6,126 research outputs found
Optimization of robustness of scale-free network to random and targeted attacks
The scale-fee networks, having connectivity distribution (where is the site connectivity), is very resilient to random
failures but fragile to intentional attack. The purpose of this paper is to
find the network design guideline which can make the robustness of the network
to both random failures and intentional attack maximum while keeping the
average connectivity per node constant. We find that when $=3$ the
robustness of the scale-free networks reach its maximum value if the minimal
connectivity $m=1$, but when is larger than four, the networks will
become more robust to random failures and targeted attacks as the minimal
connectivity gets larger
Optimization of scale-free network for random failures
It has been found that the networks with scale-free distribution are very
resilient to random failures. The purpose of this work is to determine the
network design guideline which maximize the network robustness to random
failures with the average number of links per node of the network is constant.
The optimal value of the distribution exponent and the minimum connectivity to
different network size are given in this paper. Finally, the optimization
strategy how to improve the evolving network robustness is given.Comment: 6 pages, 1 figur
Nonvolatile memory with molecule-engineered tunneling barriers
We report a novel field-sensitive tunneling barrier by embedding C60 in SiO2
for nonvolatile memory applications. C60 is a better choice than ultra-small
nanocrystals due to its monodispersion. Moreover, C60 provides accessible
energy levels to prompt resonant tunneling through SiO2 at high fields.
However, this process is quenched at low fields due to HOMO-LUMO gap and large
charging energy of C60. Furthermore, we demonstrate an improvement of more than
an order of magnitude in retention to program/erase time ratio for a metal
nanocrystal memory. This shows promise of engineering tunnel dielectrics by
integrating molecules in the future hybrid molecular-silicon electronics.Comment: to appear in Applied Physics Letter
More on QCD Ghost Dark Energy
The difference between vacuum energy of quantum fields in Minkowski space and
in Friedmann-Robterson-Walker universe might be related to the observed dark
energy. The vacuum energy of the Veneziano ghost field introduced to solve the
problem in QCD is of the form, . Based on this, we
study the dynamical evolution of a phenomenological dark energy model whose
energy density is of the form . In this model, the universe
approaches to a de Sitter phase at late times. We fit the model with current
observational data including SnIa, BAO, CMB, BBN, Hubble parameter and growth
rate of matter perturbation. It shows that the universe begins to accelerate at
redshift and this model is consistent with current data. In
particular, this model fits the data of growth factor well as the
model.Comment: 14 pages, 4 figures, 2 table
Cold gas and a Milky Way-type 2175 {\AA} bump in a metal-rich and highly depleted absorption system
We report the detection of a strong Milky Way-type 2175 \AA extinction
bump at = 2.1166 in the quasar spectrum towards SDSS J121143.42+083349.7
from the Sloan Digital Sky Survey (SDSS) Data Release 10. We conduct follow up
observations with the Echelle Spectrograph and Imager (ESI) onboard the Keck-II
telescope and the Ultraviolet and Visual Echelle Spectrograph (UVES) on the
VLT. This 2175 \AA absorber is remarkable in that we simultaneously detect
neutral carbon (C I), neutral chlorine (Cl I), and carbon monoxide (CO). It
also qualifies as a damped Lyman alpha system. The J1211+0833 absorber is found
to be metal-rich and has a dust depletion pattern resembling that of the Milky
Way disk clouds. We use the column densities of the C I fine structure states
and the C II/C I ratio (under the assumption of ionization equilibrium) to
derive the temperature and volume density in the absorbing gas. A Cloudy
photoionization model is constructed, which utilizes additional atoms/ions to
constrain the physical conditions. The inferred physical conditions are
consistent with a canonical cold (T 100 K) neutral medium with a high
density ((H I) 100 cm) and a slightly higher pressure than the
local interstellar medium. Given the simultaneous presence of C I, CO, and the
2175 \AA bump, combined with the high metallicity, high dust depletion level
and overall low ionization state of the gas, the absorber towards J1211+0833
supports the scenario that the presence of the bump requires an evolved stellar
population.Comment: 18 pages, 17 figures, to be published in MNRA
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