1,258 research outputs found
Carotid plaque morphology: A review
The recent North American Symptomatic Carotid Endarterectomy Trial has answered fairly conclusively the questions concerning the optimal management of patients with symptoms who have a > 70% stenosis of the internal carotid artery. It has also had the effect of refocusing attention on carotid pathology. The main question still to be answered is whether surgical management is the optimum treatment for other groups of patients with carotid disease. From various studies done on the natural history of carotid plaques it is apparent that there are subgroups who may benefit from surgery, namely those who will progress to stroke if not treated. The problem comes in identifying these subgroups by the factors which cause them to progress. This paper aims to review the role that plaque morphology has in the development of symptoms and whether it should be included with degree of stenosis in assessing the risk of a carotid plaque. The non-invasive assessment of plaque morphology is also reviewed. The evidence from this review does not support the use of plaque morphology as a discriminating factor for carotid endarterectomy at present
Mixed States of Composite Fermions Carrying Two and Four Vortices
There now exists preliminary experimental evidence for some fractions, such
as = 4/11 and 5/13, that do not belong to any of the sequences
, and being integers. We propose that these states
are mixed states of composite fermions of different flavors, for example,
composite fermions carrying two and four vortices. We also obtain an estimate
of the lowest-excitation dispersion curve as well as the transport gap; the
gaps for 4/11 are smaller than those for 1/3 by approximately a factor of 50.Comment: Accepted for PRB rapid communication (scheduled to appear in Nov 15,
2000 issue
Effects of culling on badger abundance : implications for tuberculosis control
Culling is often considered as a tool for controlling wildlife diseases that can also infect people or livestock. Culling European badgers Meles meles can cause both positive and negative effects on the incidence of bovine tuberculosis (TB) in cattle. One factor likely to influence the outcome of different badger culling strategies for cattle TB is the reduction in badger population density achieved. However, this reduction is difficult to measure because badgers, being nocturnal and fossorial, are difficult to count. Here, we use indices of badger abundance to measure the population impacts of two culling strategies tested in Britain. The densities of badger setts and latrines recorded before culling were correlated with the densities of badgers captured on initial culls, suggesting that both were indices of actual badger abundance. Widespread 'proactive' culling was associated with a 73% reduction in the density of badger latrines, a 69% reduction in the density of active burrows and a 73% reduction in the density of road killed badgers. This population reduction was achieved by a coordinated effort entailing widespread and repeated trapping over several years. However, this strategy caused only modest reductions in cattle TB incidence in culled areas and elevated incidence in neighbouring unculled areas. Localized 'reactive' culling caused a 26% reduction in latrine density, a 32% reduction in active burrow density and a 10% reduction in the density of road killed badgers, but apparently increased the incidence of cattle TB. These results indicate that the relationship between badger population reduction and TB transmission to cattle is strongly non linear, probably because culling prompts changes in badger behaviour that influence transmission rates. These findings raise serious questions about the capacity of badger culling to contribute to the control of cattle TB in Britain
The intrinsic features of the specific heat at half-filled Landau levels of two-dimensional electron systems
The specific heat capacity of a two-dimensional electron gas is derived for
two types of the density of states, namely, the Dirac delta function spectrum
and that based on a Gaussian function. For the first time, a closed form
expression of the specific heat for each case is obtained at half-filling. When
the chemical potential is temperature-independent, the temperature is
calculated at which the specific heat is a maximum. Here the effects of the
broadening of the Landau levels are distinguished from those of the different
filling factors. In general, the results derived herein hold for any
thermodynamic system having similar resonant states.Comment: 11 pages, 1 figure, to appear in J Low Temp Phys (2010
Cosmic Numbers: A Physical Classification for Cosmological Models
We introduce the notion of the cosmic numbers of a cosmological model, and
discuss how they can be used to naturally classify models according to their
ability to solve some of the problems of the standard cosmological model.Comment: 3 pages, no figures. v2: Two references added, cosmetic changes.
Version to appear in Phys. Rev. D (Brief reports
Limits on Cosmological Variation of Strong Interaction and Quark Masses from Big Bang Nucleosynthesis, Cosmic, Laboratory and Oklo Data
Recent data on cosmological variation of the electromagnetic fine structure
constant from distant quasar (QSO) absorption spectra have inspired a more
general discussion of possible variation of other constants. We discuss
variation of strong scale and quark masses. We derive the limits on their
relative change from (i) primordial Big-Bang Nucleosynthesis (BBN); (ii)
Oklo natural nuclear reactor, (iii) quasar absorption spectra, and (iv)
laboratory measurements of hyperfine intervals.Comment: 10 pages 2 figurs: second version have several references added and
some new comment
The existence problem for dynamics of dissipative systems in quantum probability
Motivated by existence problems for dissipative systems arising naturally in
lattice models from quantum statistical mechanics, we consider the following
-algebraic setting: A given hermitian dissipative mapping is
densely defined in a unital -algebra . The identity
element in is also in the domain of . Completely
dissipative maps are defined by the requirement that the induced maps,
, are dissipative on the by complex
matrices over for all . We establish the existence of different
types of maximal extensions of completely dissipative maps. If the enveloping
von Neumann algebra of is injective, we show the existence of an
extension of which is the infinitesimal generator of a quantum
dynamical semigroup of completely positive maps in the von Neumann algebra. If
is a given well-behaved *-derivation, then we show that each of the
maps and is completely dissipative.Comment: 24 pages, LaTeX/REVTeX v. 4.0, submitted to J. Math. Phys.; PACS 02.,
02.10.Hh, 02.30.Tb, 03.65.-w, 05.30.-
Area metric gravity and accelerating cosmology
Area metric manifolds emerge as effective classical backgrounds in quantum
string theory and quantum gauge theory, and present a true generalization of
metric geometry. Here, we consider area metric manifolds in their own right,
and develop in detail the foundations of area metric differential geometry.
Based on the construction of an area metric curvature scalar, which reduces in
the metric-induced case to the Ricci scalar, we re-interpret the
Einstein-Hilbert action as dynamics for an area metric spacetime. In contrast
to modifications of general relativity based on metric geometry, no continuous
deformation scale needs to be introduced; the extension to area geometry is
purely structural and thus rigid. We present an intriguing prediction of area
metric gravity: without dark energy or fine-tuning, the late universe exhibits
a small acceleration.Comment: 52 pages, 1 figure, companion paper to hep-th/061213
Resonant transmission through an open quantum dot
We have measured the low-temperature transport properties of a quantum dot
formed in a one-dimensional channel. In zero magnetic field this device shows
quantized ballistic conductance plateaus with resonant tunneling peaks in each
transition region between plateaus. Studies of this structure as a function of
applied perpendicular magnetic field and source-drain bias indicate that
resonant structure deriving from tightly bound states is split by Coulomb
charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure
2d Stringy Black Holes and Varying Constants
Motivated by the recent interest on models with varying constants and whether
black hole physics can constrain such theories, two-dimensional charged stringy
black holes are considered. We exploit the role of two-dimensional stringy
black holes as toy models for exploring paradoxes which may lead to constrains
on a theory. A two-dimensional charged stringy black hole is investigated in
two different settings. Firstly, the two-dimensional black hole is treated as
an isolated object and secondly, it is contained in a thermal environment. In
both cases, it is shown that the temperature and the entropy of the
two-dimensional charged stringy black hole are decreased when its electric
charge is increased in time. By piecing together our results and previous ones,
we conclude that in the context of black hole thermodynamics one cannot derive
any model independent constraints for the varying constants. Therefore, it
seems that there aren't any varying constant theories that are out of favor
with black hole thermodynamics.Comment: 12 pages, LaTeX, to appear in JHE
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