812 research outputs found
Collective modes of CP(3) Skyrmion crystals in quantum Hall ferromagnets
The two-dimensional electron gas in a bilayer quantum Hall system can sustain
an interlayer coherence at filling factor nu=1 even in the absence of tunneling
between the layers. This system has low-energy charged excitations which may
carry textures in real spin or pseudospin. Away from filling factor nu =1 a
finite density of these is present in the ground state of the 2DEG and forms a
crystal. Depending on the relative size of the various energy scales, such as
tunneling (Delta_SAS), Zeeman coupling (Delta_Z) or electrical bias (Delta_b),
these textured crystal states can involve spin, pseudospin, or both
intertwined. In this article, we present a comprehensive numerical study of the
collective excitations of these textured crystals using the GRPA. For the pure
spin case, at finite Zeeman coupling the state is a Skyrmion crystal with a
gapless phonon mode, and a separate Goldstone mode that arises from a broken
U(1) symmetry. At zero Zeeman coupling, we demonstrate that the constituent
Skyrmions break up, and the resulting state is a meron crystal with 4 gapless
modes. In contrast, a pure pseudospin Skyrme crystal at finite tunneling has
only the phonon mode. For Delta_SAS=0, the state evolves into a meron crystal
and supports an extra gapless U(1) mode in addition to the phonon. For a CP(3)
Skyrmion crystal, we find a U(1) gapless mode in the presence of the
symmetry-breaking fields. In addition, a second mode with a very small gap is
present in the spectrum.Comment: 16 pages and 12 eps figure
Effects of Forest Fire on Young-of-the-year Northern Pike, Esox lucius, in the Northwest Territories
In 1998, a forest fire burned 58% of the forested shoreline surrounding Tibbitt Lake, Northwest Territories, including riparian vegetation used by Northern Pike (Esox lucius) as spawning habitat. This presented an opportunity to investigate the effects that habitat disturbance from a natural forest fire had on young-of-the-year (Y-O-Y) Northern Pike. Pike fry were collected from three burned and three unburned sites around Tibbitt Lake in 1999 (the first post-fire spawning season) and again in 2001. Differences in size and relative abundance were evaluated between sites. Y-O-Y Northern Pike were significantly larger at the unburned sites (P<0.01) and the relative abundance of Y-O-Y Northern Pike increased significantly at burned sites (alpha=0.1; P<0.07) following re-vegetation two years post fire. These differences may be due to fire-induced changes in physical habitat or food availability. Forest fires decrease the density of riparian vegetation, which likely provides better spawning and rearing habitat for Northern Pike in the long-term.En 1998, un feu de forĂȘt a brĂ»lĂ© 58% de la rive boisĂ©e entourant le lac Tibbitt, situĂ© dans les Territoires du Nord-Ouest, y compris la vĂ©gĂ©tation riveraine utilisĂ© par le grand brochet (Esox lucius) comme habitat de fraie. Cet Ă©vĂ©nement a prĂ©sentĂ© une occasion pour Ă©tudier les effets causĂ©s par un feu de foret sur l'habitat de jeunes de l'annĂ©e du grand brochet. Les jeunes brochets ont Ă©tĂ© recueillis sur trois sites brĂ»lĂ©es et trois sites non brĂ»lĂ©s autour du lac Tibbitt en 1999 (la premiĂšre annĂ©e de fraie aprĂšs le feu de forĂȘt) et Ă nouveau en 2001. La diffĂ©rence de grandeur et l'abondance relative des poissons ont Ă©tĂ© analysĂ©s entre les diffĂ©rents sites. Les jeunes grands brochets aux sites non-atteints par le feu Ă©taient considĂ©rablement plus grands (P < 0,01) que ceux aux sites brulĂ©s. Les sites brulĂ©s, subissant une rapide revĂ©gĂ©tation dans les deux annĂ©es aprĂšs le feu, ont augmentĂ© en abondance relative de jeunes grands brochets (alpha = 0,1, P < 0,07). Ces diffĂ©rences peuvent ĂȘtre dues aux changements induits par le feu sur l'aspect physique de l'habitat ou sur la disponibilitĂ© de nourriture. Les incendies de forĂȘt rĂ©duisent la densitĂ© de la vĂ©gĂ©tation riveraine, qui fournit probablement de meilleure habitat de fraie et d'alevinage pour le grand brochet Ă long terme
Networks of nonlinear superconducting transmission line resonators
We investigate a network of coupled superconducting transmission line
resonators, each of them made nonlinear with a capacitively shunted Josephson
junction coupling to the odd flux modes of the resonator. The resulting
eigenmode spectrum shows anticrossings between the plasma mode of the shunted
junction and the odd resonator modes. Notably, we find that the combined device
can inherit the complete nonlinearity of the junction, allowing for a
description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID
instead of a single junction, the nonlinearity can be tuned between 10 kHz and
4 MHz while maintaining resonance frequencies of a few gigahertz for realistic
device parameters. An array of such nonlinear resonators can be considered a
scalable superconducting quantum simulator for a Bose-Hubbard Hamiltonian. The
device would be capable of accessing the strongly correlated regime and be
particularly well suited for investigating quantum many-body dynamics of
interacting particles under the influence of drive and dissipation.Comment: 18 pages, 3 figure
Estimating Wind Stress at the Ocean Surface From Scatterometer Observations
AbstractâWind stress is the most important ocean forcing for driving tropical surface currents. Stress can be estimated from scatterometer-reported wind measurements at 10 m that have been extrapolated to the surface, assuming a neutrally stable atmosphere and no surface current. Scatterometer calibration is designed to account for the assumption of neutral stability; however, the assumption of a particular sea state and negligible current often introduces an error in wind stress estimations. Since the fundamental scatterometer measurement is of the surface radar backscatter (sigma-0) which is related to surface roughness and, thus, stress, we develop a method to estimate wind stress directly from the scatterometer measurements of sigma-0 and their associated azimuth angle and incidence angle using a neural network approach. We compare the results with in situ estimations and observe that the wind stress estimations from this approach are more accurate compared with those obtained from the conventional estimations using 10-m-height wind measurements. Index TermsâAtmospheric stability, neutral stability, scatterometer, wind stress. I
Restenosis and its determinants in first and repeat coronary angioplasty
Restenosis is the main problem limiting long-term success of percutaneous transluminal coronary angioplasty (PTCA) and is most accurately evaluated by follow-up angiography. We compared the primary and long-term results of angioplasty in 268 consecutive patients (293 segments) with first PTCA (PTCA 1, angiographic follow-up 98%) and in 66 patients (76 segments) with repeat PTCA after restenosis (PTCA 2, angiographic follow-up 92%). Forty clinical, angiographic and procedural factors were assessed in relation to outcome. Primary success rate was higher in PTCA 2 (91% vs 67.5%) and major complications were fewer (4.5% vs 16%).Higher inflation pressure (7.9 ± 2.3 vs 6.8 ± 1.8 atm, P70%) after PTCA 1 and after PTC A 2 (27% vs 36%, P = NS) and the mean time to recurrence (4.7 vs 5.3 months, P = NS) were similar. Procedural factors were the main determinants of long-term success in primary PTCA. The restenosis risk was independently related to residual stenosis >45% (P<0.001), variant angina (P<0.05) and multivessel disease (P<0.05) after PTCA 1 and to male sex (P<0.001) and higher inflation pressure (P<0.05) after PTCA 2. Mild to moderate intimal tearing was associated with less restenosis after PTC A 1, but not after PTCA 2. Including 9 patients (10 segments) with a third PTCA, 70% of the 66 patients with repeat PTCA had a successful long-term outcome. Repeat angioplasty should therefore be considered as an integral part of PTCA therapy. Restenosis however remains a major concern. An optimal primary result with a minimal residual stenosis is decisive for first PTCA, whereas avoidance of a dissection by using lower inflation pressure on a restenosis might improve the long-term outcome of repeat PTC
Dynamics of electrons in the quantum Hall bubble phases
In Landau levels N > 1, the ground state of the two-dimensional electron gas
(2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for
small filling of the partially filled Landau level, into a succession of bubble
states with increasing number of guiding centers per bubble as the filling
increases, to a modulated stripe state near half filling. In this work, we show
that these first-order phase transitions between the bubble states lead to
measurable discontinuities in several physical quantities such as the density
of states and the magnetization of the 2DEG. We discuss in detail the behavior
of the collective excitations of the bubble states and show that their spectra
have higher-energy modes besides the pinned phonon mode. The frequencies of
these modes, at small wavevector k, have a discontinuous evolution as a
function of filling factor that should be measurable in, for example, microwave
absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40
Trend and variability in ozone in the tropical lower stratosphere over 2.5 solar cycles observed by SAGE II and OSIRIS
We have extended the satellite-based ozone anomaly time series to the
present (December 2012) by merging SAGE II (Stratospheric Aerosol and Gas Experiment
II) with OSIRIS (Optical Spectrograph and Infrared Imager System)
and correcting for the small bias (~0.5%) between them,
determined using their temporal overlap of 4 years. Analysis of the merged
data set (1984â2012) shows a statistically significant negative trend at all
altitudes in the 18â25 km range, including a trend of (â4.6 ± 2.6)% decade<sup>â1</sup>
at 19.5 km where the relative standard error is a minimum. We are
also able to replicate previously reported decadal trends in the tropical
lower-stratospheric ozone anomaly based on SAGE II observations.
Uncertainties are smaller on the merged trend than the SAGE II trend at all
altitudes. Underlying strong fluctuations in ozone anomaly due to El NiñoâSouthern Oscillation (ENSO), the altitude-dependent quasi-biennial
oscillation, and tropopause pressure need to be taken into account to reduce
trend uncertainties and, in the case of ENSO, to accurately determine the
linear trend just above the tropopause. We also compare the observed ozone
trend with a calculated trend that uses information on tropical upwelling
and its temporal trend from model simulations, tropopause pressure trend
information derived from reanalysis data, and vertical profiles from SAGE II
and OSIRIS to determine the vertical gradient of ozone and its trend. We
show that the observed trend agrees with the calculated trend and that the
magnitude of the calculated trend is dominated by increased tropical
upwelling, with minor but increasing contribution from the vertical ozone
gradient trend as the tropical tropopause is approached. Improvements are
suggested for future regression modelling efforts which could reduce trend
uncertainties and biases in trend magnitudes, thereby allowing accurate
trend detection to extend below 18 km
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