1,928 research outputs found
Conical defects in growing sheets
A growing or shrinking disc will adopt a conical shape, its intrinsic
geometry characterized by a surplus angle at the apex. If growth is slow,
the cone will find its equilibrium. Whereas this is trivial if , the
disc can fold into one of a discrete infinite number of states if is
positive. We construct these states in the regime where bending dominates,
determine their energies and how stress is distributed in them. For each state
a critical value of is identified beyond which the cone touches itself.
Before this occurs, all states are stable; the ground state has two-fold
symmetry.Comment: 4 pages, 4 figures, LaTeX, RevTeX style. New version corresponds to
the one published in PR
Experimentally induced incomplete burst fractures - a novel technique for calf and human specimens
Background: Fracture morphology is crucial for the clinical decision-making process preceding spinal fracture treatment. The presented experimental approach was designed in order to ensure reproducibility of induced fracture morphology.
Results: The presented method resulted in fracture morphology, found in clinical classification systems like the Magerl classification. In the calf spine samples, 70% displayed incomplete burst fractures corresponding to type A3.1 and A3.2 fractures. In all human samples, superior incomplete burst fractures (Magerl A3.1) were identified by an independent radiologist and spine surgeon.
Conclusions: The presented set up enables the first experimental means to reliably model and study distinct incomplete burst fracture patterns in an in vitro setting. Thus, we envisage this protocol to facilitate further studies on spine fracture treatment of incomplete burst fractures
De Branges spaces and Krein's theory of entire operators
This work presents a contemporary treatment of Krein's entire operators with
deficiency indices and de Branges' Hilbert spaces of entire functions.
Each of these theories played a central role in the research of both renown
mathematicians. Remarkably, entire operators and de Branges spaces are
intimately connected and the interplay between them has had an impact in both
spectral theory and the theory of functions. This work exhibits the
interrelation between Krein's and de Branges' theories by means of a functional
model and discusses recent developments, giving illustrations of the main
objects and applications to the spectral theory of difference and differential
operators.Comment: 37 pages, no figures. The abstract was extended. Typographical errors
were corrected. The bibliography style was change
Remarks on the method of comparison equations (generalized WKB method) and the generalized Ermakov-Pinney equation
The connection between the method of comparison equations (generalized WKB
method) and the Ermakov-Pinney equation is established. A perturbative scheme
of solution of the generalized Ermakov-Pinney equation is developed and is
applied to the construction of perturbative series for second-order
differential equations with and without turning points.Comment: The collective of the authors is enlarged and the calculations in
Sec. 3 are correcte
Inflationary Perturbations: the Cosmological Schwinger Effect
This pedagogical review aims at presenting the fundamental aspects of the
theory of inflationary cosmological perturbations of quantum-mechanical origin.
The analogy with the well-known Schwinger effect is discussed in detail and a
systematic comparison of the two physical phenomena is carried out. In
particular, it is demonstrated that the two underlying formalisms differ only
up to an irrelevant canonical transformation. Hence, the basic physical
mechanisms at play are similar in both cases and can be reduced to the
quantization of a parametric oscillator leading to particle creation due to the
interaction with a classical source: pair production in vacuum is therefore
equivalent to the appearance of a growing mode for the cosmological
fluctuations. The only difference lies in the nature of the source: an electric
field in the case of the Schwinger effect and the gravitational field in the
case of inflationary perturbations. Although, in the laboratory, it is
notoriously difficult to produce an electric field such that pairs extracted
from the vacuum can be detected, the gravitational field in the early universe
can be strong enough to lead to observable effects that ultimately reveal
themselves as temperature fluctuations in the Cosmic Microwave Background.
Finally, the question of how quantum cosmological perturbations can be
considered as classical is discussed at the end of the article.Comment: 49 pages, 6 figures, to appear in a LNP volume "Inflationary
Cosmology
Predicted gamma-ray line emission from the Cygnus complex
The Cygnus region harbours a huge complex of massive stars at a distance of
1.0-2.0kpc from us. About 170 O stars are distributed over several OB
associations, among which the Cyg OB2 cluster is by far the most important with
about 100-120 O stars. These massive stars inject large quantities of
radioactive nuclei into the interstellar medium, such as 26Al and 60Fe, and
their gamma-ray line decay signals can provide insight into the physics of
massive stars and core-collapse supernovae. Past studies of the nucleosynthesis
activity of Cygnus have concluded that the level of 26Al decay emission as
deduced from CGRO/COMPTEL observations was a factor 2-3 above the predictions
based on the theoretical yields available at that time and on the observed
stellar content of the Cygnus region. We reevaluate the situation from new
measurements of the gamma-ray decay fluxes with INTEGRAL/SPI and new
predictions based on recently improved stellar models including some of the
effects of stellar rotation for the higher mass stars and a coherent estimate
of the contribution from SNIb/c. We developed a population synthesis code to
predict the nucleosynthesis activity and corresponding decay fluxes of a given
stellar population of massive stars. The observed decay fluxes from the Cygnus
complex are found to be consistent with the values predicted by population
synthesis at solar metallicity. The observed extent of the 1809keV emission
from Cygnus is found to be consistent with the result of a numerical simulation
of the diffusion of 26Al inside the superbubble blown by Cyg OB2. Our work
indicates that the past dilemma regarding the gamma-ray line emission from
Cygnus resulted from an overestimate of the 1809keV flux of the Cygnus complex,
combined with an underestimate of the nucleosynthesis yields.Comment: 13 pages, 9 figures, accepted for publication in A&
Bohr-Sommerfeld quantization and meson spectroscopy
We use the Bohr-Sommerfeld quantization approach in the context of
constituent quark models. This method provides, for the Cornell potential,
analytical formulae for the energy spectra which closely approximate numerical
exact calculations performed with the Schrodinger or the spinless Salpeter
equations. The Bohr-Sommerfeld quantization procedure can also be used to
calculate other observables such as r.m.s. radius or wave function at the
origin. Asymptotic dependence of these observables on quantum numbers are also
obtained in the case of potentials which behave asymptotically as a power-law.
We discuss the constraints imposed by these formulae on the dynamics of the
quark-antiquark interaction.Comment: 13 page
New observations on test architecture and construction of Jullienella foetida Schlumberger, 1890, the largest shallow-water agglutinated foraminifer in modern oceans
We present new observations on Jullienella foetida Schlumberger, 1890, a giant agglutinated foraminifer with a leaf- or fan-like test reaching a maximum dimension of 14 cm, that is common on some parts of the west African continental shelf. The test wall comprises a smooth, outer veneer of small (7.0 g wet weight m−2 for the seafloor biomass of J. foetida in areas where it is particularly abundant. The relatively restricted distribution of this species off the north-west African coast at depths above 100 m is probably related to the elevated, upwelling-related surface productivity along this margin, which provides enough food to sustain this high biomass. This remarkable species appears to play an important, perhaps keystone, role in benthic ecosystems where it is abundant, providing the only common hard substrate on which sessile organisms can settle
Sharp interface limits of phase-field models
The use of continuum phase-field models to describe the motion of
well-defined interfaces is discussed for a class of phenomena, that includes
order/disorder transitions, spinodal decomposition and Ostwald ripening,
dendritic growth, and the solidification of eutectic alloys. The projection
operator method is used to extract the ``sharp interface limit'' from phase
field models which have interfaces that are diffuse on a length scale . In
particular,phase-field equations are mapped onto sharp interface equations in
the limits and , where and are
respectively the interface curvature and velocity and is the diffusion
constant in the bulk. The calculations provide one general set of sharp
interface equations that incorporate the Gibbs-Thomson condition, the
Allen-Cahn equation and the Kardar-Parisi-Zhang equation.Comment: 17 pages, 9 figure
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