4,079 research outputs found
Strongly Localized State of a Photon at the Intersection of the Phase Slips in 2D Photonic Crystal with Low Contrast of Dielectric Constant
Two-dimensional photonic crystal with a rectangular symmetry and low contrast
(< 1) of the dielectric constant is considered. We demonstrate that, despite
the {\em absence} of a bandgap, strong localization of a photon can be achieved
for certain ``magic'' geometries of a unit cell by introducing two
phase slips along the major axes. Long-living photon mode is bound to the
intersection of the phase slips. We calculate analytically the lifetime of this
mode for the simplest geometry -- a square lattice of cylinders of a radius,
. We find the magic radius, , of a cylinder to be 43.10 percent of the
lattice constant. For this value of , the quality factor of the bound mode
exceeds . Small () deviation of from results in a
drastic damping of the bound mode.Comment: 6 pages, 2 figure
Evidence for Extremely High Dust Polarization Efficiency in NGC 3184
Recent studies have found the Type II-plateau supernova (SN) 1999gi to be
highly polarized (p_max = 5.8%, where p_max is the highest degree of
polarization measured in the optical bandpass; Leonard & Filippenko 2001) and
minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From
multiple lines of evidence, including the convincing fit of a ``Serkowski''
interstellar polarization (ISP) curve to the continuum polarization shape, we
conclude that the bulk of the observed polarization is likely due to dust along
the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new
spectropolarimetric observations of four distant Galactic stars close to the
l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to
within 0.2%, effectively eliminating Galactic dust as the cause of the high
polarization. The high ISP coupled with the low reddening implies an
extraordinarily high polarization efficiency for the dust along this l-o-s in
NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with
the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the
highest polarization efficiency yet confirmed for a single sight line in either
the Milky Way or an external galaxy.Comment: 27 pages, accepted for publication by the Astronomical Journa
Selecting coastal hotspots to storm impacts at the regional scale: a Coastal Risk Assessment Framework
Managing coastal risk at the regional scale requires a prioritization of resources along the shoreline. A transparent and rigorous risk assessment should inform managers and stakeholders in their choices. This requires advances in modelling assessment (e.g., consideration of source and pathway conditions to define the probability of occurrence, nonlinear dynamics of the physical processes, better recognition of systemic impacts and non-economic losses) and open-source tools facilitating stakeholders’ engagement in the process.
This paper discusses how the Coastal Risk Assessment Framework (CRAF) has been developed as part of the Resilience Increasing Strategies for Coasts Toolkit (RISC-KIT). The framework provides two levels of analysis. A coastal index approach is first recommended to narrow down the risk analysis to a reduced number of sectors which are subsequently geographically grouped into potential hotspots. For the second level of analysis an integrated modelling approach improves the regional risk assessment of the identified hotspots by increasing the spatial resolution of the hazard modelling by using innovative process-based multi-hazard models, by including generic vulnerability indicators in the impact assessment, and by calculating regional systemic impact indicators. A multi-criteria analysis of these indicators is performed to rank the hotspots and support the stakeholders in their selection.
The CRAF has been applied and validated on ten European case studies with only small deviation to areas already recognised as high risk. The flexibility of the framework is essential to adapt the assessment to the specific region characteristics. The involvement of stakeholders is crucial not only to select the hotpots and validate the results, but also to support the collection of information and the valuation of assets at risk. As such, the CRAF permits a comprehensive and systemic risk analysis of the regional coast in order to identify and to select higher risk areas. Yet efforts still need to be amplified in the data collection process, in particular for socio-economic and environmental impacts
Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized -Pu
The effects on the local structure due to self-irradiation damage of Ga
stabilized -Pu stored at cryogenic temperatures have been examined
using extended x-ray absorption fine structure (EXAFS) experiments. Extensive
damage, seen as a loss of local order, was evident after 72 days of storage
below 15 K. The effect was observed from both the Pu and Ga sites, although
less pronounced around Ga. Isochronal annealing was performed on this sample to
study the annealing processes that occur between cryogenic and room temperature
storage conditions, where damage is mostly reversed. Damage fractions at
various points along the annealing curve have been determined using an
amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite
model, and provide information complementary to previous electrical
resistivity- and susceptibility-based isochronal annealing studies. The use of
a spherical crystallite model accounts for the changes in EXAFS spectra using
just two parameters, namely, the crystalline fraction and the particle radius.
Together, these results are discussed in terms of changes to the local
structure around Ga and Pu throughout the annealing process and highlight the
unusual role of Ga in the behavior of the lowest temperature anneals.Comment: 13 pages, 10 figure
Spectropolarimetry of the Type IIb Supernova 2001ig
We present spectropolarimetric observations of the Type IIb SN 2001ig in NGC
7424; conducted with the ESO VLT FORS1 on 2001 Dec 16, 2002 Jan 3 and 2002 Aug
16 or 13, 31 and 256 days post-explosion. These observations are at three
different stages of the SN evolution: (1) The hydrogen-rich photospheric phase,
(2) the Type II to Type Ib transitional phase and (3) the nebular phase. At
each of these stages, the observations show remarkably different polarization
properties as a function of wavelength. We show that the degree of interstellar
polarization is 0.17%. The low intrinsic polarization (~0.2%) at the first
epoch is consistent with an almost spherical (<10% deviation from spherical
symmetry) hydrogen dominated ejecta. Similar to SN 1987A and to Type IIP SNe, a
sharp increase in the degree of the polarization (~1%) is observed when the
outer hydrogen layer becomes optically thin by day 31; only at this epoch is
the polarization well described by a ``dominant axis.'' The polarization angle
of the data shows a rotation through ~40 degrees between the first and second
epochs, indicating that the asymmetries of the first epoch were not directly
coupled with those observed at the second epoch. For the most polarized lines,
we observe wavelength-dependent loop structures in addition to the dominant
axis on the Q-U plane. We show that the polarization properties of Type IIb SNe
are roughly similar to one another, but with significant differences arising
due to line blending effects especially with the high velocities observed for
SN 2001ig. This suggests that the geometry of SN 2001ig is related to SN 1993J
and that these events may have arisen from a similar binary progenitor system.Comment: 42 pages, 12 figures (figs. 11 and 12 are both composed of four
subpanels, figs. 6,7,8,11 and 12 are in color, fig. 1 is low res and a high
res version is available at http://www.as.utexas.edu/~jrm/), ApJ Accepte
Superconductivity in the Correlated Pyrochlore Cd_2Re_2O_7
We report the observation of superconductivity in high-quality
CdReO single crystals with room-temperature pyrochlore structure.
Resistivity and ac susceptibility measurements establish an onset transition
temperature T = 1.47 K with transition width T = 0.25
K. In applied magnetic field, the resistive transition shows a type-II
character, with an approximately linear temperature-dependence of the upper
critical field H. The bulk nature of the superconductivity is confirmed
by the specific heat jump with C = 37.9 mJ/mol-K. Using the
value extracted from normal-state specific heat data, we obtain
C/T = 1.29, close to the weak coupling BCS value. In the
normal state, a negative Hall coefficient below 100 K suggests electron-like
conduction in this material. The resistivity exhibits a quadratic T-dependence
between 2 and 60 K, i.e., +AT, indicative of Fermi-liquid
behavior. The values of the Kadowaki-Woods ratio A/ and the Wilson
ratio are comparable to that for strongly correlated materials.Comment: 4 pages, 5 figure
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
Spectroscopy and dissociative recombination of the lowest rotational states of H3+
The dissociative recombination of the lowest rotational states of H3+ has
been investigated at the storage ring TSR using a cryogenic 22-pole
radiofrequency ion trap as injector. The H3+ was cooled with buffer gas at ~15
K to the lowest rotational levels, (J,G)=(1,0) and (1,1), which belong to the
ortho and para proton-spin symmetry, respectively. The rate coefficients and
dissociation dynamics of H3+(J,G) populations produced with normal- and para-H2
were measured and compared to the rate and dynamics of a hot H3+ beam from a
Penning source. The production of cold H3+ rotational populations was
separately studied by rovibrational laser spectroscopy using chemical probing
with argon around 55 K. First results indicate a ~20% relative increase of the
para contribution when using para-H2 as parent gas. The H3+ rate coefficient
observed for the para-H2 source gas, however, is quite similar to the H3+ rate
for the normal-H2 source gas. The recombination dynamics confirm that for both
source gases, only small populations of rotationally excited levels are
present. The distribution of 3-body fragmentation geometries displays a broad
part of various triangular shapes with an enhancement of ~12% for events with
symmetric near-linear configurations. No large dependences on internal state or
collision energy are found.Comment: 10 pages, 9 figures, to be published in Journal of Physics:
Conference Proceeding
Muon spin rotation measurements of the superfluid density in fresh and aged superconducting PuCoGa
We have measured the temperature dependence and magnitude of the superfluid
density via the magnetic field penetration depth
in PuCoGa (nominal critical temperature K) using the muon
spin rotation technique in order to investigate the symmetry of the order
parameter, and to study the effects of aging on the superconducting properties
of a radioactive material. The same single crystals were measured after 25 days
( K) and 400 days ( K) of aging at room temperature.
The temperature dependence of the superfluid density is well described in both
materials by a model using d-wave gap symmetry. The magnitude of the muon spin
relaxation rate in the aged sample, , where is the effective mass, is reduced by
about 70% compared to fresh sample. This indicates that the scattering from
self-irradiation induced defects is not in the limit of the conventional
Abrikosov-Gor'kov pair-breaking theory, but rather in the limit of short
coherence length (about 2 nm in PuCoGa) superconductivity.Comment: 11 page
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