35,348 research outputs found
Statistical Modelling of Information Sharing: Community, Membership and Content
File-sharing systems, like many online and traditional information sharing
communities (e.g. newsgroups, BBS, forums, interest clubs), are dynamical
systems in nature. As peers get in and out of the system, the information
content made available by the prevailing membership varies continually in
amount as well as composition, which in turn affects all peers' join/leave
decisions. As a result, the dynamics of membership and information content are
strongly coupled, suggesting interesting issues about growth, sustenance and
stability.
In this paper, we propose to study such communities with a simple statistical
model of an information sharing club. Carrying their private payloads of
information goods as potential supply to the club, peers join or leave on the
basis of whether the information they demand is currently available.
Information goods are chunked and typed, as in a file sharing system where
peers contribute different files, or a forum where messages are grouped by
topics or threads. Peers' demand and supply are then characterized by
statistical distributions over the type domain.
This model reveals interesting critical behaviour with multiple equilibria. A
sharp growth threshold is derived: the club may grow towards a sustainable
equilibrium only if the value of an order parameter is above the threshold, or
shrink to emptiness otherwise. The order parameter is composite and comprises
the peer population size, the level of their contributed supply, the club's
efficiency in information search, the spread of supply and demand over the type
domain, as well as the goodness of match between them.Comment: accepted in International Symposium on Computer Performance,
Modeling, Measurements and Evaluation, Juan-les-Pins, France, October-200
Fast beam stacking using RF barriers
Two barrier RF systems were fabricated, tested and installed in the Fermilab
Main Injector. Each can provide 8 kV rectangular pulses (the RF barriers) at 90
kHz. When a stationary barrier is combined with a moving barrier, injected
beams from the Booster can be continuously deflected, folded and stacked in the
Main Injector, which leads to doubling of the beam intensity. This paper gives
a report on the beam experiment using this novel technology.Comment: 2007 Particle Accelerator Conference (PAC07
Factor structure and psychometric properties of the body appreciation scale among adults in Hong Kong
Previous research has suggested that the factor structure of Body Appreciation Scale (BAS), a widely-used measure of positive body image, may not be cross-culturally equivalent. Here, we used confirmatory factor analysis to evaluate the conceptual equivalence of a Chinese (Cantonese) translation of the BAS among women (n = 1,319) and men (n = 1,084) in Hong Kong. Results showed that neither the one-dimensional nor proposed two-dimensional factor structures had adequate fit. Instead, a modified two-dimensional structure, which retained 9 of the 13 BAS items in two factors, had the best fit. However, only one these factors, reflective of General Body Appreciation, had adequate internal consistency. This factor also had good patterns of construct validity, as indicated through significant correlations with participant body mass index, self-esteem, and (among women) actual-ideal weight discrepancy. The present results suggest that there may be cultural differences in the concept and experience of body appreciation
Prediction of Orbital Ordering in Single-Layered Ruthenates
The key role of the orbital degree of freedom to understand the magnetic
properties of layered ruthenates is here discussed. In the G-type
antiferromagnetic phase of CaRuO, recent X-ray experiments reported the
presence of 0.5 hole per site in the orbital, while the
and orbitals contain 1.5 holes. This unexpected hole
distribution is explained by a novel state with orbital ordering (OO),
stabilized by a combination of Coulomb interactions and lattice distortions. In
addition, the rich phase diagram presented here suggests the possibility of
large magnetoresistance effects, and predicts a new ferromagnetic OO phase in
ruthenates.Comment: 4 pages, Revtex, with 2 figures embedded in the text. Submitted to
Phys. Rev. Let
Tilting instability and other anomalies in the flux-lattice in some magnetic superconductors
The flux-line lattice in the compound , which has a tendency to
ferromagnetic order in the a-b plane is studied with external magnetic field
direction close to the c-axis. We show the existence of an instability where
the direction of flux-lines spontaneously tilts away from that of the applied
field near the onset of ferromagnetic order. The enhanced fluctuations in the
flux lattice and the square flux lattice recently observed are explained and
further experiments suggested.Comment: 12 pages, Latex file, no figur
Chiral symmetry breaking in a uniform external magnetic field II. Symmetry restoration at high temperatures and chemical potentials
Chiral symmetry is dynamically broken in quenched, ladder QED at weak gauge
couplings when an external magnetic field is present. In this paper, we show
that chiral symmetry is restored above a critical chemical potential and the
corresponding phase transition is of first order. In contrast, the chiral
symmetry restoration at high temperatures (and at zero chemical potential) is a
second order phase transition.Comment: Latex; 12 pages; 8 postscript figures include
Decoherence of Einstein-Podolsky-Rosen steering
We consider two systems A and B that share Einstein-Podolsky-Rosen (EPR)
steering correlations and study how these correlations will decay, when each of
the systems are independently coupled to a reservoir. EPR steering is a
directional form of entanglement, and the measure of steering can change
depending on whether the system A is steered by B, or vice versa. First, we
examine the decay of the steering correlations of the two-mode squeezed state.
We find that if the system B is coupled to a reservoir, then the decoherence of
the steering of A by B is particularly marked, to the extent that there is a
sudden death of steering after a finite time. We find a different directional
effect, if the reservoirs are thermally excited. Second, we study the
decoherence of the steering of a Schr\"odinger cat state, modeled as the
entangled state of a spin and harmonic oscillator, when the macroscopic system
(the cat) is coupled to a reservoir
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