1,524 research outputs found
Multidimensional optical fractionation with holographic verification
The trajectories of colloidal particles driven through a periodic potential
energy landscape can become kinetically locked in to directions dictated by the
landscape's symmetries. When the landscape is realized with forces exerted by a
structured light field, the path a given particle follows has been predicted to
depend exquisitely sensitively on such properties as the particle's size and
refractive index These predictions, however, have not been tested
experimentally. Here, we describe measurements of colloidal silica spheres'
transport through arrays of holographic optical traps that use holographic
video microscopy to track individual spheres' motions in three dimensions and
simultaneously to measure each sphere's radius and refractive index with
part-per-thousand resolution. These measurements confirm previously untested
predictions for the threshold of kinetically locked-in transport, and
demonstrate the ability of optical fractionation to sort colloidal spheres with
part-per-thousand resolution on multiple characteristics simultaneously.Comment: 4 pages, 2 figures. Accepted for publication in Physical Review
Letter
The Importance of Broad Emission-Line Widths in Single Epoch Black Hole Mass Estimates
Estimates of the mass of super-massive black holes (BHs) in distant active
galactic nuclei (AGNs) can be obtained efficiently only through single-epoch
spectra, using a combination of their broad emission-line widths and continuum
luminosities. Yet the reliability and accuracy of the method, and the resulting
mass estimates, M_BH, remain uncertain. A recent study by Croom using a sample
of SDSS, 2QZ and 2SLAQ quasars suggests that line widths contribute little
information about the BH mass in these single-epoch estimates and can be
replaced by a constant value without significant loss of accuracy. In this
Letter, we use a sample of nearby reverberation-mapped AGNs to show that this
conclusion is not universally applicable. We use the bulge luminosity (L_Bulge)
of these local objects to test how well the known M_BH - L_Bulge correlation is
recovered when using randomly assigned line widths instead of the measured ones
to estimate M_BH. We find that line widths provide significant information
about M_BH, and that for this sample, the line width information is just as
significant as that provided by the continuum luminosities. We discuss the
effects of observational biases upon the analysis of Croom and suggest that the
results can probably be explained as a bias of flux-limited, shallow quasar
samples.Comment: 10 text pages + 4 Figures + 1 Table. Accepted for publication in ApJ
Letter
The Computational Complexity of the Game of Set and its Theoretical Applications
The game of SET is a popular card game in which the objective is to form Sets
using cards from a special deck. In this paper we study single- and multi-round
variations of this game from the computational complexity point of view and
establish interesting connections with other classical computational problems.
Specifically, we first show that a natural generalization of the problem of
finding a single Set, parameterized by the size of the sought Set is W-hard;
our reduction applies also to a natural parameterization of Perfect
Multi-Dimensional Matching, a result which may be of independent interest.
Second, we observe that a version of the game where one seeks to find the
largest possible number of disjoint Sets from a given set of cards is a special
case of 3-Set Packing; we establish that this restriction remains NP-complete.
Similarly, the version where one seeks to find the smallest number of disjoint
Sets that overlap all possible Sets is shown to be NP-complete, through a close
connection to the Independent Edge Dominating Set problem. Finally, we study a
2-player version of the game, for which we show a close connection to Arc
Kayles, as well as fixed-parameter tractability when parameterized by the
number of rounds played
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
Bright Source of Cold Ions for Surface-Electrode Traps
We produce large numbers of low-energy ions by photoionization of
laser-cooled atoms inside a surface-electrode-based Paul trap. The
isotope-selective trap loading rate of Yb ions/s exceeds
that attained by photoionization (electron impact ionization) of an atomic beam
by four (six) orders of magnitude. Traps as shallow as 0.13 eV are easily
loaded with this technique. The ions are confined in the same spatial region as
the laser-cooled atoms, which will allow the experimental investigation of
interactions between cold ions and cold atoms or Bose-Einstein condensates.Comment: Paper submitted to PRL for review on 2/1/0
Anomalous interactions in confined charge-stabilized colloid
Charge-stabilized colloidal spheres dispersed in weak 1:1 electrolytes are
supposed to repel each other. Consequently, experimental evidence for anomalous
long-ranged like-charged attractions induced by geometric confinement inspired
a burst of activity. This has largely subsided because of nagging doubts
regarding the experiments' reliability and interpretation. We describe a new
class of thermodynamically self-consistent colloidal interaction measurements
that confirm the appearance of pairwise attractions among colloidal spheres
confined by one or two bounding walls. In addition to supporting previous
claims for this as-yet unexplained effect, these measurements also cast new
light on its mechanism.Comment: 8 pages, 5 figures, RevTeX4. Conference proceedings for CODEF-04,
Colloidal Dispersions in External Fields, March 29 - April 1, 200
Correlated particle dynamics in concentrated quasi-two-dimensional suspensions
We investigate theoretically and experimentally how the hydrodynamically
correlated lateral motion of particles in a suspension confined between two
surfaces is affected by the suspension concentration. Despite the long range of
the correlations (decaying as 1/r^2 with the inter-particle distance r), the
concentration effect is present only at short inter-particle distances for
which the static pair correlation is nonuniform. This is in sharp contrast with
the effect of hydrodynamic screening present in unconfined suspensions, where
increasing the concentration changes the prefactor of the large-distance
correlation.Comment: 13 page
Variability of Low-ionization Broad Absorption Line Quasars Based on Multi-epoch Spectra from The Sloan Digital Sky Survey
We present absorption variability results for 134 bona fide \mgii\ broad
absorption line (BAL) quasars at 0.46~~2.3 covering days
to 10 yr in the rest frame. We use multiple-epoch spectra from the Sloan
Digital Sky Survey, which has delivered the largest such BAL-variability sample
ever studied. \mgii-BAL identifications and related measurements are compiled
and presented in a catalog. We find a remarkable time-dependent asymmetry in EW
variation from the sample, such that weakening troughs outnumber strengthening
troughs, the first report of such a phenomenon in BAL variability. Our
investigations of the sample further reveal that (i) the frequency of BAL
variability is significantly lower (typically by a factor of 2) than that from
high-ionization BALQSO samples; (ii) \mgii\ BAL absorbers tend to have
relatively high optical depths and small covering factors along our line of
sight; (iii) there is no significant EW-variability correlation between \mgii\
troughs at different velocities in the same quasar; and (iv) the EW-variability
correlation between \mgii\ and \aliii\ BALs is significantly stronger than that
between \mgii\ and \civ\ BALs at the same velocities. These observational
results can be explained by a combined transverse-motion/ionization-change
scenario, where transverse motions likely dominate the strengthening BALs while
ionization changes and/or other mechanisms dominate the weakening BALs.Comment: 24 pages, accepted for publication in ApJ
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