1,177 research outputs found
Substrate-independent method for growing and modulating the density of polymer brushes from surfaces by ATRP
Published: April 19, 2012We describe a method for grafting PEG-based polymer chains of variable surface density using a substrate independent approach, allowing grafting from virtually any material substrate. The approach relies upon initial coupling of a macroinitiator to plasma polymer treated surfaces. The macroinitiator is a novel random terpolymer containing ATRP initiator residues, strongly negatively charged groups, and carboxylic acid moieties that facilitate covalent surface anchoring. Surface-initiated ATRP (SI-ATRP) using polyethylene glycol methyl ether methacrylate (PEGMA) at different concentrations led to grafted surfaces of controlled thickness in either the âbrushâ or âmushroomâ morphology, which was controlled by the abundance of initiator residues in the macroinitiator. Grafted polymer layer structure was investigated via direct interaction force measurements using colloid probe atomic force microscopy (AFM). Equilibrium, hydrated graft layer thicknesses inferred from the highly repulsive AFM force data suggest that the polymer brush graft layer contained polymer chains which were fully stretched. Since the degree of stretching resulted in layer thicknesses approaching the polymer contour length, the polymer brushes studied must be very close to maximum graft density. Grafted layers where the polymer molecules were in the mushroom regime resulted in much thinner layers but the chains had greater chain entropic freedom as indicated by strongly attractive bridging interactions between tethered chains and the silica colloid probe. Use of this experimental methodology would be suitable for preparing grafted polymer layers of a preferred density free from substrate-specific linking chemistries.Bryan R. Coad, Yi Lu, Veronica Glattauer, and Laurence Meaghe
The State of Self-Organized Criticality of the Sun During the Last 3 Solar Cycles. I. Observations
We analyze the occurrence frequency distributions of peak fluxes , total
fluxes , and durations of solar flares over the last three solar cycles
(during 1980--2010) from hard X-ray data of HXRBS/SMM, BATSE/CGRO, and RHESSI.
From the synthesized data we find powerlaw slopes with mean values of
for the peak flux, for the total
flux, and for flare durations. We find a systematic
anti-correlation of the powerlaw slope of peak fluxes as a function of the
solar cycle, varying with an approximate sinusoidal variation
, with a
mean of , a variation of , a solar cycle
period yrs, and a cycle minimum time . The
powerlaw slope is flattest during the maximum of a solar cycle, which indicates
a higher magnetic complexity of the solar corona that leads to an
overproportional rate of powerful flares.Comment: subm. to Solar Physic
Deterministically Driven Avalanche Models of Solar Flares
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting of a
coronal loop. They share similarities with other avalanche models, such as the
classical stick--slip self-organized critical model of earthquakes, in that
they are driven globally by a fully deterministic energy loading process. The
model design leads to a systematic deficit of small scale avalanches. In some
portions of model space, mid-size and large avalanching behavior is scale-free,
being characterized by event size distributions that have the form of
power-laws with index values, which, in some parameter regimes, compare
favorably to those inferred from solar EUV and X-ray flare data. For models
using conservative or near-conservative redistribution rules, a population of
large, quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy release,
this latter behavior may be relevant to recurrent flaring in individual coronal
loops. This class of models could provide a basis for the prediction of large
solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar
Physic
Type IIB Solutions with Interpolating Supersymmetries
We study type IIB supergravity solutions with four supersymmetries that
interpolate between two types widely considered in the literature: the dual of
Becker and Becker's compactifications of M-theory to 3 dimensions and the dual
of Strominger's torsion compactifications of heterotic theory to 4 dimensions.
We find that for all intermediate solutions the internal manifold is not
Calabi-Yau, but has SU(3) holonomy in a connection with a torsion given by the
3-form flux. All 3-form and 5-form fluxes, as well as the dilaton, depend on
one function appearing in the supersymmetry spinor, which satisfies a nonlinear
differential equation. We check that the fields corresponding to a flat bound
state of D3/D5-branes lie in our class of solutions. The relations among
supergravity fields that we derive should be useful in studying new gravity
duals of gauge theories, as well as possibly compactifications.Comment: 27pp, v2 REVTeX4, typographical fixes and minor clarifications, v3
added ref, modified discussion of RR axion slightl
The fermion dynamical symmetry model for the even--even and even--odd nuclei in the Xe--Ba region
The even--even and even--odd nuclei Xe-Xe and
Ba-Ba are shown to have a well-realized fermion dynamical symmetry. Their low-lying energy levels can be
described by a unified analytical expression with two (three) adjustable
parameters for even--odd (even--even) nuclei that is derived from the fermion
dynamical symmetry model. Analytical expressions are given for wavefunctions
and for transition rates that agree well with data. The distinction
between the FDSM and IBM limits is discussed. The experimentally
observed suppression of the the energy levels with increasing quantum
number can be explained as a perturbation of the pairing interaction on
the symmetry, which leads to an Pairing effect for nuclei.Comment: submitted to Phys. Rev. C, LaTeX, 31 pages, 8 figures with postscript
files available on request at [email protected]
Extremal black holes in the Ho\v{r}ava-Lifshitz gravity
We study the near-horizon geometry of extremal black holes in the
Ho\v{r}ava-Lifshitz gravity with a flow parameter . For ,
near-horizon geometry of extremal black holes are AdS with
different radii, depending on the (modified) Ho\v{r}ava-Lifshitz gravity. For
, the radius of is negative, which means
that the near-horizon geometry is ill-defined and the corresponding
Bekenstein-Hawking entropy is zero. We show explicitly that the entropy
function approach does not work for obtaining the Bekenstein-Hawking entropy of
extremal black holes.Comment: 18 pages, v2:some points on Lifshitz black holes claified, v3:
version to appear in EJP
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Observation of Orbitally Excited B_s Mesons
We report the first observation of two narrow resonances consistent with
states of orbitally excited (L=1) B_s mesons using 1 fb^{-1} of ppbar
collisions at sqrt{s} = 1.96 TeV collected with the CDF II detector at the
Fermilab Tevatron. We use two-body decays into K^- and B^+ mesons reconstructed
as B^+ \to J/\psi K^+, J/\psi \to \mu^+ \mu^- or B^+ \to \bar{D}^0 \pi^+,
\bar{D}^0 \to K^+ \pi^-. We deduce the masses of the two states to be m(B_{s1})
= 5829.4 +- 0.7 MeV/c^2 and m(B_{s2}^*) = 5839.7 +- 0.7 MeV/c^2.Comment: Version accepted and published by Phys. Rev. Let
Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS
We present the results of a search for new, heavy particles that decay at a
significant distance from their production point into a final state containing
charged hadrons in association with a high-momentum muon. The search is
conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV
and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS
detector operating at the Large Hadron Collider. Production of such particles
is expected in various scenarios of physics beyond the standard model. We
observe no signal and place limits on the production cross-section of
supersymmetric particles in an R-parity-violating scenario as a function of the
neutralino lifetime. Limits are presented for different squark and neutralino
masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final
version to appear in Physics Letters
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
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