991 research outputs found
Self-organization on social media: endo-exo bursts and baseline fluctuations
A salient dynamic property of social media is bursting behavior. In this
paper, we study bursting behavior in terms of the temporal relation between a
preceding baseline fluctuation and the successive burst response using a
frequency time series of 3,000 keywords on Twitter. We found that there is a
fluctuation threshold up to which the burst size increases as the fluctuation
increases and that above the threshold, there appears a variety of burst sizes.
We call this threshold the critical threshold. Investigating this threshold in
relation to endogenous bursts and exogenous bursts based on peak ratio and
burst size reveals that the bursts below this threshold are endogenously caused
and above this threshold, exogenous bursts emerge. Analysis of the 3,000
keywords shows that all the nouns have both endogenous and exogenous origins of
bursts and that each keyword has a critical threshold in the baseline
fluctuation value to distinguish between the two. Having a threshold for an
input value for activating the system implies that Twitter is an excitable
medium. These findings are useful for characterizing how excitable a keyword is
on Twitter and could be used, for example, to predict the response to
particular information on social media.Comment: Presented at WebAL-1: Workshop on Artificial Life and the Web 2014
(arXiv:1406.2507
The Concentration-Density Relation of Galaxies in Las Campanas Redshift Survey
We report the results of the evaluation of the ``concentration-density''
relation of galaxies in the local universe, taking advantage of the very large
and homogeneous data set available from the Las Campanas Redshift Survey
(Shectman et al. 1996). This data set consists of galaxies inhabiting the
entire range of galactic environments, from the sparsest field to the densest
clusters, thus allowing us to study environmental variations without combining
multiple data sets with inhomogeneous characteristics. Concentration is
quantified by the automatically-measured concentration index , which is a
good measure of a galaxy's bulge-to-disk ratio. The environment of the sample
galaxies is characterized both by the three-space local galaxy density and by
membership in groups and clusters. We find that the distribution of C in galaxy
populations varies both with local density and with cluster/group membership:
the fraction of centrally-concentrated galaxies increases with local galaxy
density, and is higher in clusters than in the field. A comparison of the
concentration-local density relation in clusters and the field shows that the
two connect rather smoothly at the intermediate density regime, implying that
the apparent cluster/field difference is only a manifestation of the variation
with the local density. We conclude that the structure of galaxies is
predominantly influenced by the local density and not by the broader
environments characterized by cluster/field memberships.Comment: 11 pages, 4 figures, ApJ in press, uses psfig.st
Magnetization dependent current rectification in (Ga,Mn)As magnetic tunnel junctions
We have found that the current rectification effect in triple layer (double
barrier) (Ga,Mn)As magnetic tunnel junctions strongly depends on the
magnetization alignment. The direction as well as the amplitude of the
rectification changes with the alignment, which can be switched by
bi-directional spin-injection with very small threshold currents. A possible
origin of the rectification is energy dependence of the density of states
around the Fermi level. Tunneling density of states in (Ga,Mn)As shows
characteristic dip around zero-bias indicating formation of correlation gap,
the asymmetry of which would be a potential source of the energy dependent
density of states
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