7,559 research outputs found
Energy Relaxation in the Integer Quantum Hall Regime
We investigate the energy exchanges along an electronic quantum channel
realized in the integer quantum Hall regime at filling factor . One of
the two edge channels is driven out-of-equilibrium and the resulting electronic
energy distribution is measured in the outer channel, after several propagation
lengths mm. Whereas there are no discernable energy
transfers toward thermalized states, we find efficient energy redistribution
between the two channels without particle exchanges. At long distances
m, the measured energy distribution is a hot Fermi function whose
temperature is lower than expected for two interacting channels, which suggests
the contribution of extra degrees of freedom. The observed short energy
relaxation length challenges the usual description of quantum Hall excitations
as quasiparticles localized in one edge channel.Comment: To be published in PRL, 10 pages including supplementary materia
Nanofiber-Based Double-Helix Dipole Trap for Cold Neutral Atoms
A double-helix optical trapping potential for cold atoms can be
straightforwardly created inside the evanescent field of an optical nanofiber.
It suffices to send three circularly polarized light fields through the
nanofiber; two counterpropagating and far red-detuned with respect to the
atomic transition and the third far blue-detuned. Assuming realistic
experimental parameters, the transverse confinement of the resulting potential
allows one to reach the one-dimensional regime with cesium atoms for
temperatures of several \muK. Moreover, by locally varying the nanofiber
diameter, the radius and pitch of the double-helix can be modulated, thereby
opening a realm of applications in cold-atom physics.Comment: 9 pages, 4 figure
Fracture of Zircaloy-4 fuel cladding tubes with hydride blisters
International audienceThe influence of hydride blister on cold worked stress relieved Zircaloy-4 cladding tubes fracture toughness at room temperature was studied using hoop tensile tests and plane strain tensile tests. The experimental macroscopic fracture stress and strain values and an elastic-plastic finite element analysis of the mechanical tests with the CAST3 M code were used to calculate the J-integral and estimate the fracture toughness for several blister depths from 120 to 240 μm
Defining Relations of Noncommutative Trace Algebra of Two Matrices
The noncommutative (or mixed) trace algebra is generated by
generic matrices and by the algebra generated by all
traces of products of generic matrices, . It is known that over a
field of characteristic 0 this algebra is a finitely generated free module over
a polynomial subalgebra of the center . For and we have
found explicitly such a subalgebra and a set of free generators of the
-module . We give also a set of defining relations of as an
algebra and a Groebner basis of the corresponding ideal. The proofs are based
on easy computer calculations with standard functions of Maple, the explicit
presentation of in terms of generators and relations, and methods of
representation theory of the general linear group.Comment: 19 page
Fracture of Zircaloy-4 cladding tubes with or without hydride blisters in uniaxial to plane strain conditions with standard and optimized expansion due to compression tests
International audienceTwo optimizations of the Expansion Due to Compression (EDC) test, which induces a near uniaxial loading, were proposed and developed to reach higher biaxiality ratios (ratio between mechanical quantities in axial and in circumferential direction). The first optimization, named HB-EDC for High-Biaxiality EDC, allowed to reach transverse plane strain conditions. The second optimization, named VHB-EDC for Very High Biaxiality EDC, was designed to reach higher loading biaxiality ratios. These optimized EDC tests were performed at 25 °C, 350 °C and 480 °C on unirradiated hydrided Cold Worked Stress Relieved (CWSR) Zircaloy-4 samples. First, samples unhydrided or uniformly hydrided up to 1130 wppm were tested. Second, samples hydrided at 310 wppm with a hydride blister were tested. A large ductility reduction is induced by the increase in biaxiality level in the absence of a hydride blister or with small blisters (View the MathML source<50μm deep). The fracture strain decreases quickly with the blister depth at 25 °C, but more progressively at higher temperature. An equation that quantifies the fracture strain reduction with the blister depth is proposed. Eventually, one of the tests developed in the present study, the HB-EDC test, was proven to be a good compromise between the test complexity and the stress state reached. It is a good candidate to characterize the mechanical behaviour of irradiated cladding
Four-point renormalized coupling constant and Callan-Symanzik beta-function in O(N) models
We investigate some issues concerning the zero-momentum four-point
renormalized coupling constant g in the symmetric phase of O(N) models, and the
corresponding Callan-Symanzik beta-function. In the framework of the 1/N
expansion we show that the Callan- Symanzik beta-function is non-analytic at
its zero, i.e. at the fixed-point value g^* of g. This fact calls for a check
of the actual accuracy of the determination of g^* from the resummation of the
d=3 perturbative g-expansion, which is usually performed assuming analyticity
of the beta-function. Two alternative approaches are exploited. We extend the
\epsilon-expansion of g^* to O(\epsilon^4). Quite accurate estimates of g^* are
then obtained by an analysis exploiting the analytic behavior of g^* as
function of d and the known values of g^* for lower-dimensional O(N) models,
i.e. for d=2,1,0. Accurate estimates of g^* are also obtained by a reanalysis
of the strong-coupling expansion of lattice N-vector models allowing for the
leading confluent singularity. The agreement among the g-, \epsilon-, and
strong-coupling expansion results is good for all N. However, at N=0,1,
\epsilon- and strong-coupling expansion favor values of g^* which are sligthly
lower than those obtained by the resummation of the g-expansion assuming
analyticity in the Callan-Symanzik beta-function.Comment: 35 pages (3 figs), added Ref. for GRT, some estimates are revised,
other minor change
KP line solitons and Tamari lattices
The KP-II equation possesses a class of line soliton solutions which can be
qualitatively described via a tropical approximation as a chain of rooted
binary trees, except at "critical" events where a transition to a different
rooted binary tree takes place. We prove that these correspond to maximal
chains in Tamari lattices (which are poset structures on associahedra). We
further derive results that allow to compute details of the evolution,
including the critical events. Moreover, we present some insights into the
structure of the more general line soliton solutions. All this yields a
characterization of possible evolutions of line soliton patterns on a shallow
fluid surface (provided that the KP-II approximation applies).Comment: 49 pages, 36 figures, second version: section 4 expande
Red blood cell transfusion in patients with subarachnoid hemorrhage: a multidisciplinary North American survey
Abstract
Introduction
Anemia is associated with poor outcomes in patients with aneurysmal subarachnoid hemorrhage (SAH). It remains unclear whether this association can be modified with more aggressive use of red blood cell (RBC) transfusions. The degree to which restrictive thresholds have been adopted in neurocritical care patients remains unknown.
Methods
We performed a survey of North American academic neurointensivists, vascular neurosurgeons and multidisciplinary intensivists who regularly care for patients with SAH to determine hemoglobin (Hb) concentrations which commonly trigger a decision to initiate transfusion. We also assessed minimum and maximum acceptable Hb goals in the context of a clinical trial and how decision-making is influenced by advanced neurological monitoring, clinician characteristics and patient-specific factors.
Results
The survey was sent to 531 clinicians, of whom 282 (53%) responded. In a hypothetical patient with high-grade SAH (WFNS 4), the mean Hb concentration at which clinicians administered RBCs was 8.19 g/dL (95% CI, 8.07 to 8.30 g/dL). Transfusion practices were comparatively more restrictive in patients with low-grade SAH (mean Hb 7.85 g/dL (95% CI, 7.73 to 7.97 g/dL)) (P < 0.0001) and more liberal in patients with delayed cerebral ischemia (DCI) (mean Hb 8.58 g/dL (95% CI, 8.45 to 8.72 g/dL)) (P < 0.0001). In each setting, there was a broad range of opinions. The majority of respondents expressed a willingness to study a restrictive threshold of ≤8 g/dL (92%) and a liberal goal of ≥10 g/dl (75%); in both cases, the preferred transfusion thresholds were significantly higher for patients with DCI (P < 0.0001). Neurosurgeons expressed higher minimum Hb goals than intensivists, especially for patients with high-grade SAH (β = 0.46, P = 0.003), and were more likely to administer two rather than one unit of RBCs (56% vs. 19%; P < 0.0001). Institutional use of transfusion protocols was associated with more restrictive practices. More senior clinicians preferred higher Hb goals in the context of a clinical trial. Respondents were more likely to transfuse patients with brain tissue oxygen tension values <15 mmHg and lactate-to-pyruvate ratios >40.
Conclusions
There is widespread variation in the use of RBC transfusions in SAH patients. Practices are heavily influenced by the specific dynamic clinical characteristics of patients and may be further modified by clinician specialty and seniority, the use of protocols and advanced neurological monitoring
The Oxford-Dartmouth Thirty Degree Survey II: Clustering of Bright Lyman Break Galaxies - Strong Luminosity Dependent Bias at z=4
We present measurements of the clustering properties of bright ()
z4 Lyman Break Galaxies (LBGs) selected from the Oxford-Dartmouth Thirty
Degree Survey (ODT). We describe techniques used to select and evaluate our
candidates and calculate the angular correlation function which we find best
fitted by a power law, with
(with in arcseconds), using a constrained slope of . Using
a redshift distribution consistent with photometric models, we deproject this
correlation function and find a comoving
h Mpc in a flat cosmology for
. This corresponds to a linear bias value of
(assuming ). These data show a
significantly larger and than previous studies at . We
interpret this as evidence that the brightest LBGs have a larger bias than
fainter ones, indicating a strong luminosity dependence for the measured bias
of an LBG sample. Comparing this against recent results in the literature at
fainter (sub-) limiting magnitudes, and with simple models describing
the relationship between LBGs and dark matter haloes, we discuss the
implications on the implied environments and nature of LBGs. It seems that the
brightest LBGs (in contrast with the majority sub- population), have
clustering properties, and host dark matter halo masses, that are consistent
with them being progenitors of the most massive galaxies today.Comment: Accepted for Publication in MNRAS. 15 Pages, 13 Figure
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