4,308 research outputs found
Schwinger-Keldysh approach to out of equilibrium dynamics of the Bose Hubbard model with time varying hopping
We study the real time dynamics of the Bose Hubbard model in the presence of
time-dependent hopping allowing for a finite temperature initial state. We use
the Schwinger-Keldysh technique to find the real-time strong coupling action
for the problem at both zero and finite temperature. This action allows for the
description of both the superfluid and Mott insulating phases. We use this
action to obtain dynamical equations for the superfluid order parameter as
hopping is tuned in real time so that the system crosses the superfluid phase
boundary. We find that under a quench in the hopping, the system generically
enters a metastable state in which the superfluid order parameter has an
oscillatory time dependence with a finite magnitude, but disappears when
averaged over a period. We relate our results to recent cold atom experiments.Comment: 22 pages, 7 figure
Colton Formation (Eocene: Fluviatile) and Associated Lacustrine Beds, Gunnison Plateau, Central Utah
Author Institution: Department of Geology, The Ohio State University, Columbus, Ohio 43210The Colton Formation consists mostly of fluvial (floodplain and channel) deposits that are less resistant and more highly colored than the adjacent lacustrine beds of the Flagstaff and Green River Formations. The contrast between the fluviatile and lacustrine strata is indicated by several lithic and faunal characteristics that occur in a short stratigraphic interval, and that permit precise and regionally persistent recognition of the limits of the Colton Formation. The Colton is distinguished by: 1) red, yellow, or brown, fine- to medium-grained, micaceous, feldspathic subangular sandstones that are crossbedded, thick-bedded, laterally non-persistent, and typically lens-shaped in cross-section; 2) red, green, or purple, somewhat silty or sandy mudrocks and shales that are poorly fossiliferous and poorly bedded; 3) limestone beds, locally of pastel colors, that are generally less than 1 foot thick, non-persistent, tough, platy and poorly fossiliferous; 4) far fewer fossil snails and clams than the regionally continuous carbonate units of the middle and upper Flagstaff Formation.
In contrast, lacustrine beds that bound the Colton Formation are characterized by the following features; 1) pale grey, fine-grained, subrounded sandstones, without crossbedding, that are rather thin and widespread (Flagstaff only); 2) yellow-brown or gray, thin-bedded, tough and platy limy shales that are fossiliferous and interbedded with limestone and dolomite units; 3) limestones and dolomites of several petrographic types that are two feet thick or more, and laterally persistent.
When distinguished by the sum of these characteristics, the Colton consists in the central Gunnison Plateau of a northward-thinning wedge. From a thickness of 800 feet at South Maple Canyon, it thins to less that 550 feet at Wales Canyon, north of which it has been removed by erosion. The formation becomes much more clastic toward the north in the area studied
Following microscopic motion in a two dimensional glass-forming binary fluid
The dynamics of a binary mixture of large and small discs are studied at
temperatures approaching the glass transition using an analysis based on the
topology of the Voronoi polygon surrounding each atom. At higher temperatures
we find that dynamics is dominated by fluid-like motion that involves particles
entering and exiting the nearest-neighbour shells of nearby particles. As the
temperature is lowered, the rate of topological moves decreases and motion
becomes localised to regions of mixed pentagons and heptagons. In addition we
find that in the low temperature state particles may translate significant
distances without undergoing changes in their nearest neig hbour shell. These
results have implications for dynamical heterogeneities in glass forming
liquids.Comment: 12 pages, 7 figure
Elbow Extension Predicts Motor Impairment and Performance after Stroke
Background and Purpose. Kinematic motion analysis has helped to characterize poststroke reaching strategies with the hemiparetic arm. However, the relationships between reaching strategy and performance on common functional outcome measures remain unclear.
Methods. Thirty-five participants were tested for motor performance and motor impairment using the Wolf Motor Function Test (time and functional ability measure) and Fugl-Meyer assessment, respectively. Kinematic motion analysis of a forward reaching paradigm provided potential predictors of reaching strategy including shoulder flexion, elbow extension, and trunk displacement. A stepwise linear regression model with three potential predictors was used in addition to Pearson-product moment correlations.
Results. Kinematic analysis of elbow extension predicted performance on both the Wolf Motor Function Test and Fugl-Meyer assessment. Shoulder flexion and trunk displacement did not significantly predict functional or reaching time outcomes. The Wolf Motor Function Test and the Fugl-Meyer assessment were highly correlated.
Conclusions. The ability to incorporate elbow extension during reach is a significant predictor of motor performance and hemiparetic arm motor capacity after stroke
Quantum and classical surface acoustic wave induced magnetoresistance oscillations in a 2D electron gas
We study theoretically the geometrical and temporal commensurability
oscillations induced in the resistivity of 2D electrons in a perpendicular
magnetic field by surface acoustic waves (SAWs). We show that there is a
positive anisotropic dynamical classical contribution and an isotropic
non-equilibrium quantum contribution to the resistivity. We describe how the
commensurability oscillations modulate the resonances in the SAW-induced
resistivity at multiples of the cyclotron frequency. We study the effects of
both short-range and long-range disorder on the resistivity corrections for
both the classical and quantum non-equilibrium cases. We predict that the
quantum correction will give rise to zero-resistance states with associated
geometrical commensurability oscillations at large SAW amplitude for
sufficiently large inelastic scattering times. These zero resistance states are
qualitatively similar to those observed under microwave illumination, and their
nature depends crucially on whether the disorder is short- or long-range.
Finally, we discuss the implications of our results for current and future
experiments on two dimensional electron gases.Comment: 16 pages, 8 figure
Heterogeneous slow dynamics in a two dimensional doped classical antiferromagnet
We introduce a lattice model for a classical doped two dimensional
antiferromagnet which has no quenched disorder, yet displays slow dynamics
similar to those observed in supercooled liquids. We calculate two-time spatial
and spin correlations via Monte Carlo simulations and find that for
sufficiently low temperatures, there is anomalous diffusion and
stretched-exponential relaxation of spin correlations. The relaxation times
associated with spin correlations and diffusion both diverge at low
temperatures in a sub-Arrhenius fashion if the fit is done over a large
temperature-window or an Arrhenius fashion if only low temperatures are
considered. We find evidence of spatially heterogeneous dynamics, in which
vacancies created by changes in occupation facilitate spin flips on
neighbouring sites. We find violations of the Stokes-Einstein relation and
Debye-Stokes-Einstein relation and show that the probability distributions of
local spatial correlations indicate fast and slow populations of sites, and
local spin correlations indicate a wide distribution of relaxation times,
similar to observ ations in other glassy systems with and without quenched
disorder.Comment: 12 pages, 17 figures, corrected erroneous figure, and improved
quality of manuscript, updated reference
The Atmospheric Effects of Stratospheric Aircraft: a First Program Report
Studies have indicated that, with sufficient technology development, high speed civil transport aircraft could be economically competitive with long haul subsonic aircraft. However, uncertainty about atmospheric pollution, along with community noise and sonic boom, continues to be a major concern; and this is addressed in the planned 6 yr HSRP begun in 1990. Building on NASA's research in atmospheric science and emissions reduction, the AESA studies particularly emphasizing stratospheric ozone effects. Because it will not be possible to directly measure the impact of an HSCT aircraft fleet on the atmosphere, the only means of assessment will be prediction. The process of establishing credibility for the predicted effects will likely be complex and involve continued model development and testing against climatological patterns. Lab simulation of heterogeneous chemistry and other effects will continue to be used to improve the current models
International Olympic Committee Consensus Statement: Molecular Basis of Connective Tissue and Muscle Injuries in Sport
Tendon and ligament injures cause significant loss of performance in sport and decreased functional capacity in the workplace. Many of these injures remain difficult to treat, and many individuals have long-term pain and discomfort. Animal studies of growth factor and cell-based therapies have shown promising results, but these treatments also can be misused to enhance athletic performance. The International Olympic Committee (IOC) now has high-level scientific advisors who can advise the IOC as to the use and abuse of these technologies
Optical Spectra of 73 Stripped-Envelope Core-Collapse Supernovae
We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic,
and broad-lined Ic. All of these types are attributed to the core collapse of
massive stars, with varying degrees of intact H and He envelopes before
explosion. The SNe in our sample have a mean redshift = 4200 km/s. Most of
these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics
(CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra.
The data coverage range from mere identification (1-3 spectra) for a few SNe to
extensive series of observations (10-30 spectra) that trace the spectral
evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73
SNe presented here, we have well-determined dates of maximum light to determine
the phase of each spectrum. Our sample constitutes the most extensive spectral
library of stripped-envelope SNe to date. We provide very early coverage (as
early as 30 days before V-band max) for photospheric spectra, as well as
late-time nebular coverage when the innermost regions of the SNe are visible
(as late as 2 years after explosion, while for SN1993J, we have data as late as
11.6 years). This data set has homogeneous observations and reductions that
allow us to study the spectroscopic diversity of these classes of stripped SNe
and to compare these to SNe associated with gamma-ray bursts. We undertake
these matters in follow-up papers.Comment: Published by the Astronomical Journal in May 2015. All spectra are
publicly available at the CfA SN archive:
http://www.cfa.harvard.edu/supernova/SNarchive.html . A companion paper on
constructing SNID templates based on these spectra is by Liu & Modjaz (2014)
and the resulting SNID templates are available from the NYU website:
http://cosmo.nyu.edu/SNYU/spectra
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