19,492 research outputs found
Promoting information spreading by using contact memory
Promoting information spreading is a booming research topic in network
science community. However, the exiting studies about promoting information
spreading seldom took into account the human memory, which plays an important
role in the spreading dynamics. In this paper we propose a non-Markovian
information spreading model on complex networks, in which every informed node
contacts a neighbor by using the memory of neighbor's accumulated contact
numbers in the past. We systematically study the information spreading dynamics
on uncorrelated configuration networks and a group of real-world networks,
and find an effective contact strategy of promoting information spreading,
i.e., the informed nodes preferentially contact neighbors with small number of
accumulated contacts. According to the effective contact strategy, the high
degree nodes are more likely to be chosen as the contacted neighbors in the
early stage of the spreading, while in the late stage of the dynamics, the
nodes with small degrees are preferentially contacted. We also propose a
mean-field theory to describe our model, which qualitatively agrees well with
the stochastic simulations on both artificial and real-world networks.Comment: 6 pages, 6 figure
Topological phase transition in a generalized Kane-Mele-Hubbard model: A combined Quantum Monte Carlo and Green's function study
We study a generalized Kane-Mele-Hubbard model with third-neighbor hopping,
an interacting two-dimensional model with a topological phase transition as a
function of third-neighbor hopping, by means of the determinant projector
Quantum Monte Carlo (QMC) method. This technique is essentially numerically
exact on models without a fermion sign problem, such as the one we consider. We
determine the interaction-dependence of the Z2 topological insulator/trivial
insulator phase boundary by calculating the Z2 invariants directly from the
single-particle Green's function. The interactions push the phase boundary to
larger values of third-neighbor hopping, thus stabilizing the topological
phase. The observation of boundary shifting entirely stems from quantum
{\deg}uctuations. We also identify qualitative features of the single-particle
Green's function which are computationally useful in numerical searches for
topological phase transitions without the need to compute the full topological
invariant
Vertical distributions of non-methane hydrocarbons and halocarbons in the lower troposphere over northeast China
Vertical distributions of air pollutants are crucial for understanding the key processes of atmospheric transport and for evaluating chemical transport models. In this paper, we present measurements of non-methane hydrocarbons (NMHCs) and halocarbons obtained from an intensive aircraft study over northeast (NE) China in summer 2007. Most compounds exhibited a typical negative profile of decreasing mixing ratios with increasing altitude, although the gradients differed with different species. Three regional plumes with enhanced VOC mixing ratios were discerned and characterized. An aged plume transported from the northern part of the densely populated North China Plain (NCP; i.e. Beijing-Tianjin area) showed relatively higher levels of HCFC-22, 1,2-dichloroethane (1,2-DCE) and toluene. In comparison, the plume originating from Korea had higher abundances of CFC-12, tetrachloroethene (C2Cl4) and methyl chloride (CH3Cl), while regional air masses from NE China contained more abundant light alkanes. By comparing these results with the earlier PEM-West B (1994) and TRACE-P (2001) aircraft measurements, continuing declining trends were derived for methyl chloroform (CH3CCl3), tetrachloromethane (CCl4) and C2Cl4 over the greater China-northwestern Pacific region, indicating the accomplishment of China in reducing these compounds under the Montreal protocol. However, the study also provided evidence for the continuing emissions of several halocarbons in China in 2007, such as CFCs (mainly from materials in stock) and HCFCs. © 2011 Elsevier Ltd
Large-Scale Spectroscopic Mapping of the Ophiuchi Molecular Cloud Complex I. The CH to NH Ratio as a Signpost of Cloud Characteristics
We present 2.5-square-degree CH N=1-0 and NH J=1-0 maps of the
Ophiuchi molecular cloud complex. These are the first large-scale maps
of the Ophiuchi molecular cloud complex with these two tracers. The
CH emission is spatially more extended than the NH emission. One
faint NH clump Oph-M and one CH ring Oph-RingSW are identified
for the first time. The observed CH to NH abundance ratio
([CH]/[NH]) varies between 5 and 110. We modeled the CH
and NH abundances with 1-D chemical models which show a clear decline
of [CH]/[NH] with chemical age. Such an evolutionary trend is
little affected by temperatures when they are below 40 K. At high density
(n 10 cm), however, the time it takes for the abundance
ratio to drop at least one order of magnitude becomes less than the dynamical
time (e.g., turbulence crossing time 10 years). The observed
[CH]/[NH] difference between L1688 and L1689 can be explained by
L1688 having chemically younger gas in relatively less dense regions. The
observed [CH]/[NH] values are the results of time evolution,
accelerated at higher densities. For the relative low density regions in L1688
where only CH emission was detected, the gas should be chemically younger.Comment: Accepted by ApJ, 45 pages, 10 figure
Mitigation of artifacts due to isolated acoustic heterogeneities in photoacoustic computed tomography using a variable data truncation-based reconstruction method
Photoacoustic computed tomography (PACT) is an emerging computed imaging
modality that exploits optical contrast and ultrasonic detection principles to
form images of the absorbed optical energy density within tissue. If the object
possesses spatially variant acoustic properties that are unaccounted for by the
reconstruction method, the estimated image can contain distortions. While
reconstruction methods have recently been developed to compensate for this
effect, they generally require the object's acoustic properties to be known a
priori. To circumvent the need for detailed information regarding an object's
acoustic properties, we previously proposed a half-time reconstruction method
for PACT. A half-time reconstruction method estimates the PACT image from a
data set that has been temporally truncated to exclude the data components that
have been strongly aberrated. However, this method can be improved upon when
the approximate sizes and locations of isolated heterogeneous structures, such
as bones or gas pockets, are known. To address this, we investigate PACT
reconstruction methods that are based on a variable data truncation (VDT)
approach. The VDT approach represents a generalization of the half-time
approach, in which the degree of temporal truncation for each measurement is
determined by the distance between the corresponding ultrasonic transducer
location and the nearest known bone or gas void location. Computer-simulated
and experimental data are employed to demonstrate the effectiveness of the
approach in mitigating artifacts due to acoustic heterogeneities
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