2,196 research outputs found
Translation Representations and Scattering By Two Intervals
Studying unitary one-parameter groups in Hilbert space (U(t),H), we show that
a model for obstacle scattering can be built, up to unitary equivalence, with
the use of translation representations for L2-functions in the complement of
two finite and disjoint intervals.
The model encompasses a family of systems (U (t), H). For each, we obtain a
detailed spectral representation, and we compute the scattering operator, and
scattering matrix. We illustrate our results in the Lax-Phillips model where (U
(t), H) represents an acoustic wave equation in an exterior domain; and in
quantum tunneling for dynamics of quantum states
Green functions and propagation of waves in strongly inhomogeneous media
We show that Green functions of second-order differential operators with
singular or unbounded coefficients can have an anomalous behaviour in
comparison to the well-known properties of Green functions of operators with
bounded coefficients. We discuss some consequences of such an anomalous short
or long distance behaviour for a diffusion and wave propagation in an
inhomogeneous medium
A quantitative assessment of shoot flammability for 60 tree and shrub species supports rankings based on expert opinion
Fire is an important ecological disturbance in vegetated ecosystems across the globe, and also has considerable impacts on human infrastructure. Vegetation flammability is a key bottom-up control on fire regimes, and on the nature of individual fires. Although New Zealand (NZ) historically had low fire frequencies, anthropogenic fires have considerably impacted indigenous vegetation as humans used fire extensively to clear forests. Few studies of vegetation flammability have been undertaken in NZ, and only one has compared the flammability of indigenous plants; this was a qualitative assessment derived from expert opinion. We addressed this knowledge gap by measuring the flammability of terminal shoots from a range of trees and shrubs found in NZ. We quantified shoot flammability of 60 indigenous and exotic species, and compared our experimentally derived ranking with expert opinion. The most flammable species was the invasive exotic shrub Ulex europaeus, followed by Eucalyptus viminalis, Pomaderris kumeraho, Dacrydium cupressinum, and Lophozonia menziesii. Our experimentally derived ranking was strongly correlated with expert opinion, lending support to both methods. Our results are useful to ecologists seeking to understand how fires have and will influence NZâs ecosystems, and for fire managers identifying high-risk landscapes, and low flammability species for âgreen firebreaksâ
Comparison of absolute gain photometric calibration between Planck/HFI and Herschel/SPIRE at 545 and 857 GHz
We compare the absolute gain photometric calibration of the Planck/HFI and
Herschel/SPIRE instruments on diffuse emission. The absolute calibration of HFI
and SPIRE each relies on planet flux measurements and comparison with
theoretical far-infrared emission models of planetary atmospheres. We measure
the photometric cross calibration between the instruments at two overlapping
bands, 545 GHz / 500 m and 857 GHz / 350 m. The SPIRE maps used have
been processed in the Herschel Interactive Processing Environment (Version 12)
and the HFI data are from the 2015 Public Data Release 2. For our study we used
15 large fields observed with SPIRE, which cover a total of about 120 deg^2. We
have selected these fields carefully to provide high signal-to-noise ratio,
avoid residual systematics in the SPIRE maps, and span a wide range of surface
brightness. The HFI maps are bandpass-corrected to match the emission observed
by the SPIRE bandpasses. The SPIRE maps are convolved to match the HFI beam and
put on a common pixel grid. We measure the cross-calibration relative gain
between the instruments using two methods in each field, pixel-to-pixel
correlation and angular power spectrum measurements. The SPIRE / HFI relative
gains are 1.047 ( 0.0069) and 1.003 ( 0.0080) at 545 and 857 GHz,
respectively, indicating very good agreement between the instruments. These
relative gains deviate from unity by much less than the uncertainty of the
absolute extended emission calibration, which is about 6.4% and 9.5% for HFI
and SPIRE, respectively, but the deviations are comparable to the values 1.4%
and 5.5% for HFI and SPIRE if the uncertainty from models of the common
calibrator can be discounted. Of the 5.5% uncertainty for SPIRE, 4% arises from
the uncertainty of the effective beam solid angle, which impacts the adopted
SPIRE point source to extended source unit conversion factor (Abridged)Comment: 13 pages, 10 figures; Incorporates revisions in response to referee
comments; cross calibration factors unchange
Bounds on the heat kernel of the Schroedinger operator in a random electromagnetic field
We obtain lower and upper bounds on the heat kernel and Green functions of
the Schroedinger operator in a random Gaussian magnetic field and a fixed
scalar potential. We apply stochastic Feynman-Kac representation, diamagnetic
upper bounds and the Jensen inequality for the lower bound. We show that if the
covariance of the electromagnetic (vector) potential is increasing at large
distances then the lower bound is decreasing exponentially fast for large
distances and a large time.Comment: some technical improvements, new references, to appear in
Journ.Phys.
Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2
Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the
pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc
reduction, but inconsistent with the expectations of the dirty limit.
Dirty-limit theory for undoped MgB2 predicts a single gap about three times
larger than measured and a reduced Tc comparable to that measured. Our
heavily-doped samples exhibit a uniform dispersion of C suggestive of
significantly enhanced scattering, and we conclude that the retention of
two-band superconductivity in these samples is caused by a selective
suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor
changes to the text, manuscript accepted for publication as a Phys. Rev. B
Rapid Communicatio
A magnetization and B NMR study of MgAlB superconductors
We demonstrate for the first time the magnetic field distribution of the pure
vortex state in lightly doped MgAlB () powder
samples, by using B NMR in magnetic fields of 23.5 and 47 kOe. The
magnetic field distribution at T=5 K is Al-doping dependent, revealing a
considerable decrease of anisotropy in respect to pure MgB. This result
correlates nicely with magnetization measurements and is consistent with
-band hole driven superconductivity for MgB
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
Indirect signals from light neutralinos in supersymmetric models without gaugino mass unification
We examine indirect signals produced by neutralino self-annihilations, in the
galactic halo or inside celestial bodies, in the frame of an effective MSSM
model without gaugino-mass unification at a grand unification scale. We compare
our theoretical predictions with current experimental data of gamma-rays and
antiprotons in space and of upgoing muons at neutrino telescopes. Results are
presented for a wide range of the neutralino mass, though our discussions are
focused on light neutralinos. We find that only the antiproton signal is
potentially able to set constraints on very low-mass neutralinos, below 20 GeV.
The gamma-ray signal, both from the galactic center and from high galactic
latitudes, requires significantly steep profiles or substantial clumpiness in
order to reach detectable levels. The up-going muon signal is largely below
experimental sensitivities for the neutrino flux coming from the Sun; for the
flux from the Earth an improvement of about one order of magnitude in
experimental sensitivities (with a low energy threshold) can make accessible
neutralino masses close to O, Si and Mg nuclei masses, for which resonant
capture is operative.Comment: 17 pages, 1 tables and 5 figures, typeset with ReVTeX4. The paper may
also be found at http://www.to.infn.it/~fornengo/papers/indirect04.ps.gz or
through http://www.astroparticle.to.infn.it/. Limit from BR(Bs--> mu+ mu-)
adde
The Atacama Cosmology Telescope: The polarization-sensitive ACTPol instrument
The Atacama Cosmology Telescope (ACT) is designed to make high angular
resolution measurements of anisotropies in the Cosmic Microwave Background
(CMB) at millimeter wavelengths. We describe ACTPol, an upgraded receiver for
ACT, which uses feedhorn-coupled, polarization-sensitive detector arrays, a 3
degree field of view, 100 mK cryogenics with continuous cooling, and meta
material anti-reflection coatings. ACTPol comprises three arrays with separate
cryogenic optics: two arrays at a central frequency of 148 GHz and one array
operating simultaneously at both 97 GHz and 148 GHz. The combined instrument
sensitivity, angular resolution, and sky coverage are optimized for measuring
angular power spectra, clusters via the thermal Sunyaev-Zel'dovich and kinetic
Sunyaev-Zel'dovich signals, and CMB lensing due to large scale structure. The
receiver was commissioned with its first 148 GHz array in 2013, observed with
both 148 GHz arrays in 2014, and has recently completed its first full season
of operations with the full suite of three arrays. This paper provides an
overview of the design and initial performance of the receiver and related
systems
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