984 research outputs found
Quantum Statistics of Interacting Dimer Spin Systems
The compound TlCuCl3 represents a model system of dimerized quantum spins
with strong interdimer interactions. We investigate the triplet dispersion as a
function of temperature by inelastic neutron scattering experiments on single
crystals. By comparison with a number of theoretical approaches we demonstrate
that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515
(1994)] provides an appropriate quantum statistical model for dimer spin
systems at finite temperatures, where many-body correlations become
particularly important.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Lyapunov modes in three-dimensional Lennard-Jones fluids
Recent studies on the phase-space dynamics of a one-dimensional Lennard-Jones
fluid reveal the existence of regular collective perturbations associated with
the smallest positive Lyapunov exponents of the system, called hydrodynamic
Lyapunov modes, which previously could only be identified in hard-core fluids.
In this work we present a systematic study of the Lyapunov exponents and
Lyapunov vectors, i.e. perturbations along each direction of phase space, of a
three-dimensional Lennard-Jones fluid. By performing the Fourier transform of
the spatial density of the coordinate part of the Lyapunov vector components
and then time-averaging this result we find convincing signatures of
longitudinal modes, with inconclusive evidence of transverse modes for all
studied densities. Furthermore, the longitudinal modes can be more clearly
identified for the higher density values. Thus, according to our results, the
mixing of modes induced both by the dynamics and the dimensionality induce a
hitherto unknown type of order in the tangent space of the model herein studied
at high density values.Comment: 28 Pages, 13 b&w Figure
Positivity of relative canonical bundles and applications
Given a family of canonically polarized manifolds, the
unique K\"ahler-Einstein metrics on the fibers induce a hermitian metric on the
relative canonical bundle . We use a global elliptic
equation to show that this metric is strictly positive on , unless
the family is infinitesimally trivial.
For degenerating families we show that the curvature form on the total space
can be extended as a (semi-)positive closed current. By fiber integration it
follows that the generalized Weil-Petersson form on the base possesses an
extension as a positive current. We prove an extension theorem for hermitian
line bundles, whose curvature forms have this property. This theorem can be
applied to a determinant line bundle associated to the relative canonical
bundle on the total space. As an application the quasi-projectivity of the
moduli space of canonically polarized varieties
follows.
The direct images , , carry natural hermitian metrics. We prove an
explicit formula for the curvature tensor of these direct images. We apply it
to the morphisms that are induced by the Kodaira-Spencer map and obtain a differential
geometric proof for hyperbolicity properties of .Comment: Supercedes arXiv:0808.3259v4 and arXiv:1002.4858v2. To appear in
Invent. mat
Probing Shock Breakout with Serendipitous GALEX Detections of Two SNLS Type II-P Supernovae
We report the serendipitous detection by GALEX of fast (1
mag) UV emission from two Type II plateau (II-P) supernovae (SNe) at z=0.185
and 0.324 discovered by the Supernova Legacy Survey. Optical photometry and VLT
spectroscopy 2 weeks after the GALEX detections link the onset of UV emission
to the time of shock breakout. Using radiation hydrodynamics and non-LTE
radiative transfer simulations, and starting from a standard red supergiant
(RSG; Type II-P SN progenitor) star evolved self-consistently from the main
sequence to iron core collapse, we model the shock breakout phase and the 55 hr
that follow. The small scale height of our RSG atmosphere model suggests that
the breakout signature is a thermal soft X-ray burst (lambda_peak ~ 90\AA) with
a duration of <~ 2000 s. Longer durations are possible but require either an
extended and tenuous non-standard envelope, or an unusually dense RSG wind with
\dot{M} ~ 10^(-3) Msun yr^(-1). The GALEX observations miss the peak of the
luminous (M_FUV ~ -20) UV burst but unambiguously capture the rise of the
emission and a subsequent 2 day long plateau. The postbreakout, UV-bright
plateau is a prediction of our model in which the shift of the peak of the
spectral energy distribution (SED) from ~100 to ~1000\AA and the ejecta
expansion both counteract the decrease in bolometric luminosity from ~10^11 to
~10^9 L_sun over that period. Based on the observed detection efficiency of our
study we make predictions for the breakout detection rate of the GALEX Time
Domain Survey.Comment: 4 pages, 3 color figures, accepted to ApJ Letters, emulateapj,
corrections from proofs adde
Spin-transfer in an open ferromagnetic layer: from negative damping to effective temperature
Spin-transfer is a typical spintronics effect that allows a ferromagnetic
layer to be switched by spin-injection. Most of the experimental results about
spin transfer are described on the basis of the Landau-Lifshitz-Gilbert
equation of the magnetization, in which additional current-dependent damping
factors are added, and can be positive or negative. The origin of the damping
can be investigated further by performing stochastic experiments, like one shot
relaxation experiments under spin-injection in the activation regime of the
magnetization. In this regime, the N\'eel-Brown activation law is observed
which leads to the introduction of a current-dependent effective temperature.
In order to justify the introduction of these counterintuitive parameters
(effective temperature and negative damping), a detailed thermokinetic analysis
of the different sub-systems involved is performed. We propose a thermokinetic
description of the different forms of energy exchanged between the electric and
the ferromagnetic sub-systems at a Normal/Ferromagnetic junction. The
corresponding Fokker Planck equations, including relaxations, are derived. The
damping coefficients are studied in terms of Onsager-Casimir transport
coefficients, with the help of the reciprocity relations. The effective
temperature is deduced in the activation regime.Comment: 65 pages, 10 figure
Risk assessment tools to predict location of discharge and need for supportive services for medical patients after discharge from hospital: a systematic review protocol
Early pyloric stenosis: a case control study
Pyloric stenosis (PS) is rare in the first 2Â weeks of life, often leading to delays in diagnosis and treatment. We conducted a case control study to delineate the characteristics of patients with early PS (EPS). In addition, we tested the hypothesis that patients with EPS present with a smaller pylorus than older patients.
A database of all patients presenting with PS to a children’s hospital over a 5-year period (2002–2006) was obtained. Each patient admitted during the first 2 weeks of life (subject) was matched to a patient admitted after 4 weeks of age (control), with the same gender, electrolyte status, and treating surgeon. A single pediatric radiologist, blinded to patient age, reviewed all available ultrasounds retrospectively. Demographic, clinical, diagnostic, therapeutic, and outcome data were compared.
During the study period, 278 pyloromyotomies were performed for PS. Sixteen patients (5.8%) presented with EPS between 2 and 14 days of life. EPS patients had a higher prevalence of positive family history (31 vs. 0%, P = 0.043), and breast milk feeding (75 vs. 31%, P = 0.045). Sonographic measurements showed a pylorus that was of significantly less length (17.1 ± 0.6 vs. 20.5 ± 0.9 mm, P = 0.006) and muscle thickness (3.5 ± 0.2 vs. 4.9 ± 0.2 mm, P < 0.001) in patients with EPS. Hospital stay was significantly longer for EPS patients (4.3 ± 0.9 vs. 2.0 ± 0.1 days, P = 0.19)
Babies presenting with EPS are more likely to be breast fed and to have a positive family history. EPS is associated with a longer hospital stay. Use of sonographic diagnostic measurements specific to this age group may prevent delays in diagnosis and treatment, and improve outcomes
"May I Buy a Pack of Marlboros, Please?" A Systematic Review of Evidence to Improve the Validity and Impact of Youth Undercover Buy Inspections
Most smokers become addicted to tobacco products before they are legally able to pur- chase these products. We systematically reviewed the literature on protocols to assess underage purchase and their ecological validity. We conducted a systematic search in May 2015 in PubMed and PsycINFO. We independently screened records for inclusion. We con- ducted a narrative review and examined implications of two types of legal authority for proto- cols that govern underage buy enforcement in the United States: criminal (state-level
laws prohibiting sales to youth) and administrative (federal regulations prohibiting sales to youth). Ten studies experimentally assessed underage buy protocols and 44 studies assessed the association between youth characteristics and tobacco sales. Protocols that mimicked real-world youth behaviors were consistently associated with substantially greater likelihood of a sale to a youth. Many of the tested protocols appear to be designed for compliance with criminal law rather than administrative enforcement in ways that limited ecological validity. This may be due to concerns about entrapment. For administrative enforcement in particular, entrapment may be less of an issue than commonly thought. Commonly used underage buy protocols poorly represent the reality of youths' access to tobacco from retailers. Compliance check programs should allow youth to present them- selves naturally and attempt to match the community’s demographic makeup
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