32,046 research outputs found
Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets
Using a model of nonmagnetic impurity potential, we have examined the
behavior of planar vortex solutions in the classical two-dimensional XY
ferromagnets in the presence of a spin vacancy localized out of the vortex
core. Our results show that a spinless atom impurity gives rise to an effective
potential that repels the vortex structure.Comment: 6 pages, 2 figures, RevTex
Orthogonality catastrophe and Kondo effect in graphene
Anderson's orthogonality catastrophe in graphene, at energies close to the
Dirac point, is analyzed. It is shown that, in clean systems, the orthogonality
catastrophe is suppressed, due to the vanishing density of states at the Dirac
point. In the presence of preexisting localized states at the Dirac energy, the
orthogonality catastrophe shows similar features to those found in normal
metals with a finite density of states at the Fermi level. The implications for
the Kondo effect induced by magnetic impurities, and for the Fermi edge
singularities in tunneling processes are also discussed.Comment: 7 pages, 7 figure
Global Alfven Wave Heating of the Magnetosphere of Young Stars
Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to
explain plasma heating in the magnetosphere of young stars. The wave and basic
plasma parameters are compatible with the requirement that the dissipation
length of GAWs be comparable to the distance between the shocked region at the
star's surface and the truncation region in the accretion disk. A two-fluid
magnetohydrodynamic plasma model is used in the analysis. A current carrying
filament along magnetic field lines acts as a waveguide for the GAW. The
current in the filament is driven by plasma waves along the magnetic field
lines and/or by plasma crossing magnetic field lines in the truncated region of
the disk of the accreting plasma. The conversion of a small fraction of the
kinetic energy into GAW energy is sufficient to heat the plasma filament to
observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure
The multi-thermal and multi-stranded nature of coronal rain
In this work, we analyse coordinated observations spanning chromospheric, TR
and coronal temperatures at very high resolution which reveal essential
characteristics of thermally unstable plasmas. Coronal rain is found to be a
highly multi-thermal phenomenon with a high degree of co-spatiality in the
multi-wavelength emission. EUV darkening and quasi-periodic intensity
variations are found to be strongly correlated to coronal rain showers.
Progressive cooling of coronal rain is observed, leading to a height dependence
of the emission. A fast-slow two-step catastrophic cooling progression is
found, which may reflect the transition to optically thick plasma states. The
intermittent and clumpy appearance of coronal rain at coronal heights becomes
more continuous and persistent at chromospheric heights just before impact,
mainly due to a funnel effect from the observed expansion of the magnetic
field. Strong density inhomogeneities on spatial scales of 0.2"-0.5" are found,
in which TR to chromospheric temperature transition occurs at the lowest
detectable scales. The shape of the distribution of coronal rain widths is
found to be independent of temperature with peaks close to the resolution limit
of each telescope, ranging from 0.2" to 0.8". However we find a sharp increase
of clump numbers at the coolest wavelengths and especially at higher
resolution, suggesting that the bulk of the rain distribution remains
undetected. Rain clumps appear organised in strands in both chromospheric and
TR temperatures, suggesting an important role of thermal instability in the
shaping of fundamental loop substructure. We further find structure reminiscent
of the MHD thermal mode. Rain core densities are estimated to vary between
2x10^{10} cm^{-3} and 2.5x10^{11} cm^{-3} leading to significant downward mass
fluxes per loop of 1-5x10^{9} g s^{-1}, suggesting a major role in the
chromosphere-corona mass cycle.Comment: Abstract is only short version. See paper for full. Countless pages,
figures (and movies, but not included here). Accepted for publication in the
Astrophysical Journa
On the far-infrared metallicity diagnostics: applications to high-redshift galaxies
In an earlier paper we modeled the far-infrared emission from a star-forming
galaxy using the photoionisation code CLOUDY and presented metallicity
sensitive diagnostics based on far-infrared fine structure line ratios. Here,
we focus on the applicability of the [OIII]88/[NII]122 microns line ratio as a
gas phase metallicity indicator in high redshift submillimetre luminous
galaxies. The [OIII]88/[NII]122 microns ratio is strongly dependent on the
ionization parameter (which is related to the total number of ionizing photons)
as well as the gas electron density. We demonstrate how the ratio of 88/$122
continuum flux measurements can provide a reasonable estimate of the ionization
parameter while the availability of the [NII]205 microns line can constrain the
electron density. Using the [OIII]88/[NII]122 microns line ratios from a sample
of nearby normal and star-forming galaxies we measure their gas phase
metallicities and find that their mass metallicity relation is consistent with
the one derived using optical emission lines. Using new, previously
unpublished, Herschel spectroscopic observations of key far-infrared fine
structure lines of the z~3 galaxy HLSW-01 and additional published measurements
of far-infrared fine structure lines of high-z submillimetre luminous galaxies
we derive gas phase metallicities using their [OIII]88/[NII]122 microns line
ratio. We find that the metallicities of these z~3 submm luminous galaxies are
consistent with solar metallicities and that they appear to follow the
mass-metallicity relation expected for z~3 systems.Comment: 10 pages, 7 figures, MNRAS in pres
Transition from Collisionless to Hydrodynamic Behaviour in an Ultracold Atomic Gas
Relative motion in a two-component, trapped atomic gas provides a sensitive
probe of interactions. By studying the lowest frequency excitations of a two
spin-state gas confined in a magnetic trap, we have explored the transition
from the collisionless to the hydrodynamic regime. As a function of collision
rate, we observe frequency shifts as large as 6% as well as a dramatic,
non-monotonic dependence of the damping rate. The measurements agree
qualitatively with expectations for behavior in the collisionless and
hydrodynamic limits and are quantitatively compared to a classical kinetic
model.Comment: 5 pages, 4 figure
Interfacial profiles of mismatched lamellae in thin diblock copolymer films
A number of recent experiments have allowed direct study of diblock copolymer morphology on patternedsurfaces. Here we examine, by computer simulation, the morphology of a diblock thin film in contact with a substrate having large stripes. The diblock aligns parallel to the surface but is frustrated at the point where two stripes join. We show here that at the joint one of the phases is continuous while the other is discontinuous. In the region of the join the lamellae form a kink with a length scale of the same size as the lamellar period. Within the kink the ordering is suppressed.G.G.P. and D.R.M.W. acknowledge support from an
ARC Large Grant and D.R.M.W. is supported by an ARC
QEII. AC acknowledges the Donors of the Petroleum Research
Fund, administered by the American Chemical Society
for support of this work. This work has also been supported
by National Science Foundation Grant No. CDA-
9724289 which has funded the purchase of computational
equipments ~including the Convex-Exemplar machine! at
Kansas State University
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