3,307 research outputs found
Resistance scaling at the Kosterlitz-Thouless transition
We study the linear resistance at the Kosterlitz-Thouless transition by Monte
Carlo simulation of vortex dynamics. Finite size scaling analysis of our data
show excellent agreement with scaling properties of the Kosterlitz-Thouless
transition. We also compare our results for the linear resistance with
experiments. By adjusting the vortex chemical potential to an optimum value,
the resistance at temperatures above the transition temperature agrees well
with experiments over many decades.Comment: 7 pages, 4 postscript figures included, LATEX, KTH-CMT-94-00
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
Monte Carlo calculation of the linear resistance of a three dimensional lattice Superconductor model in the London limit
We have studied the linear resistance of a three dimensional lattice
Superconductor model in the London limit London lattice model by Monte Carlo
simulation of the vortex loop dynamics. We find excellent finite size scaling
at the phase transition. We determine the dynamical exponent for the
isotropic London lattice model.Comment: 4 pages, RevTeX with 3 postscript figures include
Scaling behavior in the -relaxation regime of a supercooled Lennard-Jones mixture
We report the results of a molecular dynamics simulation of a supercooled
binary Lennard-Jones mixture. By plotting the self intermediate scattering
functions vs. rescaled time, we find a master curve in the -relaxation
regime. This master curve can be fitted well by a power-law for almost three
decades in rescaled time and the scaling time, or relaxation time, has a
power-law dependence on temperature. Thus the predictions of
mode-coupling-theory on the existence of a von Schweidler law are found to hold
for this system; moreover, the exponents in these two power-laws are very close
to satisfying the exponent relationship predicted by the mode-coupling-theory.
At low temperatures, the diffusion constants also show a power-law behavior
with the same critical temperature. However, the exponent for diffusion differs
from that of the relaxation time, a result that is in disagreement with the
theory.Comment: 8 pages, RevTex, four postscript figures available on request,
MZ-Physics-10
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Evidence of Two Distinct Dynamic Critical Exponents in Connection with Vortex Physics
The dynamic critical exponent is determined from numerical simulations
for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models
with relaxational dynamics. It is suggested that the dynamics is characterized
by two distinct dynamic critical indices and related to the
divergence of the relaxation time by and
, where is the correlation length and the
wavevector. The values determined are and for the
3D LCG and and for the 3D XY model. It is argued
that the nonlinear exponent relates to , whereas the usual
Hohenberg-Halperin classification relates to . Possible implications for the
interpretation of experiments are pointed out. Comparisons with other existing
results are discussed.Comment: to appear in PR
Automated Inspection Device for Explosive Charge in Shells - AIDECS
Certain defects in the explosive charge of an artillery shell can cause the projectile to explode prematurely in the barrel of the launcher from which it is fired. The sensitivity of the radiographic technique presently used is limited by the large influence of the steel shell casing on the transmitted radiation. A filmless radiometric technique utilizing the basic radiation principle of Compton scattering, which will detect cavities in the explosive filler with minimal interference from the steel casing, has been identified and tested. By scanning the shell with a beam of radiation and observing the Compton scattering through a unique collimating system, it has been possible to detect voids as small as 1/16 inch in cross section. The hardware consists of the source, beam collimator, detector collimator, and a large plastic scintillator detector system. The projectile is inserted into the beam path and moved through a fixed scanning pattern by a mechanical handling system. The scanning sequence is computer contra ll ed and results in a threedimensional data matrix giving a direct representation of density within the projectile. Voids are identified and classified by computer analysis, and shell acceptability decisions are automatically generated. An engineering prototype system is currently being assembled and tested. (A production prototype conceptual design is concurrently under development.) This new technique will replace an existing film radiography inspection procedure and eliminate the need for human interpretation of the defects, while providing more consistent and reliable inspections at lower costs
Black hole masses and enrichment of z ~ 6 SDSS quasars
We present sensitive near-infrared spectroscopic observations for a sample of
five z ~ 6 quasars. These are amongst the most distant, currently known quasars
in the universe. The spectra have been obtained using ISAAC at the VLT and
include the CIV, MgII and FeII lines. We measure the FeII/MgII line ratio, as
an observational proxy for the Fe/alpha element ratio. We derive a ratio of
2.7+/-0.8 for our sample, which is similar to that found for lower redshift
quasars, i.e., we provide additional evidence for the lack of evolution in the
FeII/MgII line ratio of quasars up to the highest redshifts. This result
demonstrates that the sample quasars must have undergone a major episode of
iron enrichment in less than one Gyr and star formation must have commenced at
z > 8. The linewidths of the MgII and CIV lines give two estimates for the
black hole masses. A third estimate is given by assuming that the quasars emit
at their Eddington luminosity. The derived masses using these three methods
agree well, implying that the quasars are not likely to be strongly lensed. We
derive central black hole masses of 0.3-5.2 10^9 solar masses. We use the
difference between the redshift of MgII (a proxy for the systemic redshift of
the quasar) and the onset of the Gunn Peterson trough to derive the extent of
the ionized Stromgren spheres around our target quasars. The derived physical
radii are about five Mpc. Using a simple ionization model, the emission of the
central quasars would need of order 10^6-10^8 year to create these cavities in
a surrounding intergalactic medium with a neutral fraction between 0.1 and 1.0.
As the e-folding time scale for the central accreting black hole is on the
order of a few times 10^7 year, it can grow by one e-folding or less within
this time span.Comment: Accepted by ApJ, 15 pages, 8 figure
Regulatory T Cells and IL-10 Independently Counterregulate Cytotoxic T Lymphocyte Responses Induced by Transcutaneous Immunization
The imidazoquinoline derivate imiquimod induces inflammatory responses and protection against transplanted tumors when applied to the skin in combination with a cognate peptide epitope (transcutaneous immunization, TCI). Here we investigated the role of regulatory T cells (T(reg)) and the suppressive cytokine IL-10 in restricting TCI-induced cytotoxic T lymphocyte (CTL) responses.TCI was performed with an ointment containing the TLR7 agonist imiquimod and a CTL epitope was applied to the depilated back skin of C57BL/6 mice. Using specific antibodies and FoxP3-diphteria toxin receptor transgenic (DEREG) mice, we interrogated inhibiting factors after TCI: by depleting FoxP3(+) regulatory T cells we found that specific CTL-responses were greatly enhanced. Beyond this, in IL-10 deficient (IL-10(-/-)) mice or after blocking of IL-10 signalling with an IL-10 receptor specific antibody, the TCI induced CTL response is greatly enhanced indicating an important role for this cytokine in TCI. However, by transfer of T(reg) in IL-10(-/-) mice and the use of B cell deficient JHT(-/-) mice, we can exclude T(reg) and B cells as source of IL-10 in the setting of TCI.We identify T(reg) and IL-10 as two important and independently acting suppressors of CTL-responses induced by transcutaneous immunization. Advanced vaccination strategies inhibiting T(reg) function and IL-10 release may lead the development of effective vaccination protocols aiming at the induction of T cell responses suitable for the prophylaxis or treatment of persistent infections or tumors
Possible first order transition in the two-dimensional Ginzburg-Landau model induced by thermally fluctuating vortex cores
We study the two-dimensional Ginzburg-Landau model of a neutral superfluid in
the vicinity of the vortex unbinding transition. The model is mapped onto an
effective interacting vortex gas by a systematic perturbative elimination of
all fluctuating degrees of freedom (amplitude {\em and} phase of the order
parameter field) except the vortex positions. In the Coulomb gas descriptions
derived previously in the literature, thermal amplitude fluctuations were
neglected altogether. We argue that, if one includes the latter, the vortices
still form a two- dimensional Coulomb gas, but the vortex fugacity can be
substantially raised. Under the assumption that Minnhagen's generic phase
diagram of the two- dimensional Coulomb gas is correct, our results then point
to a first order transition rather than a Kosterlitz-Thouless transition,
provided the Ginzburg-Landau correlation length is large enough in units of a
microscopic cutoff length for fluctuations. The experimental relevance of these
results is briefly discussed. [Submitted to J. Stat. Phys.]Comment: 36 pages, LaTeX, 6 figures upon request, UATP2-DB1-9
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