1,973 research outputs found
Tunneling conduction in graphene/(poly)vinyl alcohol composites
Graphene/(Poly)vinyl alcohol (PVA) composite film with thickness
were synthesized by solidification of a PVA solution comprising of dispersed
graphene nanosheets. The close proximity of the graphene sheets enables the
fluctuation induced tunneling of electrons to occur from one sheet to another.
The dielectric data show that the present system can be simulated to a parallel
resistance-capacitor network. The high frequency exponent of the frequency
variation of the ac conductivity indicates that the charge carriers move in a
two-dimensional space. The sample preparation technique will be helpful for
synthesizing flexible conductors.Comment: 10 pages, 8 figure
Outlaw Community Innovations
Recent studies of outlaw communities provide qualitative evidence of their existence and the organisation of the underlying innovation processes. We provide descriptive results from a large scale survey of two online outlaw communities focussing on Microsoft's XBox. In line with previous findings, we identify two types of participants in outlaw communities - user innovators and adopters. Based on 2,256 responses, we find that users modify their XBox mainly to be able to increase the set of available functions of their XBox. Users are also motivated to modify their XBox for the sake of having fun and to conduct pirate behaviour. Finally, the results from our survey suggest that user innovators are largely intrinsically motivated by fun and the intellectual stimulation of writing code for homebrew software
A BAYESIAN ANALYSIS OF THE AGES OF FOUR OPEN CLUSTERS
In this paper we apply a Bayesian technique to determine the best fit of stellar evolution models to find the main sequence turn off age and other cluster parameters of four intermediate-age open clusters: NGC 2360, NGC 2477, NGC 2660, and NGC 3960. Our algorithm utilizes a Markov chain Monte Carlo technique to fit these various parameters, objectively finding the best fit isochrone for each cluster. The result is a high precision isochrone fit. We compare these results with the those of traditional “by eye” isochrone fitting methods. By applying this Bayesian technique to NGC 2360, NGC 2477, NGC 2660, and NGC 3960 we determine the ages of these clusters to be 1.35 ± 0.05, 1.02 ± 0.02, 1.64 ± 0.04, and 0.860 ± 0.04 Gyr, respectively. The results of this paper continue our effort to determine cluster ages to higher precision than that offered by these traditional methods of isochrone fitting
BAYESIAN ANALYSIS OF TWO STELLAR POPULATIONS IN GALACTIC GLOBULAR CLUSTERS I: STATISTICAL AND COMPUTATIONAL METHODS
We develop a Bayesian model for globular clusters composed of multiple stellar populations, extending earlier statistical models for open clusters composed of simple (single) stellar populations (e.g., van Dyk et al. 2009; Stein et al. 2013). Specifically, we model globular clusters with two populations that differ in helium abundance. Our model assumes a hierarchical structuring of the parameters in which physical properties—age, metallicity, helium abundance, distance, absorption, and initial mass—are common to (i) the cluster as a whole or to (ii) individual populations within a cluster, or are unique to (iii) individual stars. An adaptive Markov chain Monte Carlo (MCMC) algorithm is devised for model fitting that greatly improves convergence relative to its precursor non-adaptive MCMC algorithm. Our model and computational tools are incorporated into an open-source software suite known as BASE-9. We use numerical studies to demonstrate that our method can recover parameters of two-population clusters, and also show model misspecification can potentially be identified. As a proof of concept, we analyze the two stellar populations of globular cluster NGC 5272 using our model and methods. (BASE-9 is available from GitHub: https://github.com/argiopetech/base/releases)
Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters II: NGC 5024, NGC 5272, and NGC 6352
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival
ACS Treasury observations of Galactic Globular Clusters to find and
characterize two stellar populations in NGC 5024 (M53), NGC 5272 (M3), and NGC
6352. For these three clusters, both single and double-population analyses are
used to determine a best fit isochrone(s). We employ a sophisticated Bayesian
analysis technique to simultaneously fit the cluster parameters (age, distance,
absorption, and metallicity) that characterize each cluster. For the
two-population analysis, unique population level helium values are also fit to
each distinct population of the cluster and the relative proportions of the
populations are determined. We find differences in helium ranging from
0.05 to 0.11 for these three clusters. Model grids with solar
-element abundances ([/Fe] =0.0) and enhanced -elements
([/Fe]=0.4) are adopted.Comment: ApJ, 21 pages, 14 figures, 7 table
Exact Bond Ordered Ground State for the Transition Between the Band and the Mott Insulator
We derive an effective Hamiltonian for an ionic Hubbard chain,
valid for , where is the hopping, the Coulomb
repulsion, and the charge transfer energy. is the minimal
model for describing the transition from the band insulator (BI) () and the Mott insulator (MI) (). Using spin-particle
transformations (Phys. Rev. Lett. \textbf{86}, 1082 (2001)), we map
into an SU(3) antiferromagnetic Heisenberg model whose
exact ground state is known. In this way, we show rigorously that a
spontaneously dimerized insulating ferroelectric phase appears in the
transition region between the BI and MI
B-physics with Wilson fermions
We report the final results of the ALPHA collaboration for some B-physics
observables: , and . We employ CLS configurations with 2
flavors of improved Wilson fermions in the sea and pion masses ranging
down to 190 MeV. The b-quark is treated in HQET to order . The
renormalization, the matching and the improvement were performed
non-perturbatively, and three lattice spacings reaching fm are used
in the continuum extrapolation
New Techniques to Determine Ages of Open Clusters Using White Dwarfs
Currently there are two main techniques for independently determining the
ages of stellar populations: main sequence evolution theory (via cluster
isochrones) and white dwarf cooling theory. Open clusters provide the ideal
environment for the calibration of these two clocks. Because current techniques
to derive cluster ages from white dwarfs are observationally challenging, we
discuss the feasibility of determining white dwarf ages from the brighter white
dwarfs alone. This would eliminate the requirement of observing the coolest
(i.e., faintest) white dwarfs. We discuss our method for testing this new idea,
as well as the required photometric precision and prior constraints on
metallicity, distance, and reddening. We employ a new Bayesian statistical
technique to obtain and interpret results.Comment: 15 pages, 5 figures. accepted to Astrophysical Journa
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