24,273 research outputs found
A generalized spatiotemporal covariance model for stationary background in analysis of MEG data
Using a noise covariance model based on a single Kronecker product of spatial
and temporal covariance in the spatiotemporal analysis of MEG data was
demonstrated to provide improvement in the results over that of the commonly
used diagonal noise covariance model. In this paper we present a model that is
a generalization of all of the above models. It describes models based on a
single Kronecker product of spatial and temporal covariance as well as more
complicated multi-pair models together with any intermediate form expressed as
a sum of Kronecker products of spatial component matrices of reduced rank and
their corresponding temporal covariance matrices. The model provides a
framework for controlling the tradeoff between the described complexity of the
background and computational demand for the analysis using this model. Ways to
estimate the value of the parameter controlling this tradeoff are also
discussedComment: 4 pages, EMBS 2006 conferenc
Dynamical Mean Field Theory for the Bose-Hubbard Model
The dynamical mean field theory (DMFT), which is successful in the study of
strongly correlated fermions, was recently extended to boson systems [Phys.
Rev. B {\textbf 77}, 235106 (2008)]. In this paper, we employ the bosonic DMFT
to study the Bose-Hubbard model which describes on-site interacting bosons in a
lattice. Using exact diagonalization as the impurity solver, we get the DMFT
solutions for the Green's function, the occupation density, as well as the
condensate fraction on a Bethe lattice. Various phases are identified: the Mott
insulator, the Bose-Einstein condensed (BEC) phase, and the normal phase. At
finite temperatures, we obtain the crossover between the Mott-like regime and
the normal phase, as well as the BEC-to-normal phase transition. Phase diagrams
on the plane and on the plane are
produced ( is the scaled hopping amplitude). We compare our results
with the previous ones, and discuss the implication of these results to
experiments.Comment: 11 pages, 8 figure
Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate
Planetary rotation rate is a key parameter in determining atmospheric
circulation and hence the spatial pattern of clouds. Since clouds can exert a
dominant control on planetary radiation balance, rotation rate could be
critical for determining mean planetary climate. Here we investigate this idea
using a three-dimensional general circulation model with a sophisticated cloud
scheme. We find that slowly rotating planets (like Venus) can maintain an
Earth-like climate at nearly twice the stellar flux as rapidly rotating planets
(like Earth). This suggests that many exoplanets previously believed to be too
hot may actually be habitable, depending on their rotation rate. The
explanation for this behavior is that slowly rotating planets have a weak
Coriolis force and long daytime illumination, which promotes strong convergence
and convection in the substellar region. This produces a large area of
optically thick clouds, which greatly increases the planetary albedo. In
contrast, on rapidly rotating planets a much narrower belt of clouds form in
the deep tropics, leading to a relatively low albedo. A particularly striking
example of the importance of rotation rate suggested by our simulations is that
a planet with modern Earth's atmosphere, in Venus' orbit, and with modern
Venus' (slow) rotation rate would be habitable. This would imply that if Venus
went through a runaway greenhouse, it had a higher rotation rate at that time.Comment: 7 pages, 4 figures, accepted at Astrophysical Journal Letter
production off the proton in a Regge-plus-chiral quark approach
A chiral constituent quark model approach, embodying s- and u-channel
exchanges,complemented with a Reggeized treatment for t-channel is presented. A
model is obtained allowing data for and to be describe satisfactorily. For the latter reaction, recently released
data by CLAS and CBELSA/TAPS Collaborations in the system total energy range
GeV are well reproduced due to the inclusion of
Reggeized trajectories instead of simple and poles.
Contribution from "missing" resonances is found to be negligible in the
considered processes.Comment: 23 pages.4 figures,4 tables, to appear in Phys.Rev.
The dynamics of loop formation in a semiflexible polymer
The dynamics of loop formation by linear polymer chains has been a topic of
several theoretical/experimental studies. Formation of loops and their opening
are key processes in many important biological processes. Loop formation in
flexible chains has been extensively studied by many groups. However, in the
more realistic case of semiflexible polymers, not much results are available.
In a recent study (K. P. Santo and K. L. Sebastian, Phys. Rev. E, \textbf{73},
031293 (2006)), we investigated opening dynamics of semiflexible loops in the
short chain limit and presented results for opening rates as a function of the
length of the chain. We presented an approximate model for a semiflexible
polymer in the rod limit, based on a semiclassical expansion of the bending
energy of the chain. The model provided an easy way to describe the dynamics.
In this paper, using this model, we investigate the reverse process, i.e., the
loop formation dynamics of a semiflexible polymer chain by describing the
process as a diffusion-controlled reaction. We perform a detailed
multidimensional analysis of the problem and calculate closing times for a
semiflexible chain which leads to results that are physically expected. Such a
multidimensional analysis leading to these results does not seem to exist in
the literature so far.Comment: 37 pages 4 figure
One-parameter extension of the Doi-Peliti formalism and relation with orthogonal polynomials
An extension of the Doi-Peliti formalism for stochastic chemical kinetics is
proposed. Using the extension, path-integral expressions consistent with
previous studies are obtained. In addition, the extended formalism is naturally
connected to orthogonal polynomials. We show that two different orthogonal
polynomials, i.e., Charlier polynomials and Hermite polynomials, can be used to
express the Doi-Peliti formalism explicitly.Comment: 10 page
Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region
Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H
images in a quiet region and a coronal hole, observed on September 14 and 16,
2004, respectively, we have explored the magnetic flux emergence, disappearance
and distribution in the two regions. The following results are obtained: (1)
The evolution of magnetic flux in the quiet region is much faster than that in
the coronal hole, as the flux appeared in the form of ephemeral regions in the
quiet region is 4.3 times as large as that in the coronal hole, and the flux
disappeared in the form of flux cancellation, 2.9 times as fast as in the
coronal hole. (2) More magnetic elements with opposite polarities in the quiet
region are connected by arch filaments, estimating from magnetograms and
H images. (3) We measured the magnetic flux of about 1000 magnetic
elements in each observing region. The flux distribution of network and
intranetwork (IN) elements is similar in both polarities in the quiet region.
For network fields in the coronal hole, the number of negative elements is much
more than that of positive elements. However for the IN fields, the number of
positive elements is much more than that of negative elements. (4) In the
coronal hole, the fraction of negative flux change obviously with different
threshold flux density. 73% of the magnetic fields with flux density larger
than 2 Gauss is negative polarity, and 95% of the magnetic fields is negative,
if we only measure the fields with their flux density larger than 20 Gauss. Our
results display that in a coronal hole, stronger fields is occupied by one
predominant polarity; however the majority of weaker fields, occupied by the
other polarity
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