119,549 research outputs found
Delay-induced multiple stochastic resonances on scale-free neuronal networks
We study the effects of periodic subthreshold pacemaker activity and
time-delayed coupling on stochastic resonance over scale-free neuronal
networks. As the two extreme options, we introduce the pacemaker respectively
to the neuron with the highest degree and to one of the neurons with the lowest
degree within the network, but we also consider the case when all neurons are
exposed to the periodic forcing. In the absence of delay, we show that an
intermediate intensity of noise is able to optimally assist the pacemaker in
imposing its rhythm on the whole ensemble, irrespective to its placing, thus
providing evidences for stochastic resonance on the scale-free neuronal
networks. Interestingly thereby, if the forcing in form of a periodic pulse
train is introduced to all neurons forming the network, the stochastic
resonance decreases as compared to the case when only a single neuron is paced.
Moreover, we show that finite delays in coupling can significantly affect the
stochastic resonance on scale-free neuronal networks. In particular,
appropriately tuned delays can induce multiple stochastic resonances
independently of the placing of the pacemaker, but they can also altogether
destroy stochastic resonance. Delay-induced multiple stochastic resonances
manifest as well-expressed maxima of the correlation measure, appearing at
every multiple of the pacemaker period. We argue that fine-tuned delays and
locally active pacemakers are vital for assuring optimal conditions for
stochastic resonance on complex neuronal networks.Comment: 7 two-column pages, 5 figures; accepted for publication in Chao
Hydrogen as a Source of Flux Noise in SQUIDs
Superconducting qubits are hampered by flux noise produced by surface spins
from a variety of microscopic sources. Recent experiments indicated that
hydrogen (H) atoms may be one of those sources. Using density functional theory
calculations, we report that H atoms either embedded in, or adsorbed on, an
a-Al2O3(0001) surface have sizeable spin moments ranging from 0.81 to 0.87 uB
with energy barriers for spin reorientation as low as ~10 mK. Furthermore, H
adatoms on the surface attract gas molecules such as O2, producing new spin
sources. We propose coating the surface with graphene to eliminate H-induced
surface spins and to protect the surface from other adsorbates.Comment: 12 pages, 4 figure
Abell 2111: An Optical and Radio Study of the Richest Butcher-Oemler Cluster
We present an in-depth analysis of the Butcher-Oemler cluster A2111,
including new optical spectroscopy plus a deep Very Large Array (VLA) radio
continuum observation. These are combined with optical imaging from the Sloan
Digital Sky Survey (SDSS) to assess the activity and properties of member
galaxies. Prior X-ray studies have suggested A2111 is a head-on cluster merger,
a dynamical state which might be connected to the high level of activity
inferred from its blue fraction. We are able to directly assess this claim,
using our spectroscopic data to identify 95 cluster members among 196 total
galaxy spectra. These galaxy velocities do not themselves provide significant
evidence for the merger interpretation, however they are consistent with it
provided the system is viewed near the time of core passage and at a viewing
angle >~30 degrees different from the merger axis. The SDSS data allow us to
confirm the high blue fraction for A2111, f_b = 0.15 +/- 0.03 based on
photometry alone and f_b = 0.23 +/- 0.03 using spectroscopic data to remove
background galaxies. We are able to detect 175 optical sources from the SDSS in
our VLA radio data, of which 35 have redshift information. We use the SDSS
photometry to determine photometric redshifts for the remaining 140
radio-optical sources. In total we identify up to 26 cluster radio galaxies, 14
of which have spectroscopic redshifts. The optical spectroscopy and radio data
reveal a substantial population of dusty starbursts within the cluster. The
high blue fraction and prevalence of star formation is consistent with the
hypothesis that dynamically-active clusters are associated with more active
member galaxies than relaxed clusters.Comment: To appear in AJ; 53 pages including 10 figures and several long
table
Conduction mechanisms of epitaxial EuTiO3 thin films
To investigate leakage current density versus electric field characteristics,
epitaxial EuTiO3 thin films were deposited on (001) SrTiO3 substrates by pulsed
laser deposition and were post-annealed in a reducing atmosphere. This
investigation found that conduction mechanisms are strongly related to
temperature and voltage polarity. It was determined that from 50 to 150 K the
dominant conduction mechanism was a space-charge-limited current under both
negative and positive biases. From 200 to 300 K, the conduction mechanism shows
Schottky emission and Fowler-Nordheim tunneling behaviors for the negative and
positive biases, respectively. This work demonstrates that Eu3+ is one source
of leakage current in EuTiO3 thin films.Comment: 17 pages,4 figures, conferenc
Influences of magnetic coupling process on the spectrum of a disk covered by the corona
Recently, much attention has been paid to the magnetic coupling (MC) process,
which is supported by very high emissivity indexes observed in Seyfert 1 galaxy
MCG-6-30-15 and GBHC XTE J1650-500. But the rotational energy transferred from
a black hole is simply assumed to be radiated away from the surrounding
accretion disk in black-body spectrum, which is obviously not consistent with
the observed hard power-law X-ray spectra. We intend to introduce corona into
the MC model to make it more compatible with the observations. We describe the
model and the procedure of a simplified Monte Carlo simulation, compare the
output spectra in the cases with and without the MC effects, and discuss the
influences of three parameters involved in the MC process on the output
spectra. It is shown that the MC process augments radiation fluxes in the UV or
X-ray band. The emergent spectrum is affected by the BH spin and magnetic field
strength at the BH horizon, while it is almost unaffected by the radial profile
of the magnetic field at the disk. Introducing corona into the MC model will
improve the fitting of the output spectra from AGNs and GBHCs.Comment: 15 pages, 5 figures, accepted by A&
Detection of X-ray-Emitting Hypernova Remnants in M101
Based on an ultra deep (230 ks) ROSAT HRI imaging of M101, we have detected 5
X-ray sources that coincide spatially with optical emission line features
previously classified as supernova remnants in this nearby galaxy. Two of these
coincidences (SNR MF83 and NGC5471B) most likely represent the true physical
association of X-ray emission with shock-heated interstellar gas. MF83, with a
radius of ~ 134 pc, is one of the largest remnants known. NGC5471B, with a
radius of 30 pc and a velocity of at least 350 km/s (FWZI), is extremely bright
in both radio and optical. The X-ray luminosities of these two shell-like
remnants are and (0.5-2 keV), about an order
of magnitude brighter than the brightest supernova remnants known in our Galaxy
and in the Magellanic Clouds. The inferred blastwave energy is for NGC5471B and ergs for MF83.
Therefore, the remnants likely originate in hypernovae, which are a factor of
more energetic than canonical supernovae and are postulated as
being responsible for Gamma-ray bursts observed at cosmological distances. The
study of such hypernova remnants in nearby galaxies has the potential to
provide important constraints on the progenitor type, rate, energetics, and
beaming effect of Gamma-ray bursts.Comment: 10 pages, 2 gif figures, Accepted for publication in Astrophysical
Journal Letter
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