520 research outputs found
Experimental Assessment of Proximal Stent-graft (InterVascularTM) Fixation in Human Cadaveric Infrarenal Aortas
AbstractObjectivesthis paper investigates the radial deformation load of an aortic endoluminal prosthesis and determines the longitudinal load required to cause migration in a human cadaveric aorta of the endoprosthesis.Design and methodsthe endovascular prosthesis under investigation was a 24 mm diameter, nitinol, self-expanding aortoaortic device (InterVascular, Clearwater, Florida, U.S.A.). Initially, a motorised digital force gauge developed an incremental load which was applied to the ends of five stent-grafts, to a maximum of 10 mm (42%) compression. Secondly, using a simple bench model, each end of four stent-grafts were deployed into 10 cadaveric experimental aneurysm necks and a longitudinal load applied to effect distraction.Resultsincreasing load produced increasing percentage deformation of the stent-grafts. The mean longitudinal distraction load for an aneurysm neck of 20 mm was 409 g (200–480 g), for 15 mm was 277 g (130–410 g) and for 10 mm was 218 g (130–340 g). The aneurysm diameter and aortic calcification hadpvalues of 0.002 and 0.047, respectively, while thepvalue for aneurysm neck length was less than 0.00001.Conclusionsthese results suggest that there is a theoretical advantage of oversizing an aortic prosthesis and that sufficient anchorage is achieved in an aortic neck of 10 mm to prevent migration when fully deployed
Hemodynamics at Maximum Exercise and Exercise Recovery in Freshman Football Recruits at a BCS School
To determine if blood pressures assessed during max exercise and exercise recovery differ in Freshman football player recruits classified according to body mass index categories (BMICAT). A group of 107 freshman football recruits (mean age=18.2yrs, mean height=187.2 cm, mean weight = 103.0 kg, mean BMI=27.4 kg/meters squared, mean percent fat= 18.3%.) underwent graded exercise testing on a treadmill. Height and weight were assessed and BMI was calculated as weight (kg) / height (m) squared. All 97 subjects were classified as either “normal weight”(NW) (N=38), “overweight” (OW)(N=41) or “obese” (OB)(N=28) according to the National Institutes of Health guidelines. Body fat percentage (PCTFAT) was assessed using dual x-ray absorptiometry. Resting systolic (SBP) and diastolic (DBP) blood pressures were taken following a 3 minute rest period. Mean arterial pressure (MAP) was estimated as [.3(SBP-DBP)]+DBP. Pressures were also assessed at max exercise and at 1, 3, and 5 minutes post exercise. An analysis of covariance (ANCOVA) with PCTFAT as a covariate was used to determine if differences remained among BMICAT in adjusted values for max exercise and all recovery pressures. After ACOVA adjustment, all maximum and recovery pressures were different among BMICAT with statistical significance found at max exercise and 1 minute post exercise. After adjustment for PCTFAT, differences remain in blood pressures among BMI categories
Self-consistent scattering description of transport in normal-superconductor structures
We present a scattering description of transport in several
normal-superconductor structures. We show that the related requirements of
self-consistency and current conservation introduce qualitative changes in the
transport behavior when the current in the superconductor is not negligible.
The energy thresholds for quasiparticle propagation in the superconductor are
sensitive to the existence of condensate flow (). This dependence is
responsible for a rich variety of transport regimes, including a voltage range
in which only Andreev transmission is possible at the interfaces, and a state
of gapless superconductivity which may survive up to high voltages if
temperature is low. The two main effects of current conservation are a shift
towards lower voltages of the first peak in the differential conductance and an
enhancement of current caused by the greater availability of charge
transmitting scattering channels.Comment: 31 pages, 10 PS figures, Latex file, psfig.sty file is added. To
appear in Phys. Rev. B (Jan 97
Pressure and linear heat capacity in the superconducting state of thoriated UBe13
Even well below Tc, the heavy-fermion superconductor (U,Th)Be13 has a large
linear term in its specific heat. We show that under uniaxial pressure, the
linear heat capacity increases in magnitude by more than a factor of two. The
change is reversible and suggests that the linear term is an intrinsic property
of the material. In addition, we find no evidence of hysteresis or of latent
heat in the low-temperature and low-pressure portion of the phase diagram,
showing that all transitions in this region are second order.Comment: 5 pages, 4 figure
Quantifying contributions of chlorofluorocarbon banks to emissions and impacts on the ozone layer and climate
Chlorofluorocarbon (CFC) banks from uses such as air conditioners or foams can be emitted after global production stops. Recent reports of unexpected emissions of CFC-11 raise the need to better quantify releases from these banks, and associated impacts on ozone depletion and climate change. Here we develop a Bayesian probabilistic model for CFC-11, 12, and 113 banks and their emissions, incorporating the broadest range of constraints to date. We find that bank sizes of CFC-11 and CFC-12 are larger than recent international scientific assessments suggested, and can account for much of current estimated CFC-11 and 12 emissions (with the exception of increased CFC-11 emissions after 2012). Left unrecovered, these CFC banks could delay Antarctic ozone hole recovery by about six years and contribute 9 billion metric tonnes of equivalent CO2 emission. Derived CFC-113 emissions are subject to uncertainty, but are much larger than expected, raising questions about its sources
Visualization of Frequent Itemsets with Nested Circular Layout and Bundling Algorithm
International audienceFrequent itemset mining is one of the major data mining issues. Once generated by algorithms, the itemsets can be automatically processed, for instance to extract association rules. They can also be explored with visual tools, in order to analyze the emerging patterns. Graphical itemsets representation is a convenient way to obtain an overview of the global interaction structure. However, when the complexity of the database increases, the network may become unreadable. In this paper, we propose to display itemsets on concentric circles, each one being organized to lower the intricacy of the graph through an optimization process. Thanks to a graph bundling algorithm, we finally obtain a compact representation of a large set of itemsets that is easier to exploit. Colors accumulation and interaction operators facilitate the exploration of the new bundle graph and to illustrate how much an itemset is supported by the data
Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems
We present a theory of Andreev reflection in a
ferromagnet/superconductor/ferromagnet double junction system. The spin
polarized quasiparticles penetrate to the superconductor in the range of
penetration depth from the interface by the Andreev reflection. When the
thickness of the superconductor is comparable to or smaller than the
penetration depth, the spin polarized quasiparticles pass through the
superconductor and therefore the electric current depends on the relative
orientation of magnetizations of the ferromagnets. The dependences of the
magnetoresistance on the thickness of the superconductor, temperature, the
exchange field of the ferromagnets and the height of the interfacial barriers
are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures
We study a simple microscopic model for thin, ferromagnetic, metallic layers
on semi-infinite bulk superconductor. We find that for certain values of the
exchange spliting, on the ferromagnetic side, the ground states of such
structures feature spontaneously induced spin polarized currents. Using a
mean-field theory, which is selfconsistent with respect to the pairing
amplitude , spin polarization and the spontaneous current
, we show that not only there are Andreev bound states in the
ferromagnet but when their energies are near zero they support
spontaneous currents parallel to the ferromagnetic-superconducting interface.
Moreover, we demonstrate that the spin-polarization of these currents depends
sensitively on the band filling.Comment: 4 pages, 5 Postscript figures (included
Off-shell extension of S-matrix elements and tachyonic effective actions
We show that the on-shell S-matrix elements of four open string massless
scalars, two scalars and two tachyons, and four open string tachyons in the
super string theory can be written in a unique form. We then propose an
off-shell extension for the S-matrix element of four scalars which is
consistent, in the low energy limit, with the Dirac-Born-Infeld effective
action. Using a similar off-shell extension for the S-matrix element of two
scalars and two tachyons and for the S-matrix element of four tachyons, we show
that they are fully consistent with the tachyonic DBI action.Comment: Latex, 17 pages, v3:a paragraph comparing off-shell and on- shell
amplitudes added, reference adde
SD-brane gravity fields and rolling tachyons
S(pacelike)D-branes are objects arising naturally in string theory when
Dirichlet boundary conditions are imposed on the time direction. SD-brane
physics is inherently time-dependent. Previous investigations of gravity fields
of SD-branes have yielded undesirable naked spacelike singularities. We set up
the problem of coupling the most relevant open-string tachyonic mode to
massless closed-string modes in the bulk, with backreaction and Ramond-Ramond
fields included. We find solutions numerically in a self-consistent
approximation; our solutions are naturally asymptotically flat and
time-reversal asymmetric. We find completely nonsingular evolution; in
particular, the dilaton and curvature are well-behaved for all time. The
essential mechanism for spacetime singularity resolution is the inclusion of
full backreaction between the bulk fields and the rolling tachyon. Our analysis
is not the final word on the story, because we have to make some significant
approximations, most notably homogeneity of the tachyon on the unstable branes.
Nonetheless, we provide significant progress in plugging a gaping hole in prior
understanding of the gravity fields of SD-branes.Comment: References added. Analysis for much broader range of solutions
presented. Conclusions unchanged. Time-reversal symmetric examples ruled out,
new examples are provide
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