649 research outputs found
Diagnostisches und therapeutisches Vorgehen in der Akutphase nach kranio-zervikalem Beschleunigungstrauma (sog. Schleudertrauma) : Empfehlungen einer schweizerischen Arbeitsgruppe
https://digitalrepository.unm.edu/abq_eh_news/1514/thumbnail.jp
The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil
The critical-velocity behavior of oscillatory superfluid Helium-4 flow
through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil
has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up
to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during
which the frequency remained below 400 Hz, the critical velocity was a
nearly-linearly decreasing function of increasing temperature throughout the
region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi
phase slips could be observed at the onset of dissipation. In runs with
frequencies higher than 400 Hz, downward curvature was observed in the decrease
of critical velocity with increasing temperature. In addition, above 500 Hz an
alteration in supercritical behavior was seen at the lower temperatures,
involving the appearance of large energy-loss events. These irregular events
typically lasted a few tens of half-cycles of oscillation and could involve
hundreds of times more energy loss than would have occurred in a single
complete 2 Pi phase slip at maximum flow. The temperatures at which this
altered behavior was observed rose with frequency, from ~ 0.6 K and below, at
500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203
Inflationary Perturbations: the Cosmological Schwinger Effect
This pedagogical review aims at presenting the fundamental aspects of the
theory of inflationary cosmological perturbations of quantum-mechanical origin.
The analogy with the well-known Schwinger effect is discussed in detail and a
systematic comparison of the two physical phenomena is carried out. In
particular, it is demonstrated that the two underlying formalisms differ only
up to an irrelevant canonical transformation. Hence, the basic physical
mechanisms at play are similar in both cases and can be reduced to the
quantization of a parametric oscillator leading to particle creation due to the
interaction with a classical source: pair production in vacuum is therefore
equivalent to the appearance of a growing mode for the cosmological
fluctuations. The only difference lies in the nature of the source: an electric
field in the case of the Schwinger effect and the gravitational field in the
case of inflationary perturbations. Although, in the laboratory, it is
notoriously difficult to produce an electric field such that pairs extracted
from the vacuum can be detected, the gravitational field in the early universe
can be strong enough to lead to observable effects that ultimately reveal
themselves as temperature fluctuations in the Cosmic Microwave Background.
Finally, the question of how quantum cosmological perturbations can be
considered as classical is discussed at the end of the article.Comment: 49 pages, 6 figures, to appear in a LNP volume "Inflationary
Cosmology
Spin, charge and orbital ordering in ferrimagnetic insulator YBaMnO
The oxygen-deficient (double) perovskite YBaMnO, containing
corner-linked MnO square pyramids, is found to exhibit ferrimagnetic
ordering in its ground state. In the present work we report
generalized-gradient-corrected, relativistic first-principles full-potential
density-functional calculations performed on YBaMnO in the nonmagnetic,
ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings
are explained with site-, angular momentum- and orbital-projected density of
states, charge-density plots, electronic structure and total energy studies.
YBaMnO is found to stabilize in a G-type ferrimagnetic state in
accordance with experimental results. The experimentally observed insulating
behavior appears only when we include ferrimagnetic ordering in our
calculation. We observed significant optical anisotropy in this material
originating from the combined effect of ferrimagnetic ordering and crystal
field splitting. In order to gain knowledge about the presence of different
valence states for Mn in YBaMnO we have calculated -edge x-ray
absorption near-edge spectra for the Mn and O atoms. The presence of the
different valence states for Mn is clearly established from the x-ray
absorption near-edge spectra, hyperfine field parameters and the magnetic
properties study. Among the experimentally proposed structures, the recently
reported description based on 4/ is found to represent the stable
structure
Vortices on Higher Genus Surfaces
We consider the topological interactions of vortices on general surfaces. If
the genus of the surface is greater than zero, the handles can carry magnetic
flux. The classical state of the vortices and the handles can be described by a
mapping from the fundamental group to the unbroken gauge group. The allowed
configurations must satisfy a relation induced by the fundamental group. Upon
quantization, the handles can carry ``Cheshire charge.'' The motion of the
vortices can be described by the braid group of the surface. How the motion of
the vortices affects the state is analyzed in detail.Comment: 28 pages with 10 figures; uses phyzzx and psfig; Caltech preprint
CALT-68-187
Monte Carlo reconstruction of the inflationary potential
We present Monte Carlo reconstruction, a new method for ``inverting''
observational data to constrain the form of the scalar field potential
responsible for inflation. This stochastic technique is based on the flow
equation formalism and has distinct advantages over reconstruction methods
based on a Taylor expansion of the potential. The primary ansatz required for
Monte Carlo reconstruction is simply that inflation is driven by a single
scalar field. We also require a very mild slow roll constraint, which can be
made arbitrarily weak since Monte Carlo reconstruction is implemented at
arbitrary order in the slow roll expansion. While our method cannot evade
fundamental limits on the accuracy of reconstruction, it can be simply and
consistently applied to poor data sets, and it takes advantage of the attractor
properties of single-field inflation models to constrain the potential outside
the small region directly probed by observations. We show examples of Monte
Carlo reconstruction for data sets similar to that expected from the Planck
satellite, and for a hypothetical measurement with a factor of five better
parameter discrimination than Planck.Comment: 10 pages, 5 figures (RevTeX 4) Version submitted to PRD: references
added, minor clarification
HLA-DP on Epithelial Cells Enables Tissue Damage by NKp44+ Natural Killer Cells in Ulcerative Colitis
Background & aims: Ulcerative colitis (UC) is characterized by severe inflammation and destruction of the intestinal epithelium, and is associated with specific risk single nucleotide polymorphisms in HLA class II. Given the recently discovered interactions between subsets of HLA-DP molecules and the activating natural killer (NK) cell receptor NKp44, genetic associations of UC and HLA-DP haplotypes and their functional implications were investigated.Methods: HLA-DP haplotype and UC risk association analyses were performed (UC: n = 13,927; control: n = 26,764). Expression levels of HLA-DP on intestinal epithelial cells (IECs) in individuals with and without UC were quantified. Human intestinal 3-dimensional (3D) organoid cocultures with human NK cells were used to determine functional consequences of interactions between HLA-DP and NKp44.Results: These studies identified HLA-DPA1â01:03-DPB1â04:01 (HLA-DP401) as a risk haplotype and HLA-DPA1â01:03-DPB1â03:01 (HLA-DP301) as a protective haplotype for UC in European populations. HLA-DP expression was significantly higher on IECs of individuals with UC compared with controls. IECs in human intestinal 3D organoids derived from HLA-DP401pos individuals showed significantly stronger binding of NKp44 compared with HLA-DP301pos IECs. HLA-DP401pos IECs in organoids triggered increased degranulation and tumor necrosis factor production by NKp44+ NK cells in cocultures, resulting in enhanced epithelial cell death compared with HLA-DP301pos organoids. Blocking of HLA-DP401-NKp44 interactions (anti-NKp44) abrogated NK cell activity in cocultures.Conclusions: We identified an UC risk HLA-DP haplotype that engages NKp44 and activates NKp44+ NK cells, mediating damage to intestinal epithelial cells in an HLA-DP haplotype-dependent manner. The molecular interaction between NKp44 and HLA-DP401 in UC can be targeted by therapeutic interventions to reduce NKp44+ NK cell-mediated destruction of the intestinal epithelium in UC
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