4,080 research outputs found
Anomalous strength of membranes with elastic ridges
We report on a simulational study of the compression and buckling of elastic
ridges formed by joining the boundary of a flat sheet to itself. Such ridges
store energy anomalously: their resting energy scales as the linear size of the
sheet to the 1/3 power. We find that the energy required to buckle such a ridge
is a fixed multiple of the resting energy. Thus thin sheets with elastic ridges
such as crumpled sheets are qualitatively stronger than smoothly bent sheets.Comment: 4 pages, REVTEX, 3 figure
Harmonic fields on the extended projective disc and a problem in optics
The Hodge equations for 1-forms are studied on Beltrami's projective disc
model for hyperbolic space. Ideal points lying beyond projective infinity arise
naturally in both the geometric and analytic arguments. An existence theorem
for weakly harmonic 1-fields, changing type on the unit circle, is derived
under Dirichlet conditions imposed on the non-characteristic portion of the
boundary. A similar system arises in the analysis of wave motion near a
caustic. A class of elliptic-hyperbolic boundary-value problems is formulated
for those equations as well. For both classes of boundary-value problems, an
arbitrarily small lower-order perturbation of the equations is shown to yield
solutions which are strong in the sense of Friedrichs.Comment: 30 pages; Section 3.3 has been revise
The first joint ESGAR/ ESPR consensus statement on the technical performance of cross-sectional small bowel and colonic imaging
Objectives: To develop guidelines describing a standardised approach to patient preparation and acquisition protocols for magnetic resonance imaging (MRI), computed tomography (CT) and ultrasound (US) of the small bowel and colon, with an emphasis on imaging inflammatory bowel disease. Methods: An expert consensus committee of 13 members from the European Society of Gastrointestinal and Abdominal Radiology (ESGAR) and European Society of Paediatric Radiology (ESPR) undertook a six-stage modified Delphi process, including a detailed literature review, to create a series of consensus statements concerning patient preparation, imaging hardware and image acquisition protocols. Results: One hundred and fifty-seven statements were scored for agreement by the panel of which 129 statements (82 %) achieved immediate consensus with a further 19 (12 %) achieving consensus after appropriate modification. Nine (6 %) statements were rejected as consensus could not be reached. Conclusions: These expert consensus recommendations can be used to help guide cross-sectional radiological practice for imaging the small bowel and colon. Key points: • Cross-sectional imaging is increasingly used to evaluate the bowel • Image quality is paramount to achieving high diagnostic accuracy • Guidelines concerning patient preparation and image acquisition protocols are provided
Observation of Sommerfeld precursors on a fluid surface
We report the observation of two types of Sommerfeld precursors (or
forerunners) on the surface of a layer of mercury. When the fluid depth
increases, we observe a transition between these two precursor surface waves in
good agreement with the predictions of asymptotic analysis. At depths thin
enough compared to the capillary length, high frequency precursors propagate
ahead of the ''main signal'' and their period and amplitude, measured at a
fixed point, increase in time. For larger depths, low frequency ''precursors''
follow the main signal with decreasing period and amplitude. These behaviors
are understood in the framework of the analysis first introduced for linear
transient electromagnetic waves in a dielectric medium by Sommerfeld and
Brillouin [1].Comment: to be published in Physical Review Letter
Reversal of the Charge Transfer between Host and Dopant Atoms in Semiconductor Nanocrystals
We present ab initio density functional calculations that show P (Al) dopant
atoms in small hydrogen-terminated Si crystals to be negatively (positively)
charged. These signs of the dopant charges are reversed relative to the same
dopants in bulk Si. We predict this novel reversal of the dopant charge (and
electronic character of the doping) to occur at crystal sizes of order 100 Si
atoms. We explain it as a result of competition between fundamental principles
governing charge transfer in bulk semiconductors and molecules and predict it
to occur in nanocrystals of most semiconductors.Comment: 4 pages, 4 figures (3 in color), 2 table
Statistical models for quantifying diagnostic accuracy with multiple lesions per patient
We propose random-effects models to summarize and quantify the accuracy of the diagnosis of multiple lesions on a single image without assuming independence between lesions. The number of false-positive lesions was assumed to be distributed as a Poisson mixture, and the proportion of true-positive lesions was assumed to be distributed as a binomial mixture. We considered univariate and bivariate, both parametric and nonparametric mixture models. We applied our tools to simulated data and data of a study assessing diagnostic accuracy of virtual colonography with computed tomography in 200 patients suspected of having one or more polyps
Nanostratification of optical excitation in self-interacting 1D arrays
The major assumption of the Lorentz-Lorenz theory about uniformity of local
fields and atomic polarization in dense material does not hold in finite groups
of atoms, as we reported earlier [A. E. Kaplan and S. N. Volkov, Phys. Rev.
Lett., v. 101, 133902 (2008)]. The uniformity is broken at sub-wavelength
scale, where the system may exhibit strong stratification of local field and
dipole polarization, with the strata period being much shorter than the
incident wavelength. In this paper, we further develop and advance that theory
for the most fundamental case of one-dimensional arrays, and study nanoscale
excitation of so called "locsitons" and their standing waves (strata) that
result in size-related resonances and related large field enhancement in finite
arrays of atoms. The locsitons may have a whole spectrum of spatial
frequencies, ranging from long waves, to an extent reminiscent of ferromagnetic
domains, -- to super-short waves, with neighboring atoms alternating their
polarizations, which are reminiscent of antiferromagnetic spin patterns. Of
great interest is the new kind of "hybrid" modes of excitation, greatly
departing from any magnetic analogies. We also study differences between
Ising-like near-neighbor approximation and the case where each atom interacts
with all other atoms in the array. We find an infinite number of "exponential
eigenmodes" in the lossless system in the latter case. At certain "magic"
numbers of atoms in the array, the system may exhibit self-induced (but linear
in the field) cancellation of resonant local-field suppression. We also studied
nonlinear modes of locsitons and found optical bistability and hysteresis in an
infinite array for the simplest modes.Comment: 39 pages, 5 figures; v2: Added the Conclusions section, corrected a
typo in Eq. (5.3), corrected minor stylistic and grammatical imperfection
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