2,533 research outputs found
Harnessing the Bethe Free Energy
Gibbs measures induced by random factor graphs play a prominent role in computer science, combinatorics and physics. A key problem is to calculate the typical value of the partition function. According to the "replica symmetric cavity method", a heuristic that rests on non-rigorous considerations from statistical mechanics, in many cases this problem can be tackled by way of maximising a functional called the "Bethe free energy". In this paper we prove that the Bethe free energy upper-bounds the partition function in a broad class of models. Additionally, we provide a sufficient condition for this upper bound to be tight
Proton Stopping Power of Different Density Profile Plasmas
In this work, the stopping power of a partially ionized plasma is analyzed by
means of free electron stopping and bound electron stopping. For the first one,
the RPA dielectric function is used, and for the latter one, an interpolation
of high and low projectile velocity formulas is used. The dynamical energy loss
of an ion beam inside a plasma is estimated by using an iterative scheme of
calculation. The Abel inversion is also applied when we have a plasma with
radial symmetry. Finally, we compare our methods with two kind of plasmas. In
the first one, we estimate the energy loss in a plasma created by a laser
prepulse, whose density is approximated by a piecewise function. For the latter
one, a radial electron density is supposed and the stopping is obtained as
function of radius from the calculated lateral points. In both cases, the
dependence with the density profile is observed.Comment: 5 pages, 7 figure
Belief Propagation on replica symmetric random factor graph models
According to physics predictions, the free energy of random factor graph
models that satisfy a certain "static replica symmetry" condition can be
calculated via the Belief Propagation message passing scheme [Krzakala et al.,
PNAS 2007]. Here we prove this conjecture for two general classes of random
factor graph models, namely Poisson random factor graphs and random regular
factor graphs. Specifically, we show that the messages constructed just as in
the case of acyclic factor graphs asymptotically satisfy the Belief Propagation
equations and that the free energy density is given by the Bethe free energy
formula
Galactic-scale macro-engineering: Looking for signs of other intelligent species, as an exercise in hope for our own
If we consider Big History as simply 'our' example of the process of cosmic
evolution playing out, then we can seek to broaden our view of our possible
fate as a species by asking questions about what paths or trajectories other
species' own versions of Big History might take or have taken. This paper
explores the broad outlines of possible scenarios for the evolution of
long-lived intelligent engineering species---scenarios which might have been
part of another species' own Big History story, or which may yet lie ahead in
our own distant future. A sufficiently long-lived engineering-oriented species
may decide to undertake a program of macro-engineering projects that might
eventually lead to a re-engineered galaxy so altered that its artificiality may
be detectable from Earth. We consider activities that lead ultimately to a
galactic structure consisting of a central inner core surrounded by a more
distant ring of stars separated by a relatively sparser 'gap', where star
systems and stellar materials may have been removed, 'lifted' or turned into
Dyson Spheres. When one looks to the sky, one finds that such galaxies do
indeed exist---including the beautiful ringed galaxy known as 'Hoag's Object'
(PGC 54559) in the constellation Serpens. This leads us to pose the question:
Is Hoag's Object an example of galaxy-scale macro-engineering? And this
suggests a program of possible observational activities and theoretical
explorations, several of which are presented here, that could be carried out in
order to begin to investigate this beguiling question.Comment: 17 pages. Published in: Teaching and Researching Big History:
Exploring a New Scholarly Field; L. Grinin, D. Baker, E. Quaedackers and A.
Korotayev (eds). Uchitel Publ House, Volgograd, Russia, 2014, Chapter 14, pp.
283-304. ISBN: 978-5-7057-4027-
Longitudinal hydrodynamics from event-by-event Landau initial conditions
We investigate three-dimensional ideal hydrodynamic evolution, with Landau
initial conditions, incorporating event-by-event variation with many events and
transverse density inhomogeneities. We show that the transition to
boost-invariant flow occurs too late for realistic setups, with corrections of
\order{20-30\%} expected at freezeout for most scenarios. Moreover, the
deviation from boost-invariance is correlated with both transverse flow and
elliptic flow, with the more highly transversely flowing regions also showing
the most violation of boost invariance. Therefore, if longitudinal flow is not
fully developed at the early stages of heavy ion collisions, hydrodynamics
where boost-invariance holds at mid-rapidity is inadequate to extract transport
coefficients of the quark-gluon plasma. We conclude by arguing that developing
experimental probes of boost invariance is necessary, and suggest some
promising directions in this regard.Comment: Accepted for publication, Phys.Rev.
Polytope of Correct (Linear Programming) Decoding and Low-Weight Pseudo-Codewords
We analyze Linear Programming (LP) decoding of graphical binary codes
operating over soft-output, symmetric and log-concave channels. We show that
the error-surface, separating domain of the correct decoding from domain of the
erroneous decoding, is a polytope. We formulate the problem of finding the
lowest-weight pseudo-codeword as a non-convex optimization (maximization of a
convex function) over a polytope, with the cost function defined by the channel
and the polytope defined by the structure of the code. This formulation
suggests new provably convergent heuristics for finding the lowest weight
pseudo-codewords improving in quality upon previously discussed. The algorithm
performance is tested on the example of the Tanner [155, 64, 20] code over the
Additive White Gaussian Noise (AWGN) channel.Comment: 6 pages, 2 figures, accepted for IEEE ISIT 201
Nonlinear photocurrents in two-dimensional systems based on graphene and boron nitride
DC photoelectrical currents can be generated purely as a non-linear effect in
uniform media lacking inversion symmetry without the need for a material
junction or bias voltages to drive it, in what is termed photogalvanic effect.
These currents are strongly dependent on the polarization state of the
radiation, as well as on topological properties of the underlying Fermi surface
such as its Berry curvature. In order to study the intrinsic photogalvanic
response of gapped graphene (GG), biased bilayer graphene (BBG), and hexagonal
boron nitride (hBN), we compute the non-linear current using a perturbative
expansion of the density matrix. This allows a microscopic description of the
quadratic response to an electromagnetic field in these materials, which we
analyze as a function of temperature and electron density. We find that the
intrinsic response is robust across these systems and allows for currents in
the range of pA cm/W to nA cm/W. At the independent-particle level, the
response of hBN-based structures is significant only in the ultra-violet due to
their sizeable band-gap. However, when Coulomb interactions are accounted for
by explicit solution of the Bethe-Salpeter equation, we find that the
photoconductivity is strongly modified by transitions involving exciton levels
in the gap region, whose spectral weight dominates in the overall frequency
range. Biased bilayers and gapped monolayers of graphene have a strong
photoconductivity in the visible and infrared window, allowing for photocurrent
densities of several nA cm/W. We further show that the richer electronic
dispersion of BBG at low energies and the ability to change its band-gap on
demand allows a higher tunability of the photocurrent, including not only its
magnitude but also, and significantly, its polarity.Comment: Updating with published version and respective references; 14 pages,
11 figure
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