10,469 research outputs found
He Scattering from Compact Clusters and from Diffusion-Limited Aggregates on Surfaces: Observable Signatures of Structure
The angular intensity distribution of He beams scattered from compact
clusters and from diffusion limited aggregates, epitaxially grown on metal
surfaces, is investigated theoretically. The purpose is twofold: to distinguish
compact cluster structures from diffusion limited aggregates, and to find
observable {\em signatures} that can characterize the compact clusters at the
atomic level of detail. To simplify the collision dynamics, the study is
carried out in the framework of the sudden approximation, which assumes that
momentum changes perpendicular to the surface are large compared with momentum
transfer due to surface corrugation. The diffusion limited aggregates on which
the scattering calculations were done, were generated by kinetic Monte Carlo
simulations. It is demonstrated, by focusing on the example of compact Pt
Heptamers, that signatures of structure of compact clusters may indeed be
extracted from the scattering distribution. These signatures enable both an
experimental distinction between diffusion limited aggregates and compact
clusters, and a determination of the cluster structure. The characteristics
comprising the signatures are, to varying degrees, the Rainbow, Fraunhofer,
specular and constructive interference peaks, all seen in the intensity
distribution. It is also shown, how the distribution of adsorbate heights above
the metal surface can be obtained by an analysis of the specular peak
attenuation. The results contribute to establishing He scattering as a powerful
tool in the investigation of surface disorder and epitaxial growth on surfaces,
alongside with STM.Comment: 41 pages, 16 postscript figures. For more details see
http://www.fh.huji.ac.il/~dan
Splitting between Bright and Dark excitons in Transition Metal Dichalcogenide Monolayers
The optical properties of transition metal dichalcogenide monolayers such as
the two-dimensional semiconductors MoS and WSe are dominated by
excitons, Coulomb bound electron-hole pairs. The light emission yield depends
on whether the electron-hole transitions are optically allowed (bright) or
forbidden (dark). By solving the Bethe Salpeter Equation on top of wave
functions in density functional theory calculations, we determine the sign and
amplitude of the splitting between bright and dark exciton states. We evaluate
the influence of the spin-orbit coupling on the optical spectra and clearly
demonstrate the strong impact of the intra-valley Coulomb exchange term on the
dark-bright exciton fine structure splitting.Comment: 6 pages, 2 figure
Magnetization driven metal - insulator transition in strongly disordered Ge:Mn magnetic semiconductors
We report on the temperature and field driven metal-insulator transition in
disordered Ge:Mn magnetic semiconductors accompanied by magnetic ordering,
magnetoresistance reaching thousands of percents and suppression of the
extraordinary Hall effect by a magnetic field. Magnetoresistance isotherms are
shown to obey a universal scaling law with a single scaling parameter depending
on temperature and fabrication. We argue that the strong magnetic disorder
leads to localization of charge carriers and is the origin of the unusual
properties of Ge:Mn alloys.Comment: 10 pages, 5 figure
Toward a direct and scalable identification of reduced models for categorical processes
The applicability of many computational approaches is dwelling on the identification of reduced models defined on a small set of collective variables (colvars). A methodology for scalable probability-preserving identification of reduced models and colvars directly from the data is derived—not relying on the availability of the full relation matrices at any stage of the resulting algorithm, allowing for a robust quantification of reduced model uncertainty and allowing us to impose a priori available physical information. We show two applications of the methodology: (i) to obtain a reduced dynamical model for a polypeptide dynamics in water and (ii) to identify diagnostic rules from a standard breast cancer dataset. For the first example, we show that the obtained reduced dynamical model can reproduce the full statistics of spatial molecular configurations—opening possibilities for a robust dimension and model reduction in molecular dynamics. For the breast cancer data, this methodology identifies a very simple diagnostics rule—free of any tuning parameters and exhibiting the same performance quality as the state of the art machine-learning applications with multiple tuning parameters reported for this problem
Improving clustering by imposing network information
Cluster analysis is one of the most popular data analysis tools in a wide range of applied disciplines. We propose and justify a computationally efficient and straightforward-to-implement way of imposing the available information from networks/graphs (a priori available in many application areas) on a broad family of clustering methods. The introduced approach is illustrated on the problem of a noninvasive unsupervised brain signal classification. This task is faced with several challenging difficulties such as nonstationary noisy signals and a small sample size, combined with a high-dimensional feature space and huge noise-to-signal ratios. Applying this approach results in an exact unsupervised classification of very short signals, opening new possibilities for clustering methods in the area of a noninvasive brain-computer interface
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
You Can\u27t Stop Me From Loving You
https://digitalcommons.library.umaine.edu/mmb-vp/5433/thumbnail.jp
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
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
- …