3,439 research outputs found
Stochastic pole expansion method
In this paper, we propose a new analytic continuation method to extract real
frequency spectral functions from imaginary frequency Green's functions of
quantum many-body systems. This method is based on the pole representation of
Matsubara Green's function and a stochastic sampling procedure is utilized to
optimize the amplitudes and locations of poles. In order to capture narrow
peaks and sharp band edges in the spectral functions, a constrained sampling
algorithm and a self-adaptive sampling algorithm are developed. To demonstrate
the usefulness and performance of the new method, we at first apply it to study
the spectral functions of representative fermionic and bosonic correlators.
Then we employ this method to tackle the analytic continuation problems of
matrix-valued Green's functions. The synthetic Green's functions, as well as
realistic correlation functions from finite temperature quantum many-body
calculations, are used as input. The benchmark results demonstrate that this
method is capable of reproducing most of the key characteristics in the
spectral functions. The sharp, smooth, and multi-peak features in both
low-frequency and high-frequency regions of spectral functions could be
accurately resolved, which overcomes one of the main limitations of the
traditional maximum entropy method. More importantly, it exhibits excellent
robustness with respect to noisy and incomplete input data. The causality of
spectral function is always satisfied even in the presence of sizable noises.
As a byproduct, this method could derive a fitting formula for the Matsubara
data, which provides a compact approximation to the many-body Green's
functions. Hence, we expect that this new method could become a pivotal
workhorse for numerically analytic continuation and be broadly useful in many
applications.Comment: 26 pages, 20 figure
Reconstructing lattice QCD spectral functions with stochastic pole expansion and Nevanlinna analytic continuation
The reconstruction of spectral functions from Euclidean correlation functions
is a well-known, yet ill-posed inverse problem in the fields of many-body and
high-energy physics. In this paper, we present a comprehensive investigation of
two recently developed analytic continuation methods, namely stochastic pole
expansion and Nevanlinna analytic continuation, for extracting spectral
functions from mock lattice QCD data. We examine a range of Euclidean
correlation functions generated by representative models, including the
Breit-Wigner model, the Gaussian mixture model, the resonance-continuum model,
and the bottomonium model. Our findings demonstrate that the stochastic pole
expansion method, when combined with the constrained sampling algorithm and the
self-adaptive sampling algorithm, successfully recovers the essential features
of the spectral functions and exhibits excellent resilience to noise of input
data. In contrast, the Nevanlinna analytic continuation method suffers from
numerical instability, often resulting in the emergence of spurious peaks and
significant oscillations in the high-energy regions of the spectral functions,
even with the application of the Hardy basis function optimization algorithm.Comment: 14 pages, 8 figure
Multifunctional Bracts in the Dove Tree Davidia involucrata (Nyssaceae:Cornales)
Although there has been much experimental work on floral traits that are under selection from mutualists and antagonists, selection by abiotic environmental factors on flowers has been largely ignored. Here we test whether pollen susceptibility to rain damage could have played a role in the evolution of the reproductive architecture of Davidia involucrata, an endemic in the mountains of western China. Flowers in this tree species lack a perianth and are arranged in capitula surrounded by large (up to 10 cm#5 cm) bracts that at anthesis turn from green to white, losing their photosynthetic capability. Flowers are nectarless, and pollen grains are presented on the recurved anther walls for 5–7 days. Flower visitors, and likely pollinators, were mainly pollen-collecting bees from the genera Apis, Xylocopa, Halictus, and Lasioglossum. Capitula with natural or white paper bracts attracted significantly more bees per hour than capitula that had their bracts removed or replaced by green paper. Experimental immersion of pollen grains in water resulted in rapid loss of viability, and capitula with bracts lost less pollen to rain than did capitula that had their bracts removed, suggesting that the bracts protect the pollen from rain damage as well as attracting pollinators
Distributed Contingency Analysis over Wide Area Network among Dispatch Centers
Traditionally, a regional dispatch center uses the equivalent method to deal
with external grids, which fails to reflect the interactions among regions.
This paper proposes a distributed N-1 contingency analysis (DCA) solution,
where dispatch centers join a coordinated computation using their private data
and computing resources. A distributed screening method is presented to
determine the Critical Contingency Set (DCCS) in DCA. Then, the distributed
power flow is formulated as a set of boundary equations, which is solved by a
Jacobi-Free Newton-GMRES (JFNG) method. During solving the distributed power
flow, only boundary conditions are exchanged. Acceleration techniques are also
introduced, including reusing preconditioners and optimal resource scheduling
during parallel processing of multiple contingencies. The proposed method is
implemented on a real EMS platform, where tests using the Southwest Regional
Grid of China are carried out to validate its feasibility.Comment: 5 pages, 6 figures, 2017 IEEE PES General Meetin
Dark Energy and Fate of the Universe
We explore the ultimate fate of the Universe by using a divergence-free
parametrization for dark energy .
Unlike the CPL parametrization, this parametrization has well behaved, bounded
behavior for both high redshifts and negative redshifts, and thus can genuinely
cover many theoretical dark energy models. After constraining the parameter
space of this parametrization by using the current cosmological observations,
we find that, at the 95.4% confidence level, our Universe can still exist at
least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy
dominated Universe, we find that a gravitationally bound system will be
destroyed at a time , where
is the period of a circular orbit around this system, before the big rip.Comment: 5 pages, 3 figures; typos corrected, publication version, Sci
China-Phys Mech Astron, doi: 10.1007/s11433-012-4748-
Family Foundation Development in China: Two Case Studies
Amid the accumulation of private wealth in China, family foundations have begun to emerge in recent decades. Little research, however, has explored this nascent phenomenon. This article examines the development of two Chinese family foundations — the Lao Niu Foundation and the Lu Jiaxiang Foundation — using document analyses and semi-structured interviews with foundation leaders.
While detailed data on program effectiveness and efficiency is lacking because of underdeveloped methods of evaluation, it is evident that both foundations have generated positive impacts on social development despite an overall lack of support for the foundation sector from Chinese government policy.
The case studies indicate that Chinese family foundations are exploring new paths in an increasingly mature philanthropic environment, and suggest several development approaches for family foundations in China and other emerging philanthropic sectors
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