1,685 research outputs found
Finger patterns produced by thermomagnetic instability in superconductors
A linear analysis of thermal diffusion and Maxwell equations is applied to
study the thermomagnetic instability in a type-II superconducting slab. It is
shown that the instability can lead to formation of spatially nonuniform
distributions of magnetic field and temperature. The distributions acquire a
finger structure with fingers perpendicular to the screening current direction.
We derive the criterion for the instability, and estimate its build-up time and
characteristic finger width. The fingering instability emerges when the
background electric field is larger than a threshold field, , and the
applied magnetic field exceeds a value . Numerical
simulations support the analytical results, and allow to follow the development
of the fingering instability beyond the linear regime. The fingering
instability may be responsible for the nucleation of dendritic flux patterns
observed in superconducting films using magneto-optical imaging.Comment: 8 pages, 6 figures, accepted to Phys. Rev. B; (new version: minor
changes
On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions
Within a hydrodynamical approach we investigate the sensitivity of single
inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three
different equations of state of nuclear matter. Two of the equations of state
are based on lattice QCD results and include a phase transition to a
quark-gluon plasma. The third equation of state has been extracted from the
microscopic transport code RQMD under the assumption of complete local
thermalization. All three equations of state provide reasonable fits to data
taken by the NA44 and NA49 Collaborations. The initial conditions before the
evolution of the fireballs and the space-time evolution pictures differ
dramatically for the three equations of state when the same freeze-out
temperature is used in all calculations. However, the softest of the equations
of state results in transverse mass spectra that are too steep in the central
rapidity region. We conclude that the transverse particle momenta are
determined by the effective softness of the equation of state during the
fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of
the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm
Perturbative QCD Analysis of the Nucleon's Pauli Form Factor F_2(Q^2)
We perform a perturbative QCD analysis of the nucleon's Pauli form factor
in the asymptotically large limit. We find that the leading
contribution to has a power behavior, consistent with the
well-known result in the literature. Its coefficient depends on the leading-
and subleading-twist light-cone wave functions of the nucleon, the latter
describing the quarks with one unit of orbital angular momentum. We also derive
at the logarithmic accurary the asymptotic scaling which describes recent Jefferson Lab data well.Comment: 4 papes, 3 figures include
An unidentified TeV source in the vicinity of Cygnus OB2
Deep observation (∼113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ∼0.5° north of Cyg X-3. The source centre of gravity is RA αJ2000: 20hr32m07s± 9.2stats±2.2syss, Dec δJ2000: +41°30′30″2.0stat±0.4′sys. The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius 5.6′ at the ∼3σ level, and has a differential power-law flux with hard photon index of - 1.9 ± 0.3stat ± 0.3sys. The integral flux above 1 TeV amounts ∼3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.F. A. Aharonian, ... G. P. Rowell, ... [et al
Parallel Unsmoothed Aggregation Algebraic Multigrid Algorithms on GPUs
We design and implement a parallel algebraic multigrid method for isotropic
graph Laplacian problems on multicore Graphical Processing Units (GPUs). The
proposed AMG method is based on the aggregation framework. The setup phase of
the algorithm uses a parallel maximal independent set algorithm in forming
aggregates and the resulting coarse level hierarchy is then used in a K-cycle
iteration solve phase with a -Jacobi smoother. Numerical tests of a
parallel implementation of the method for graphics processors are presented to
demonstrate its effectiveness.Comment: 18 pages, 3 figure
Globally abundant “candidatus Udaeobacter” benefits from release of antibiotics in soil and potentially performs trace gas scavenging
Dynamics of vortex penetration, jumpwise instabilities and nonlinear surface resistance of type-II superconductors in strong rf fields
We consider nonlinear dynamics of a single vortex in a superconductor in a
strong rf magnetic field . Using the London theory, we
calculate the dissipated power , and the transient time scales
of vortex motion for the linear Bardeen-Stephen viscous drag force, which
results in unphysically high vortex velocities during vortex penetration
through the oscillating surface barrier. It is shown that penetration of a
single vortex through the ac surface barrier always involves penetration of an
antivortex and the subsequent annihilation of the vortex antivortex pairs.
Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex
velocities results in a jump-wise vortex penetration through the surface
barrier and a significant increase of the dissipated power. We calculate the
effect of dissipation on nonlinear vortex viscosity and the rf vortex
dynamics and show that it can also result in the LO-type behavior,
instabilities, and thermal localization of penetrating vortex channels. We
propose a thermal feedback model of , which not only results in the LO
dependence of for a steady-state motion, but also takes into account
retardation of temperature field around rapidly accelerating vortex, and a
long-range interaction with the surface. We also address the effect of pinning
on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on
the surface resistance calculated as a function or rf frequency and
field. It is shown that trapped flux can result in a temperature-independent
residual resistance at low , and a hysteretic low-field dependence of
, which can {\it decrease} as is increased, reaching a minimum
at much smaller than the thermodynamic critical field .Comment: 18 figure
Hydrodynamical analysis of symmetric nucleus-nucleus collisions at CERN/SPS energies
We present a coherent theoretical study of ultrarelativistic heavy-ion data
obtained at the CERN/SPS by the NA35/NA49 Collaborations using 3+1-dimensional
relativistic hydrodynamics. We find excellent agreement with the rapidity
spectra of negative hadrons and protons and with the correlation measurements
in two experiments: at 200 and at 160 (preliminary
results). Within our model this implies that for () a
quark-gluon-plasma of initial volume 174 (24 ) with a lifetime 3.4
(1.5 ) was formed. It is found that the Bose-Einstein correlation
measurements do not determine the maximal effective radii of the hadron sources
because of the large contributions from resonance decay at small momenta. Also
within this study we present an NA49 acceptance corrected two-pion
Bose-Einstein correlation function in the invariant variable, .Comment: 21 pages, 11 Postscript figures (1 File, 775654 Bytes, has to be
requested for submission via e.mail from [email protected]
Various Models for Pion Probability Distributions from Heavy-Ion Collisions
Various models for pion multiplicity distributions produced in relativistic
heavy ion collisions are discussed. The models include a relativistic
hydrodynamic model, a thermodynamic description, an emitting source pion laser
model, and a description which generates a negative binomial description. The
approach developed can be used to discuss other cases which will be mentioned.
The pion probability distributions for these various cases are compared.
Comparison of the pion laser model and Bose-Einstein condensation in a laser
trap and with the thermal model are made. The thermal model and hydrodynamic
model are also used to illustrate why the number of pions never diverges and
why the Bose-Einstein correction effects are relatively small. The pion
emission strength of a Poisson emitter and a critical density
are connected in a thermal model by , and this fact
reduces any Bose-Einstein correction effects in the number and number
fluctuation of pions. Fluctuations can be much larger than Poisson in the pion
laser model and for a negative binomial description. The clan representation of
the negative binomial distribution due to Van Hove and Giovannini is discussed
using the present description. Applications to CERN/NA44 and CERN/NA49 data are
discussed in terms of the relativistic hydrodynamic model.Comment: 12 pages, incl. 3 figures and 4 tables. You can also download a
PostScript file of the manuscript from
http://p2hp2.lanl.gov/people/schlei/eprint.htm
Analysis of Biomechanical Response After Corneal Crosslinking with Different Fluence Levels in Porcine Corneas.
PURPOSE
To evaluate corneal stiffening of porcine corneas induced by corneal crosslinking (CXL) with constant irradiance as a function of total fluence.
METHODS
Ninety corneas from freshly enucleated porcine eyes were divided into five groups of 18 eyes. Groups 1-4 underwent epi-off CXL using a dextran-based riboflavin solution and an irradiance of 18 mW/cm2, group 5 served as the control group. Groups 1 to 4 were treated with a total fluence of 20, 15, 10.8, and 5.4 J/cm2, respectively. Thereafter, biomechanical measurements were performed on 5 mm wide and 6 mm long strips using an uniaxial material tester. Pachymetry measurements were performed on each cornea.
RESULTS
At 10% strain, the stress was 76, 56, 52, and 31% higher in groups 1-4, respectively compared to the control group. The Young's modulus was 2.85 MPa for group 1, 2.53 MPa for group 2, 2.46 MPa for group 3, 2.12 MPa for group 4, and 1.62 MPa for the control group. The difference between groups 1 to 4 and the control group 5 were statistically significant (p = <0.001; p = <0.001; p = <0.001; p = 0.021). In addition, group 1 showed significantly more stiffening than group 4 (p = <0.001), no other significant differences were found. Pachymetry measurements revealed no statistically significant differences among the five groups.
CONCLUSION
Additional mechanical stiffening can be achieved by increasing the fluence of the CXL. There was no threshold detected up to 20 J/cm2. A higher fluence could compensate the weaker effect of accelerated or epi-on CXL procedures
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