1,175 research outputs found
Weak Long-Ranged Casimir Attraction in Colloidal Crystals
We investigate the influence of geometric confinement on the free energy of
an idealized model for charge-stabilized colloidal suspensions. The mean-field
Poisson-Boltzmann formulation for this system predicts pure repulsion among
macroionic colloidal spheres. Fluctuations in the simple ions' distribution
provide a mechanism for the macroions to attract each other at large
separations. Although this Casimir interaction is long-ranged, it is too weak
to influence colloidal crystals' dynamics.Comment: 5 pages 2 figures ReVTe
C IV BAL disappearance in a large SDSS QSO sample
Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs)
originate from outflowing winds along our line of sight; winds are thought to
originate from the inner regions of the QSO accretion disk, close to the
central supermassive black hole (SMBH). Winds likely play a role in galaxy
evolution and aid the accretion mechanism onto the SMBH. BAL equivalent widths
can change on typical timescales from months to years; such variability is
generally attributed to changes in the covering factor and/or in the ionization
level of the gas. We investigate BAL variability, focusing on BAL
disappearance. We analyze multi-epoch spectra of more than 1500 QSOs -the
largest sample ever used for such a study- observed by different programs from
the Sloan Digital Sky Survey-I/II/III (SDSS), and search for disappearing C IV
BALs. The spectra rest-frame time baseline ranges from 0.28 to 4.9 yr; the
source redshifts range from 1.68 to 4.27. We detect 73 disappearing BALs in the
spectra of 67 sources. This corresponds to 3.9% of disappearing BALs, and 5.1%
of our BAL QSOs exhibit at least one disappearing BAL. We estimate the average
lifetime of a BAL along our line of sight (~ 80-100 yr), which appears
consistent with the accretion disk orbital time at distances where winds are
thought to originate. We inspect properties of the disappearing BALs and
compare them to the properties of our main sample. We also investigate the
existence of a correlation in the variability of multiple troughs in the same
spectrum, and find it persistent at large velocity offsets between BAL pairs,
suggesting that a mechanism extending on a global scale is necessary to explain
the phenomenon. We select a more reliable sample of disappearing BALs following
Filiz Ak et al. (2012), where a subset of our sample was analyzed, and compare
the findings from the two works, obtaining generally consistent results.Comment: 22 pages, 9 figures. Accepted for publication in A&
Observation of Cold Collisions between Trapped Ions and Trapped Atoms
We demonstrate a double-trap system well suited to study cold collisions
between trapped ions and trapped atoms. Using Yb ions confined in a Paul
trap and Yb atoms in a magneto-optical trap, we investigate charge-exchange
collisions of several isotopes for collision energies down to 400 neV (5 mK).
The measured rate coefficient of cms, constant
over four orders of magnitude in collision energy, is in good agreement with
that derived from a semiclassical Langevin model for an atomic polarizability
of 143 a.u.Comment: 4 pages, 4 figures; Revision 1/V2: Revised in response to PRL
Referees' comment
Structure of Flux Line Lattices with Weak Disorder at Large Length Scales
Dislocation-free decoration images containing up to 80,000 vortices have been
obtained on high quality BiSrCaCuO superconducting
single crystals. The observed flux line lattices are in the random manifold
regime with a roughening exponent of 0.44 for length scales up to 80-100
lattice constants. At larger length scales, the data exhibit nonequilibrium
features that persist for different cooling rates and field histories.Comment: 4 pages, 3 gif images, to appear in PRB rapid communicatio
A proposed metric for assessing the measurement quality of individual microarrays
BACKGROUND: High-density microarray technology is increasingly applied to study gene expression levels on a large scale. Microarray experiments rely on several critical steps that may introduce error and uncertainty in analyses. These steps include mRNA sample extraction, amplification and labeling, hybridization, and scanning. In some cases this may be manifested as systematic spatial variation on the surface of microarray in which expression measurements within an individual array may vary as a function of geographic position on the array surface. RESULTS: We hypothesized that an index of the degree of spatiality of gene expression measurements associated with their physical geographic locations on an array could indicate the summary of the physical reliability of the microarray. We introduced a novel way to formulate this index using a statistical analysis tool. Our approach regressed gene expression intensity measurements on a polynomial response surface of the microarray's Cartesian coordinates. We demonstrated this method using a fixed model and presented results from real and simulated datasets. CONCLUSION: We demonstrated the potential of such a quantitative metric for assessing the reliability of individual arrays. Moreover, we showed that this procedure can be incorporated into laboratory practice as a means to set quality control specifications and as a tool to determine whether an array has sufficient quality to be retained in terms of spatial correlation of gene expression measurements
The Sloan Digital Sky Survey Reverberation Mapping Project: Velocity Shifts of Quasar Emission Lines
Quasar emission lines are often shifted from the systemic velocity due to
various dynamical and radiative processes in the line-emitting region. The
level of these velocity shifts depends both on the line species and on quasar
properties. We study velocity shifts for the line peaks of various narrow and
broad quasar emission lines relative to systemic using a sample of 849 quasars
from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. The
coadded (from 32 epochs) spectra of individual quasars have sufficient
signal-to-noise ratio (SNR) to measure stellar absorption lines to provide
reliable systemic velocity estimates, as well as weak narrow emission lines.
The sample also covers a large dynamic range in quasar luminosity (~2 dex),
allowing us to explore potential luminosity dependence of the velocity shifts.
We derive average line peak velocity shifts as a function of quasar luminosity
for different lines, and quantify their intrinsic scatter. We further quantify
how well the peak velocity can be measured for various lines as a function of
continuum SNR, and demonstrate there is no systematic bias in the line peak
measurements when the spectral quality is degraded to as low as SNR~3 per SDSS
pixel. Based on the observed line shifts, we provide empirical guidelines on
redshift estimation from [OII]3728, [OIII]5008, [NeV]3426, MgII, CIII],
HeII1640, broad Hbeta, CIV, and SiIV, which are calibrated to provide unbiased
systemic redshifts in the mean, but with increasing intrinsic uncertainties of
46, 56, 119, 205, 233, 242, 400, 415, and 477 km/s, in addition to the
measurement uncertainties. These more realistic redshift uncertainties are
generally much larger than the formal uncertainties reported by the redshift
pipelines for spectroscopic quasar surveys, and demonstrate the infeasibility
of measuring quasar redshifts to better than ~200 km/s with only broad lines.Comment: matched to the published version; minor changes and conclusions
unchange
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
Correction to “Temperature-Dependent High-Speed Dynamics of Terahertz Quantum Cascade Lasers”
Corrections to author affiliation information is presented in the above named paper
Bragg- and Moving-glasses: a theory of disordered vortex lattices
We study periodic lattices, such as vortex lattices in type II
superconductors in a random pinning potential.
For the static case we review the prediction that the phase diagram of such
systems consists of a topologically ordered Bragg glass phase, with quasi long
range translational order, at low fields. This Bragg glass phase undergoes a
transition at higher fields into another glassy phase, with dislocations, or a
liquid. This proposition is compatible with a large number of experimental
results on BSCCO or Thalium compounds. Further experimental consequences of our
results and relevance to other systems will be discussed.
When such vortex systems are driven by an external force, we show that, due
to periodicity in the direction transverse to motion, the effects of static
disorder persist even at large velocity. In , at weak disorder, or large
velocity the lattice forms a topologically ordered glass state, the ``moving
Bragg glass'', an anisotropic version of the static Bragg glass. The lattice
flows through well-defined, elastically coupled, static channels. We determine
the roughness of the manifold of channels and the positional correlation
functions. The channel structure also provides a natural starting point to
study the influence of topological defects such as dislocations. In or at
strong disorder the channels can decouple along the direction of motion leading
to a ``smectic'' like flow. We also show that such a structure exhibits an
effective transverse critical pinning force due to barriers to transverse
motion, and discuss the experimental consequences of this effect.Comment: Proceedings of M2S-HTSC-V conference (Beijing, Feb 97) to be
published in Physica C; 4 pages, 3 figures, uses espcrc2.st
Phase diagrams of flux lattices with disorder
We review the prediction, made in a previous work [Phys. Rev. B 52 (1995)],
that the phase diagram of type II superconductors consists of a topologically
ordered Bragg glass phase at low fields undergoing a transition at higher
fields into a vortex glass or a liquid. We estimate the position of the phase
boundary using a Lindemann criterion. We find that the proposed phenomenology
is compatible with recent experiments on superconductors.Comment: 7 pages 2 figures, uses epsfi
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