26,339 research outputs found
The contact conductance of a one-dimensional wire partly embedded in a superconductor
The conductance through a semi-infinite one-dimensional wire, partly embedded
in a superconducting bulk electrode, is studied. When the electron-electron
interactions within the wire are strongly repulsive, the wire effectively
decouples from the superconductor. If they are moderately or weakly repulsive,
the proximity of the superconductor induces superconducting order in the
segment of the wire embedded in it. In this case it is shown that the
conductance exhibits a crossover from conductive to insulating behavior as the
temperature is lowered down. The characteristic crossover temperature of this
transition has a stretched exponential dependence on the length of the part of
the wire embedded in the superconductor. The amount of this stretch is
determined by the nature of the electron interactions within the wire.Comment: 8 pages, 4 figure
Effects of the Charge-Dipole Interaction on the Coagulation of Fractal Aggregates
A numerical model with broad applications to complex (dusty) plasmas is
presented. The self-consistent N-body code allows simulation of the coagulation
of fractal aggregates, including the charge-dipole interaction of the clusters
due to the spatial arrangement of charge on the aggregate. It is shown that not
only does a population of oppositely charged particles increase the coagulation
rate, the inclusion of the charge-dipole interaction of the aggregates as well
as the electric dipole potential of the dust ensemble decreases the gelation
time by a factor of up to twenty. It is further shown that these interactions
can also stimulate the onset of gelation, or "runaway growth," even in a
population of particles charged to a monopotential where previously it was
believed that like-charged grains would inhibit coagulation. Gelation is
observed to occur due to the formation of high-mass aggregates with fractal
dimensions greater than two which act as seeds for runaway growth.Comment: 9 page
Research in planetary Astronomy
The focus was on the study, via near infrared observations, of the outer planets and their satellites. In the last year, these observations have emphasized imaging observations using the Cassegrain infrared camera at the f/70 focus of the 200 inch Hale telescope
Discovery of a Shell of Neutral Atomic Hydrogen Surrounding the Carbon Star IRC+10216
We have used the Robert C. Byrd Green Bank Telescope to perform the most
sensitive search to date for neutral atomic hydrogen (HI) in the circumstellar
envelope (CSE) of the carbon star IRC+10216. Our observations have uncovered a
low surface brightness HI shell of diameter ~1300" (~0.8 pc), centered on
IRC+10216. The HI shell has an angular extent comparable to the far
ultraviolet-emitting astrosphere of IRC+10216 previously detected with the
GALEX satellite, and its kinematics are consistent with circumstellar matter
that has been decelerated by the local interstellar medium. The shell appears
to completely surround the star, but the highest HI column densities are
measured along the leading edge of the shell, near the location of a previously
identified bow shock. We estimate a total mass of atomic hydrogen associated
with IRC+10216 CSE of M_HI~3x10e-3 M_sun. This is only a small fraction of the
expected total mass of the CSE (<1%) and is consistent with the bulk of the
stellar wind originating in molecular rather than atomic form, as expected for
a cool star with an effective temperature T_eff<~2200 K. HI mapping of a 2 deg
x 2 deg region surrounding IRC+10216 has also allowed us to characterize the
line-of-sight interstellar emission in the region and has uncovered a link
between diffuse FUV emission southwest of IRC+10216 and the Local Leo Cold
Cloud.Comment: Accepted to MNRAS. 16 pages. A version with full resolution figures
may be found at
http://www.haystack.mit.edu/hay/staff/lmatthew/Matthews_etal_IRC+10216.pd
HI emission from the red giant Y CVn with the VLA and FAST
Imaging studies with the VLA have revealed HI emission associated with the
extended circumstellar shells of red giants. We analyse the spectral map
obtained on Y CVn, a J-type carbon star on the AGB. The HI line profiles can be
interpreted with a model of a detached shell resulting from the interaction of
a stellar outflow with the local interstellar medium. We reproduce the spectral
map by introducing a distortion along a direction corresponding to the star's
motion in space. We then use this fitting to simulate observations expected
from the FAST radiotelescope, and discuss its potential for improving
ourdescription of the outer regions of circumstellar shells.Comment: accepted for publication in RA
Charging and coagulation of dust in protoplanetary plasma environments
Combining a particle-particle, particle-cluster and cluster-cluster
agglomeration model with an aggregate charging model, the coagulation and
charging of dust particles in various plasma environments relevant for
proto-planetary disks have been investigated. The results show that charged
aggregates tend to grow by adding small particles and clusters to larger
particles and clusters, leading to greater sizes and masses as compared to
neutral aggregates, for the same number of monomers in the aggregate. In
addition, aggregates coagulating in a Lorentzian plasma (containing a larger
fraction of high-energy plasma particles) are more massive and larger than
aggregates coagulating in a Maxwellian plasma, for the same plasma densities
and characteristic temperature. Comparisons of the grain structure, utilizing
the compactness factor, {\phi}{\sigma}, demonstrate that a Lorentzian plasma
environment results in fluffier aggregates, with small {\phi}{\sigma}, which
exhibit a narrow compactness factor distribution. Neutral aggregates are more
compact, with larger {\phi}{\sigma}, and exhibit a larger variation in
fluffiness. Measurement of the compactness factor of large populations of
aggregates is shown to provide information on the disk parameters that were
present during aggregation
Near Infrared Observations of a Redshift 5.34 Galaxy: Further Evidence for Dust Absorption in the Early Universe
Imaging at 1.25 and 2.20 microns has been obtained of the field containing
the galaxy (RD1) found at redshift 5.34 by Dey et al.(1998). This galaxy has
been detected at 1.25 microns, while the lower redshift (z=4.02) galaxy also
found in the same field by Dey et al. was detected at both 1.25 and 2.20
microns. Comparison to stellar population synthesis models indicates that if
RD1 is a young ( 0.5 mag)
is indicated. Combined with observations of other high redshift systems, these
data show that dust is likely to be an important component of young galaxies
even at redshifts of z > 5. The extinction-corrected monochromatic luminosity
of RD1 at 1500 angstroms is then a factor of about three larger than L(1500)*
as determined by Dickinson (1998) for z ~ 3 starburst galaxies. The implied
star formation rate in RD1, corrected for extinction, is ~ 50-100 solar masses
per year.Comment: plain LaTex with 1 postscript figure. ApJ Letters, accepte
Dusty plasma cavities: probe-induced and natural
A comprehensive exploration of regional dust evacuation in complex plasma
crystals is presented. Voids created in 3D crystals on the International Space
Station have provided a rich foundation for experiments, but cavities in dust
crystals formed in ground-based experiments have not received as much
attention. Inside a modified GEC RF cell, a powered vertical probe was used to
clear the central area of a dust crystal, producing a cavity with high
cylindrical symmetry. Cavities generated by three mechanisms are examined.
First, repulsion of micrometer-sized particles by a negatively charged probe is
investigated. A model of this effect developed for a DC plasma is modified and
applied to explain new experimental data in RF plasma. Second, the formation of
natural cavities is surveyed; a radial ion drag proposed to occur due to a
curved sheath is considered in conjunction with thermophoresis and a flattened
confinement potential above the center of the electrode. Finally, cavity
formation unexpectedly occurs upon increasing the probe potential above the
plasma floating potential. The cavities produced by these methods appear
similar, but each are shown to be facilitated by fundamentally different
processes.Comment: 10 pages, 12 figure
Interstate Differences in Insured Unemployment: Some Recent Evidence
Recent panel data is used to reconsider the determinants of interstate differences in the ratio of insured to total unemployment. We conclude that previous research on the influence of replacement rates, duration of jobless spells and female labor force participation is robust, but find that political affiliations and attitudes could be more important, and unionization rates less important, than once believed.insured unemployment, work norms, replacement rates, unionization
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