445 research outputs found
Duality in Shearing Rheology Near the Athermal Jamming Transition
We consider the rheology of soft-core frictionless disks in two dimensions in
the neighborhood of the athermal jamming transition. From numerical simulations
of bidisperse, overdamped, particles, we argue that the divergence of the
viscosity below jamming is characteristic of the hard-core limit, independent
of the particular soft-core interaction. We develop a mapping from soft-core to
hard-core particles that recovers all the critical behavior found in earlier
scaling analyses. Using this mapping we derive a duality relation that gives
the exponent of the non-linear Herschel-Bulkley rheology above jamming in terms
of the exponent of the diverging viscosity below jamming.Comment: 5 pages, 4 figures. Manuscript revisions: new title, additional text
concerning connections to experiment, revised Fig. 4, other minor changes and
clarifications in text. Conclusions remain essentially unchanged. Accepted
for publication in Phys. Rev. Let
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Clinical Utility of Echocardiography in Former Preterm Infants with Bronchopulmonary Dysplasia.
BackgroundThe clinical utility of echocardiography for the diagnosis of pulmonary vascular disease (PVD) in former preterm infants with bronchopulmonary dysplasia (BPD) is not established. Elevated pulmonary vascular resistance (PVR) rather than pulmonary artery pressure (PAP) is the hallmark of PVD. We evaluated the utility of echocardiography in infants with BPD in diagnosing pulmonary hypertension and PVD (PVR >3 Wood units × m2) assessed by cardiac catheterization.MethodsA retrospective single center study of 29 infants born ≤29 weeks of gestational age with BPD who underwent cardiac catheterization and echocardiography was performed. PVD was considered present by echocardiography if the tricuspid valve regurgitation jet peak velocity was >2.9 m/sec, post-tricuspid valve shunt systolic flow velocity estimated a right ventricular systolic pressure >35 mm Hg, or systolic septal flattening was present. The utility (accuracy, sensitivity, and positive predictive value [PPV]) of echocardiography in the diagnosis of PVD was tested. Subgroup analysis in patients without post-tricuspid valve shunts was performed. Echocardiographic estimations of right ventricular pressure, dimensions, function, and pulmonary flow measurements were evaluated for correlation with PVR.ResultsThe duration between echocardiography and cardiac catheterization was a median of 1 day (interquartile range, 1-4 days). Accuracy, sensitivity, and PPV of echocardiography in diagnosing PVD were 72%, 90.5%, and 76%, respectively. Accuracy, sensitivity, and PPV increased to 93%, 91.7%, and 100%, respectively, when infants with post-tricuspid valve shunts were excluded. Echocardiography had poor accuracy in estimating the degree of PAP elevation by cardiac catheterization. In infants without post-tricuspid valve shunts, there was moderate to good correlation between indexed PVR and right ventricular myocardial performance index (rho = 0.89, P = .005), systolic to diastolic time index (0.84, P < .001), right to left ventricular diameter ratio at end systole (0.66, P = .003), and pulmonary artery acceleration time (0.48, P = .05).ConclusionsEchocardiography performs well in screening for PVD in infants with BPD and may be diagnostic in the absence of a post-tricuspid valve shunt. However, cardiac catheterization is needed to assess the degree of PAP elevation and PVR. The diagnostic utility of echocardiographic measurements that correlate with PVR should be evaluated prospectively in this patient population
Boundary effects on the scaling of the superfluid density
We study numerically the influence of the substrate (boundary conditions) on
the finite--size scaling properties of the superfluid density in
superfluid films of thickness within the XY model employing the Monte Carlo
method. Our results suggest that the jump at the
Kosterlitz--Thouless transition temperature depends on the boundary
conditions.Comment: 2 pages, 1 Latex file, 1 postscript figure, 2 style file
Glassiness Vs. Order in Densely Frustrated Josephson Arrays
We carry out extensive Monte Carlo simulations on the Coulomb gas dual to the
uniformly frustrated two dimensional XY model, for a sequence of frustrations f
converging to the irraltional (3-sqrt 5)/2. We find in these systems a sharp
first order equilibrium phase transition to an ordered vortex structure at a
T_c which varies only slightly with f. This ordered vortex structure remains in
general phase incoherent until a lower pinning transition T_p(f) that varies
with f. We argue that the glassy behaviors reported for this model in earlier
simulations are dynamic effects.Comment: 4 pages, 4 eps figure
Kinetic Inductance of Josephson Junction Arrays: Dynamic and Equilibrium Calculations
We show analytically that the inverse kinetic inductance of an
overdamped junction array at low frequencies is proportional to the admittance
of an inhomogeneous equivalent impedance network. The bond in this
equivalent network has an inverse inductance
, where is the Josephson
coupling energy of the bond, is the ground-state phase
of the grain , and is the usual magnetic phase factor. We use this
theorem to calculate for square arrays as large as .
The calculated is in very good agreement with the low-temperature
limit of the helicity modulus calculated by conventional equilibrium
Monte Carlo techniques. However, the finite temperature structure of ,
as a function of magnetic field, is \underline{sharper} than the
zero-temperature , which shows surprisingly weak structure. In
triangular arrays, the equilibrium calculation of yields a series of
peaks at frustrations , where is an integer , consistent with experiment.Comment: 14 pages + 6 postscript figures, 3.0 REVTe
Patterns of Striped order in the Classical Lattice Coulomb Gas
We obtain via Monte Carlo simulations the low temperature charge
configurations in the lattice Coulomb gas on square lattices for charge filling
ratio in the range . We find a simple regularity in the low
temperature charge configurations which consist of a suitable periodic
combination of a few basic striped patterns characterized by the existence of
partially filled diagonal channels. In general there exist two separate
transitions where the lower temperature transition () corresponds to the
freezing of charges within the partially filled channels. is found to be
sensitively dependent on through the charge number density within the channels.Comment: 4 pages, 8 figure
Vortex lattce melting in 2D superconductors and Josephson arrays
Monte Carlo simulations of 2D vortex lattice melting in a thin
superconducting film (or alternatively an array of Josephson junctions) are
performed in the London limit. Finite size scaling analyses are used to make a
detailed test of the dislocation mediated melting theory of KTNHY. We find that
the melting transition is weakly first order, with a jump in the shear modulus
very close to that predicted by the KTNHY theory. No hexatic liquid phase is
found.Comment: 12 pages, 4 figures (available on request from
[email protected]), REVTEX [we revise our conclusion on the order of the
melting transition from second to first order - new figure 4 added
Current-voltage scaling of a Josephson-junction array at irrational frustration
Numerical simulations of the current-voltage characteristics of an ordered
two-dimensional Josephson junction array at an irrational flux quantum per
plaquette are presented. The results are consistent with an scaling analysis
which assumes a zero temperature vortex glass transition. The thermal
correlation length exponent characterizing this transition is found to be
significantly different from the corresponding value for vortex-glass models in
disordered two-dimensional superconductors. This leads to a current scale where
nonlinearities appear in the current-voltage characteristics decreasing with
temperature roughly as in contrast with the behavior expected
for disordered models.Comment: RevTex 3.0, 12 pages with Latex figures, to appear in Phys. Rev. B
54, Rapid. Com
The Effect of Columnar Disorder on the Superconducting Transition of a Type-II Superconductor in Zero Applied Magnetic Field
We investigate the effect of random columnar disorder on the superconducting
phase transition of a type-II superconductor in zero applied magnetic field
using numerical simulations of three dimensional XY and vortex loop models. We
consider both an unscreened model, in which the bare magnetic penetration
length is approximated as infinite, and a strongly screened model, in which the
magnetic penetration length is of order the vortex core radius. We consider
both equilibrium and dynamic critical exponents. We show that, as in the
disorder free case, the equilibrium transitions of the unscreened and strongly
screened models lie in the same universality class, however scaling is now
anisotropic. We find for the correlation length exponent , and
for the anisotropy exponent . We find different dynamic
critical exponents for the unscreened and strongly screened models.Comment: 30 pages 12 ps figure
Positional Disorder (Random Gaussian Phase Shifts) in the Fully Frustrated Josephson Junction Array (2D XY Model)
We consider the effect of positional disorder on a Josephson junction array
with an applied magnetic field of f=1/2 flux quantum per unit cell. This is
equivalent to the problem of random Gaussian phase shifts in the fully
frustrated 2D XY model. Using simple analytical arguments and numerical
simulations, we present evidence that the ground state vortex lattice of the
pure model becomes disordered, in the thermodynamic limit, by any amount of
positional disorder.Comment: 4 pages, 4 eps figures embedde
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