10 research outputs found
Accelerating Growth and Size-dependent Distribution of Human Activities Online
Research on human online activities usually assumes that total activity
increases linearly with active population , that is, . However, we find examples of systems where total
activity grows faster than active population. Our study shows that the power
law relationship is in fact ubiquitous in
online activities such as micro-blogging, news voting and photo tagging. We
call the pattern "accelerating growth" and find it relates to a type of
distribution that changes with system size. We show both analytically and
empirically how the growth rate associates with a scaling parameter
in the size-dependent distribution. As most previous studies explain
accelerating growth by power law distribution, the model of size-dependent
distribution is novel and worth further exploration.Comment: 6 pages, 2 figure
Random hypergraphs and their applications
In the last few years we have witnessed the emergence, primarily in on-line
communities, of new types of social networks that require for their
representation more complex graph structures than have been employed in the
past. One example is the folksonomy, a tripartite structure of users,
resources, and tags -- labels collaboratively applied by the users to the
resources in order to impart meaningful structure on an otherwise
undifferentiated database. Here we propose a mathematical model of such
tripartite structures which represents them as random hypergraphs. We show that
it is possible to calculate many properties of this model exactly in the limit
of large network size and we compare the results against observations of a real
folksonomy, that of the on-line photography web site Flickr. We show that in
some cases the model matches the properties of the observed network well, while
in others there are significant differences, which we find to be attributable
to the practice of multiple tagging, i.e., the application by a single user of
many tags to one resource, or one tag to many resources.Comment: 11 pages, 7 figure
Radiation damping optical enhancement in cold atoms
This work is licensed under the Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/Open Access journalThe typically tiny effect of radiation damping on a moving body can be amplified to a favorable extent by exploiting the sharp reflectivity slope at one edge of an optically induced stop-band in atoms loaded into an optical lattice. In this paper, this phenomenon is demonstrated for the periodically trapped and coherently driven cold 87Rb atoms, where radiation damping might be much larger than that anticipated in previous proposals and become comparable with radiation pressure. Such an enhancement could be observed even at speeds of only a few meters per second with less than 1.0% absorption, making radiation damping experimentally accessible
Resonant nonlinear magneto-optical effects in atoms
In this article, we review the history, current status, physical mechanisms,
experimental methods, and applications of nonlinear magneto-optical effects in
atomic vapors. We begin by describing the pioneering work of Macaluso and
Corbino over a century ago on linear magneto-optical effects (in which the
properties of the medium do not depend on the light power) in the vicinity of
atomic resonances, and contrast these effects with various nonlinear
magneto-optical phenomena that have been studied both theoretically and
experimentally since the late 1960s. In recent years, the field of nonlinear
magneto-optics has experienced a revival of interest that has led to a number
of developments, including the observation of ultra-narrow (1-Hz)
magneto-optical resonances, applications in sensitive magnetometry, nonlinear
magneto-optical tomography, and the possibility of a search for parity- and
time-reversal-invariance violation in atoms.Comment: 51 pages, 23 figures, to appear in Rev. Mod. Phys. in Oct. 2002,
Figure added, typos corrected, text edited for clarit