudden Approximation is applied to invert structural data on randomly
corrugated surfaces from inert atom scattering intensities. Several expressions
relating experimental observables to surface statistical features are derived.
The results suggest that atom (and in particular He) scattering can be used
profitably to study hitherto unexplored forms of complex surface disorder.Comment: 10 pages, no figures. Related papers available at
http://neon.cchem.berkeley.edu/~dan
Spin screening of magnetic moments in superconductors
We consider ferromagnetic particles embedded into a superconductor and study
the screening of their magnetic moments by the spins of the Cooper pairs in the
superconductor. It is shown that a magnetic moment opposite to the one of the
ferromagnetic particle is induced in the superconductor. In the case of a small
itinerant ferromagnet grain and low temperatures the full screening of the
magnetic moment takes place, \textit{% i.e} the absolute value of the total
magnetic moment induced in the superconductor is equal to the one of the
ferromagnetic particle. In type II superconductors the proposed screening by
spins of the conduction electrons can be much stronger than the conventional
screening by Meissner currents.Comment: 7 pages; 2 figure
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