43,758 research outputs found
A cluster expansion approach to exponential random graph models
The exponential family of random graphs is among the most widely-studied
network models. We show that any exponential random graph model may
alternatively be viewed as a lattice gas model with a finite Banach space norm.
The system may then be treated by cluster expansion methods from statistical
mechanics. In particular, we derive a convergent power series expansion for the
limiting free energy in the case of small parameters. Since the free energy is
the generating function for the expectations of other random variables, this
characterizes the structure and behavior of the limiting network in this
parameter region.Comment: 15 pages, 1 figur
Acylsulfonamide safety-catch linker : promise and limitations for solid-phase oligosaccharide synthesis
Safety-catch linkers are useful for solid-phase oligosaccharide synthesis as they are orthogonal to many common protective groups. A new acylsulfonamide safety-catch linker was designed, synthesized and employed during glycosylations using an automated carbohydrate synthesizer. The analysis of the cleavage products revealed shortcomings for oligosaccharide synthesis
Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure
The observed "simple cubic" (sc) phase of elemental Ca at room temperature in
the 32-109 GPa range is, from linear response calculations, dynamically
unstable. By comparing first principle calculations of the enthalpy for five
sc-related (non-close-packed) structures, we find that all five structures
compete energetically at room temperature in the 40-90 GPa range, and three do
so in the 100-130 GPa range. Some competing structures below 90 GPa are
dynamically stable, i.e., no imaginary frequency, suggesting that these
sc-derived short-range-order local structures exist locally and can account for
the observed (average) "sc" diffraction pattern. In the dynamically stable
phases below 90 GPa, some low frequency phonon modes are present, contributing
to strong electron-phonon (EP) coupling as well as arising from the strong
coupling. Linear response calculations for two of the structures over 120 GPa
lead to critical temperatures in the 20-25 K range as is observed, and do so
without unusually soft modes.Comment: 8 pages, 6 figures, 1 table, accepted for publication in Phys. Rev.
Carbon coating of the SPS dipole chambers
The Electron Multipacting (EM) phenomenon is a limiting factor for the
achievement of high luminosity in accelerators for positively charged particles
and for the performance of RF devices. At CERN, the Super Proton Synchrotron
(SPS) must be upgraded in order to feed the Large Hadron Collider (LHC) with 25
ns bunch spaced beams. At such small bunch spacing, EM may limit the
performance of the SPS and consequently that of the LHC. To mitigate this
phenomenon CERN is developing a carbon thin film coating with low Secondary
Electron Yield (SEY) to coat the internal walls of the SPS dipoles beam pipes.
This paper presents the progresses in the coating technology, the performance
of the carbon coatings and the strategy for a large scale production.Comment: 7 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Italy; CERN Yellow Report CERN-2013-002, pp.141-14
Majorana fermions in ferromagnetic chains on the surface of bulk spin-orbit coupled -wave superconductors
Majorana fermion (MF) excitations in solid state system have non-Abelian
statistics which is essential for topological quantum computation. Previous
proposals to realize MF, however, generally requires fine-tuning of parameters.
Here we explore a platform which avoids the fine-tuning problem, namely a
ferromagnetic chain deposited on the surface of a spin-orbit coupled -wave
superconductor. We show that it generically supports zero-energy topological MF
excitations near the two ends of the chain with minimal fine-tuning. Depending
on the strength of the ferromagnetic moment in the chain, the number of MFs at
each end, , can be either one or two, and should be revealed by a robust
zero-bias peak (ZBP) of height in scanning tunneling microscopy (STM)
measurements which would show strong (weak) signals at the ends (middle) of the
chain. The role of an approximate chiral symmetry which gives an integer
topological invariant to the system is discussed.Comment: 9 pages, 4 figure
The peculiar velocity field: constraining the tilt of the Universe
A large bulk flow, which is in tension with the Lambda Cold Dark Matter
(CDM) cosmological model, has been observed. In this paper, we provide
a physically plausible explanation of this bulk flow, based on the assumption
that some fraction of the observed dipole in the cosmic microwave background is
due to an intrinsic fluctuation, so that the subtraction of the observed dipole
leads to a mismatch between the cosmic microwave background (CMB) defined rest
frame and the matter rest frame. We investigate a model that takes into account
the relative velocity (hereafter the tilted velocity) between the two frames,
and develop a Bayesian statistic to explore the likelihood of this tilted
velocity.
By studying various independent peculiar velocity catalogs, we find that: (1)
the magnitude of the tilted velocity is around 400 km/s, and its direction
is close to what is found from previous bulk flow analyses; for most catalogs
analysed, u=0 is excluded at about the level;(2) constraints on
the magnitude of the tilted velocity can result in constraints on the duration
of inflation, due to the fact that inflation can neither be too long (no dipole
effect) nor too short (very large dipole effect); (3) Under the assumption of a
super-horizon isocurvature fluctuation, the constraints on the tilted velocity
require that inflation lasts at least 6 e-folds longer (at the 95% confidence
interval) than that required to solve the horizon problem. This opens a new
window for testing inflation and models of the early Universe from observations
of large scale structure.Comment: 7 pages, 7 figures, match the published version in Phys.Rev.
Augmented L1 and Nuclear-Norm Models with a Globally Linearly Convergent Algorithm
This paper studies the long-existing idea of adding a nice smooth function to
"smooth" a non-differentiable objective function in the context of sparse
optimization, in particular, the minimization of
, where is a vector, as well as the
minimization of , where is a matrix and
and are the nuclear and Frobenius norms of ,
respectively. We show that they can efficiently recover sparse vectors and
low-rank matrices. In particular, they enjoy exact and stable recovery
guarantees similar to those known for minimizing and under
the conditions on the sensing operator such as its null-space property,
restricted isometry property, spherical section property, or RIPless property.
To recover a (nearly) sparse vector , minimizing
returns (nearly) the same solution as minimizing
almost whenever . The same relation also
holds between minimizing and minimizing
for recovering a (nearly) low-rank matrix , if . Furthermore, we show that the linearized Bregman algorithm for
minimizing subject to enjoys global
linear convergence as long as a nonzero solution exists, and we give an
explicit rate of convergence. The convergence property does not require a
solution solution or any properties on . To our knowledge, this is the best
known global convergence result for first-order sparse optimization algorithms.Comment: arXiv admin note: text overlap with arXiv:1207.5326 by other author
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