31,622 research outputs found
Isotopic difference in the heteronuclear loss rate in a two-species surface trap
We have realized a two-species mirror-magneto-optical trap containing a
mixture of Rb (Rb) and Cs atoms. Using this trap, we have
measured the heteronuclear collisional loss rate due to
intra-species cold collisions. We find a distinct difference in the magnitude
and intensity dependence of for the two isotopes Rb and
Rb which we attribute to the different ground-state hyperfine splitting
energies of the two isotopes.Comment: 4 pages, 2 figure
Herschel Observations and Updated Spectral Energy Distributions of Five Sunlike Stars with Debris Disks
Observations from the Herschel Space Observatory have more than doubled the
number of wide debris disks orbiting Sunlike stars to include over 30 systems
with R > 100 AU. Here we present new Herschel PACS and re-analyzed Spitzer MIPS
photometry of five Sunlike stars with wide debris disks, from Kuiper belt size
to R > 150 AU. The disk surrounding HD 105211 is well resolved, with an angular
extent of >14" along the major axis, and the disks of HD 33636, HD 50554, and
HD 52265 are extended beyond the PACS PSF size (50% of energy enclosed within
radius 4.23"). HD 105211 also has a 24-micron infrared excess that was
previously overlooked because of a poorly constrained photospheric model.
Archival Spitzer IRS observations indicate that the disks have small grains of
minimum radius ~3 microns, though the minimum grain gradius is larger than the
radiation pressure blowout size in all systems. If modeled as
single-temperature blackbodies, the disk temperatures would all be <60 K. Our
radiative transfer models predict actual disk radii approximately twice the
radius of model blackbody disks. We find that the Herschel photometry traces
dust near the source population of planetesimals. The disk luminosities are in
the range 0.00002 <= L/L* <= 0.0002, consistent with collisions in icy
planetesimal belts stirred by Pluto-size dwarf planets.Comment: Accepted for publication in ApJ. 18 pages, including 10 figures and 3
table
Macroscopic quantum coherence in spinor condensates confined in an anisotropic potential
We investigate the macroscopic quantum coherence of a spin-1 Rb condensate
confined in an anisotropic potential. Under the single-mode approximation, we
show that the system can be modeled as a biaxial quantum magnet due to the
interplay between the magnetic dipole-dipole interaction and the anisotropic
potential. By applying a magnetic field along the hard-axis, we show that the
tunneling splitting oscillates as a function of the field strength. We also
propose an experimental scheme to detect the oscillatory behavior of the
tunneling splitting by employing the Landau-Zener tunneling.Comment: 5 pages, 4 figure
SCOOTER: A compact and scalable dynamic labeling scheme for XML updates
Although dynamic labeling schemes for XML have been the
focus of recent research activity, there are significant challenges still to be overcome. In particular, though there are labeling schemes that ensure a compact label representation when creating an XML document, when the document is subject to repeated and arbitrary deletions and insertions, the labels grow rapidly and consequently have a significant impact on query and update performance. We review the outstanding issues todate and in this paper we propose SCOOTER - a new dynamic labeling scheme for XML. The new labeling scheme can completely avoid relabeling
existing labels. In particular, SCOOTER can handle frequently skewed insertions gracefully. Theoretical analysis and experimental results confirm the scalability, compact representation, efficient growth rate and performance of SCOOTER in comparison to existing dynamic labeling schemes
Driven Pair Contact Process with Diffusion
The pair contact process with diffusion (PCPD) has been recently investigated
extensively, but its critical behavior is not yet clearly established. By
introducing biased diffusion, we show that the external driving is relevant and
the driven PCPD exhibits a mean-field-type critical behavior even in one
dimension. In systems which can be described by a single-species bosonic field
theory, the Galilean invariance guarantees that the driving is irrelevant. The
well-established directed percolation (DP) and parity conserving (PC) classes
are such examples. This leads us to conclude that the PCPD universality class
should be distinct from the DP or PC class. Moreover, it implies that the PCPD
is generically a multi-species model and a field theory of two species is
suitable for proper description
Protoplanetary and Transitional Disks in the Open Stellar Cluster IC 2395
We present new deep UBVRI images and high-resolution multi-object optical
spectroscopy of the young (~ 6 - 10 Myr old), relatively nearby (800 pc) open
cluster IC 2395. We identify nearly 300 cluster members and use the photometry
to estimate their spectral types, which extend from early B to middle M. We
also present an infrared imaging survey of the central region using the IRAC
and MIPS instruments on board the Spitzer Space Telescope, covering the
wavelength range from 3.6 to 24 microns. Our infrared observations allow us to
detect dust in circumstellar disks originating over a typical range of radii ~
0.1 to ~ 10AU from the central star. We identify 18 Class II, 8 transitional
disk, and 23 debris disk candidates, respectively 6.5%, 2.9%, and 8.3% of the
cluster members with appropriate data. We apply the same criteria for
transitional disk identification to 19 other stellar clusters and associations
spanning ages from ~ 1 to ~ 18 Myr. We find that the number of disks in the
transitional phase as a fraction of the total with strong 24 micron excesses
([8] - [24] > 1.5) increases from 8.4 +\- 1.3% at ~ 3 Myr to 46 +\- 5% at ~ 10
Myr. Alternative definitions of transitional disks will yield different
percentages but should show the same trend.Comment: accepted by the Astrophysical Journa
The Structure of the {\beta} Leonis Debris Disk
We combine nulling interferometry at 10 {\mu}m using the MMT and Keck
Telescopes with spectroscopy, imaging, and photometry from 3 to 100 {\mu}m
using Spitzer to study the debris disk around {\beta} Leo over a broad range of
spatial scales, corresponding to radii of 0.1 to ~100 AU. We have also measured
the close binary star o Leo with both Keck and MMT interferometers to verify
our procedures with these instruments. The {\beta} Leo debris system has a
complex structure: 1.) relatively little material within 1 AU; 2.) an inner
component with a color temperature of ~600 K, fitted by a dusty ring from about
2 to 3 AU; and 3.) a second component with a color temperature of ~120 K fitted
by a broad dusty emission zone extending from about ~5 AU to ~55 AU. Unlike
many other A-type stars with debris disks, {\beta} Leo lacks a dominant outer
belt near 100 AU.Comment: 14 page body, 3 page appendix, 15 figure
Density of Neutral Solitons in Weakly Disordered Peierls Chains
We study the effects of weak off-diagonal disorder on Peierls systems with a
doubly degenerate ground state. We show that for these systems disorder in the
electron hopping amplitudes induces a finite density of solitons in the
minimal-energy lattice configuration of a single chain. These disorder-induced
dimerization kinks are neutral and have spin 1/2. Using a continuum model for
the Peierls chain and treating the lattice classically, we analytically
calculate the average free energy and density of kinks. We compare these
results to numerical calculations for a discrete model and discuss the
implications of the kinks for the optical and magnetic properties of the
conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 5 Postscript figures, to appear in Phys. Rev.
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