401 research outputs found
Auxetic regions in large deformations of periodic frameworks
In materials science, auxetic behavior refers to lateral widening upon
stretching. We investigate the problem of finding domains of auxeticity in
global deformation spaces of periodic frameworks. Case studies include planar
periodic mechanisms constructed from quadrilaterals with diagonals as periods
and other frameworks with two vertex orbits. We relate several geometric and
kinematic descriptions.Comment: Presented at the International Conference on "Interdisciplinary
Applications of Kinematics" (IAK18), Lima, Peru, March 201
Root asymptotics of spectral polynomials for the Lame operator
The study of polynomial solutions to the classical Lam\'e equation in its
algebraic form, or equivalently, of double-periodic solutions of its
Weierstrass form has a long history. Such solutions appear at integer values of
the spectral parameter and their respective eigenvalues serve as the ends of
bands in the boundary value problem for the corresponding Schr\"odinger
equation with finite gap potential given by the Weierstrass -function on
the real line. In this paper we establish several natural (and equivalent)
formulas in terms of hypergeometric and elliptic type integrals for the density
of the appropriately scaled asymptotic distribution of these eigenvalues when
the integer-valued spectral parameter tends to infinity. We also show that this
density satisfies a Heun differential equation with four singularities.Comment: final version, to appear in Commun. Math. Phys.; 13 pages, 3 figures,
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Spectroscopy of P using the one-proton knockout reaction
The structure of P was studied with a one-proton knockout reaction
at88~MeV/u from a S projectile beam at NSCL. The rays from
thedepopulation of excited states in P were detected with GRETINA,
whilethe P nuclei were identified event-by-event in the focal plane of
theS800 spectrograph. The level scheme of P was deduced up to 7.5 MeV
using coincidences. The observed levels were attributed to
protonremovals from the -shell and also from the deeply-bound
orbital.The orbital angular momentum of each state was derived from the
comparisonbetween experimental and calculated shapes of individual
(-gated)parallel momentum distributions. Despite the use of different
reactions andtheir associate models, spectroscopic factors, , derived
from theS knockout reaction agree with those obtained earlier
fromS(,\nuc{3}{He}) transfer, if a reduction factor , as
deducedfrom inclusive one-nucleon removal cross sections, is applied to the
knockout transitions.In addition to the expected proton-hole configurations,
other states were observedwith individual cross sections of the order of
0.5~mb. Based on their shiftedparallel momentum distributions, their decay
modes to negative parity states,their high excitation energy (around 4.7~MeV)
and the fact that they were notobserved in the (,\nuc{3}{He}) reaction, we
propose that they may resultfrom a two-step mechanism or a nucleon-exchange
reaction with subsequent neutronevaporation. Regardless of the mechanism, that
could not yet be clarified, thesestates likely correspond to neutron core
excitations in \nuc{35}{P}. Thisnewly-identified pathway, although weak, offers
the possibility to selectivelypopulate certain intruder configurations that are
otherwise hard to produceand identify.Comment: 5 figures, 1 table, accepted for publication in Physical Review
Generalized Borcea-Voisin Construction
C. Voisin and C. Borcea have constructed mirror pairs of families of
Calabi-Yau threefolds by taking the quotient of the product of an elliptic
curve with a K3 surface endowed with a non-symplectic involution. In this
paper, we generalize the construction of Borcea and Voisin to any prime order
and build three and four dimensional Calabi-Yau orbifolds. We classify the
topological types that are obtained and show that, in dimension 4, orbifolds
built with an involution admit a crepant resolution and come in topological
mirror pairs. We show that for odd primes, there are generically no minimal
resolutions and the mirror pairing is lost.Comment: 15 pages, 2 figures. v2: typos corrected & references adde
Filtering Deterministic Layer Effects in Imaging
Sensor array imaging arises in applications such as nondestructive evaluation of materials
with ultrasonic waves, seismic exploration, and radar. The sensors probe a medium with
signals and record the resulting echoes, which are then processed to determine the location
and reflectivity of remote reflectors. These could be defects in materials such as voids,
fault lines or salt bodies in the earth, and cars, buildings, or aircraft in radar applications.
Imaging is relatively well understood when the medium through which the signals
propagate is smooth, and therefore nonscattering. But in many problems the medium is
heterogeneous, with numerous small inhomogeneities that scatter the waves. We refer to
the collection of inhomogeneities as clutter, which introduces an uncertainty in imaging
because it is unknown and impossible to estimate in detail. We model the clutter as a
random process. The array data is measured in one realization of the random medium,
and the challenge is to mitigate cumulative clutter scattering so as to obtain robust images
that are statistically stable with respect to different realizations of the inhomogeneities.
Scatterers that are not buried too deep in clutter can be imaged reliably with the coherent
interferometric (CINT) approach. But in heavy clutter the signal-to-noise ratio (SNR)
is low and CINT alone does not work. The “signal,” the echoes from the scatterers to be
imaged, is overwhelmed by the “noise,” the strong clutter reverberations. There are two
existing approaches for imaging at low SNR: The first operates under the premise that data
are incoherent so that only the intensity of the scattered field can be used. The unknown
coherent scatterers that we want to image are modeled as changes in the coefficients of
diffusion or radiative transport equations satisfied by the intensities, and the problem becomes
one of parameter estimation. Because the estimation is severely ill-posed, the results
have poor resolution, unless very good prior information is available and large arrays are
used. The second approach recognizes that if there is some residual coherence in the data,
that is, some reliable phase information is available, it is worth trying to extract it and
use it with well-posed coherent imaging methods to obtain images with better resolution.
This paper takes the latter approach and presents a first attempt at enhancing the SNR
of the array data by suppressing medium reverberations. It introduces filters, or annihilators of layer backscatter, that are designed to remove primary echoes from strong, isolated
layers in a medium with additional random layering at small, subwavelength scales. These
strong layers are called deterministic because they can be imaged from the data. However,
our goal is not to image the layers, but to suppress them and thus enhance the echoes
from compact scatterers buried deep in the medium. Surprisingly, the layer annihilators
work better than intended, in the sense that they suppress not only the echoes from the
deterministic layers, but also multiply scattered ones in the randomly layered structure.
Following the layer annihilators presented here, other filters of general, nonlayered
heavy clutter have been developed. We review these more recent developments and the
challenges of imaging in heavy clutter in the introduction in order to place the research
presented here in context. We then present in detail the layer annihilators and show with
analysis and numerical simulations how they work
The photometric properties of a vast stellar substructure in the outskirts of M33
We have surveyed sq.degrees surrounding M33 with CFHT MegaCam in the
g and i filters, as part of the Pan-Andromeda Archaeological Survey. Our
observations are deep enough to resolve the top 4mags of the red giant branch
population in this galaxy. We have previously shown that the disk of M33 is
surrounded by a large, irregular, low-surface brightness substructure. Here, we
quantify the stellar populations and structure of this feature using the PAndAS
data. We show that the stellar populations of this feature are consistent with
an old population with dex and an interquartile range in
metallicity of dex. We construct a surface brightness map of M33 that
traces this feature to mags\,arcsec. At these low surface
brightness levels, the structure extends to projected radii of kpc from
the center of M33 in both the north-west and south-east quadrants of the
galaxy. Overall, the structure has an "S-shaped" appearance that broadly aligns
with the orientation of the HI disk warp. We calculate a lower limit to the
integrated luminosity of the structure of mags, comparable to a
bright dwarf galaxy such as Fornax or AndII and slightly less than $1\$ of the
total luminosity of M33. Further, we show that there is tentative evidence for
a distortion in the distribution of young stars near the edge of the HI disk
that occurs at similar azimuth to the warp in HI. The data also hint at a
low-level, extended stellar component at larger radius that may be a M33 halo
component. We revisit studies of M33 and its stellar populations in light of
these new results, and we discuss possible formation scenarios for the vast
stellar structure. Our favored model is that of the tidal disruption of M33 in
its orbit around M31.Comment: Accepted for publication in ApJ. 17 figures. ApJ preprint forma
The nuclear shell effects near the r-process path in the relativistic Hartree-Bogoliubov theory
We have investigated the evolution of the shell structure of nuclei in going
from the r-process path to the neutron drip line within the framework of the
Relativistic Hartree-Bogoliubov (RHB) theory. By introducing the quartic
self-coupling of meson in the RHB theory in addition to the non-linear
scalar coupling of meson, we reproduce the available data on the shell
effects about the waiting-point nucleus Zn. With this approach, it is
shown that the shell effects at N=82 in the inaccessible region of the
r-process path become milder as compared to the Lagrangian with the scalar
self-coupling only. However, the shell effects remain stronger as compared to
the quenching exhibited by the HFB+SkP approach. It is also shown that in
reaching out to the extreme point at the neutron drip line, a terminal
situation arises where the shell structure at the magic number is washed out
significantly.Comment: 18 pages (revtex), 8 ps figures, to appear in Phys. Rev.
Non-Perturbative Corrections and Modularity in N=1 Type IIB Compactifications
Non-perturbative corrections and modular properties of four-dimensional type
IIB Calabi-Yau orientifolds are discussed. It is shown that certain
non-perturbative alpha' corrections survive in the large volume limit of the
orientifold and periodically correct the Kahler potential. These corrections
depend on the NS-NS two form and have to be completed by D-instanton
contributions to transform covariantely under symmetries of the type IIB
orientifold background. It is shown that generically also the D-instanton
superpotential depends on the two-form moduli as well as on the complex
dilaton. These contributions can arise through theta-functions with the dilaton
as modular parameter. An orientifold of the Enriques Calabi-Yau allows to
illustrate these general considerations. It is shown that this compactification
leads to a controlled four-dimensional N=1 effective theory due to the absence
of various quantum corrections. Making contact to the underlying topological
string theory the D-instanton superpotential is proposed to be related to a
specific modular form counting D3, D1, D(-1) degeneracies on the Enriques
Calabi-Yau.Comment: 35 page
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