121 research outputs found
Using the Uncharged Kerr Black Hole as a Gravitational Mirror
We extend the study of the possibility to use the Schwarzschild black hole as
a gravitational mirror to the more general case of an uncharged Kerr black
hole. We use the null geodesic equation in the equatorial plane to prove a
theorem concerning the conditions the impact parameter has to satisfy if there
shall exist boomerang photons. We derive an equation for these boomerang
photons and an equation for the emission angle. Finally, the radial null
geodesic equation is integrated numerically in order to illustrate boomerang
photons.Comment: 11 pages Latex, 3 Postscript figures, uufiles to compres
Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
Nanowire heterostructures define high-quality few-electron quantum dots for
nanoelectronics, spintronics and quantum information processing. We use a
cooled scanning probe microscope (SPM) to image and control an InAs quantum dot
in an InAs/InP nanowire, using the tip as a movable gate. Images of dot
conductance vs. tip position at T = 4.2 K show concentric rings as electrons
are added, starting with the first electron. The SPM can locate a dot along a
nanowire and individually tune its charge, abilities that will be very useful
for the control of coupled nanowire dots
Analyzing capacitance-voltage measurements of vertical wrapped-gated nanowires
The capacitance of arrays of vertical wrapped-gate InAs nanowires are
analyzed. With the help of a Poisson-Schr"odinger solver, information about the
doping density can be obtained directly. Further features in the measured
capacitance-voltage characteristics can be attributed to the presence of
surface states as well as the coexistence of electrons and holes in the wire.
For both scenarios, quantitative estimates are provided. It is furthermore
shown that the difference between the actual capacitance and the geometrical
limit is quite large, and depends strongly on the nanowire material.Comment: 15 pages, 6 Figures included, to appear in Nanotechnolog
State transition of a non-Ohmic damping system in a corrugated plane
Anomalous transport of a particle subjected to non-Ohmic damping of the power
in a tilted periodic potential is investigated via Monte Carlo
simulation of generalized Langevin equation. It is found that the system
exhibits two relative motion modes: the locking state and the running state.
Under the surrounding of sub-Ohmic damping (), the particle should
transfer into a running state from a locking state only when local minima of
the potential vanish; hence the particle occurs a synchronization oscillation
in its mean displacement and mean square displacement (MSD). In particular, the
two motion modes are allowed to coexist in the case of super-Ohmic damping
() for moderate driving forces, namely, where exists double centers
in the velocity distribution. This induces the particle having faster
diffusion, i.e., its MSD reads . Our result shows that the effective power index
can be enhanced and is a nonmonotonic function of the
temperature and the driving force. The mixture effect of the two motion modes
also leads to a breakdown of hysteresis loop of the mobility.Comment: 7 pages,7 figure
On the ubiquity of trivial torsion on elliptic curves
The purpose of this paper is to give a "down--to--earth" proof of the
well--known fact that a randomly chosen elliptic curve over the rationals is
most likely to have trivial torsion
Solar flare particle propagation: Comparison of a new analytic solution with spacecraft measurements
Four lectures on secant varieties
This paper is based on the first author's lectures at the 2012 University of
Regina Workshop "Connections Between Algebra and Geometry". Its aim is to
provide an introduction to the theory of higher secant varieties and their
applications. Several references and solved exercises are also included.Comment: Lectures notes to appear in PROMS (Springer Proceedings in
Mathematics & Statistics), Springer/Birkhause
Growth of Inclined GaAs Nanowires by Molecular Beam Epitaxy: Theory and Experiment
The growth of inclined GaAs nanowires (NWs) during molecular beam epitaxy (MBE) on the rotating substrates is studied. The growth model provides explicitly the NW length as a function of radius, supersaturations, diffusion lengths and the tilt angle. Growth experiments are carried out on the GaAs(211)A and GaAs(111)B substrates. It is found that 20° inclined NWs are two times longer in average, which is explained by a larger impingement rate on their sidewalls. We find that the effective diffusion length at 550°C amounts to 12 nm for the surface adatoms and is more than 5,000 nm for the sidewall adatoms. Supersaturations of surface and sidewall adatoms are also estimated. The obtained results show the importance of sidewall adatoms in the MBE growth of NWs, neglected in a number of earlier studies
Physical literacy in Europe: The current state of implementation in research, practice, and policy
Background/objective The holistic concept of physical literacy (PL) embraces different person-centered qualities (physical, cognitive, affective/psychological) necessary to lead physically active lifestyles. PL has recently gained increasing attention globally and Europe is no exception. However, scientific endeavors summarizing the current state of PL in Europe are lacking. Therefore, the goal of this study was to comprehensively assess and compare the implementation of PL in research, policy, and practice across the continent. Methods We assembled a panel of experts representing 25 European countries. Employing a complementary mixed-methods design, the experts first prepared reviews about the current state of PL in their countries (categories: research, practice/policy). The reviews underwent comparative document analysis, ensuring a transnational four-eyes principle. For re-validation purposes, the representatives completed a quantitative survey with questions reflecting the inductive themes from the document analysis. Results The document analysis resulted in ten disjunct themes (related to âconceptâ, âresearchâ, âpractice/policyâ, âfuture/prospectâ) and yielded a heterogenous PL situation in Europe. The implementation state was strongly linked to conceptual discussions (e.g., existence of competing approaches), linguistic issues (e.g., translations), and country-specific traditions. Despite growing scholarly attention, PL hesitantly permeates practice and policy in most countries. Nevertheless, the experts largely anticipate increasing popularity of PL for the future. Conclusion Despite the heterogeneous situation across Europe, the analysis has uncovered similarities among the countries, such as the presence of established yet not identical concepts. Research should intensify academic activities (conceptual-linguistic elaborations, empirical work) before PL may gain further access into practical and political spheres in the long term
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