2,768 research outputs found
Microphase separation in polyelectrolytic diblock copolymer melt : weak segregation limit
We present a generalized theory of microphase separation for charged-neutral
diblock copolymer melt. Stability limit of the disordered phase for salt-free
melt has been calculated using Random Phase Approximation (RPA) and
self-consistent field theory (SCFT). Explicit analytical free energy
expressions for different classical ordered microstructures (lamellar, cylinder
and sphere) are presented. We demonstrate that chemical mismatch required for
the onset of microphase separation () in charged-neutral
diblock melt is higher and the period of ordered microstructures is lower than
those for the corresponding neutral-neutral diblock system. Theoretical
predictions on the period of ordered structures in terms of Coulomb
electrostatic interaction strength, chain length, block length, and the
chemical mismatch between blocks are presented. SCFT has been used to go beyond
the stability limit, where electrostatic potential and charge distribution are
calculated self-consistently. Stability limits calculated using RPA are in
perfect agreement with the corresponding SCFT calculations. Limiting laws for
stability limit and the period of ordered structures are presented and
comparisons are made with an earlier theory. Also, transition boundaries
between different morphologies have been investigated
Constraining Relativistic Bow Shock Properties in Rotation-Powered Millisecond Pulsar Binaries
Multiwavelength followup of unidentified Fermi sources has vastly expanded
the number of known galactic-field "black widow" and "redback" millisecond
pulsar binaries. Focusing on their rotation-powered state, we interpret the
radio to X-ray phenomenology in a consistent framework. We advocate the
existence of two distinct modes differing in their intrabinary shock
orientation, distinguished by the phase-centering of the double-peaked X-ray
orbital modulation originating from mildly-relativistic Doppler boosting. By
constructing a geometric model for radio eclipses, we constrain the shock
geometry as functions of binary inclination and shock stand-off . We
develop synthetic X-ray synchrotron orbital light curves and explore the model
parameter space allowed by radio eclipse constraints applied on archetypal
systems B1957+20 and J1023+0038. For B1957+20, from radio eclipses the
stand-off is -- fraction of binary separation from the
companion center, depending on the orbit inclination. Constructed X-ray light
curves for B1957+20 using these values are qualitatively consistent with those
observed, and we find occultation of the shock by the companion as a minor
influence, demanding significant Doppler factors to yield double peaks. For
J1023+0038, radio eclipses imply while X-ray light curves
suggest (from the pulsar). Degeneracies in the
model parameter space encourage further development to include transport
considerations. Generically, the spatial variation along the shock of the
underlying electron power-law index should yield energy-dependence in the shape
of light curves motivating future X-ray phase-resolved spectroscopic studies to
probe the unknown physics of pulsar winds and relativistic shock acceleration
therein.Comment: Accepted to ApJ, 36 pages, 15 figures; comments welcom
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
Modeling the Emission Processes in Blazars
Blazars are the most violent steady/recurrent sources of high-energy
gamma-ray emission in the known Universe. They are prominent emitters of
electromagnetic radiation throughout the entire electromagnetic spectrum. The
observable radiation most likely originates in a relativistic jet oriented at a
small angle with respect to the line of sight. This review starts out with a
general overview of the phenomenology of blazars, including results from a
recent multiwavelength observing campaign on 3C279. Subsequently, issues of
modeling broadband spectra will be discussed. Spectral information alone is not
sufficient to distinguish between competing models and to constrain essential
parameters, in particular related to the primary particle acceleration and
radiation mechanisms in the jet. Short-term spectral variability information
may help to break such model degeneracies, which will require snap-shot
spectral information on intraday time scales, which may soon be achievable for
many blazars even in the gamma-ray regime with the upcoming GLAST mission and
current advances in Atmospheric Cherenkov Telescope technology. In addition to
pure leptonic and hadronic models of gamma-ray emission from blazars,
leptonic/hadronic hybrid models are reviewed, and the recently developed
hadronic synchrotron mirror model for TeV gamma-ray flares which are not
accompanied by simultaneous X-ray flares (``orphan TeV flares'') is revisited.Comment: Invited Review at "The Multimessenger Approach to Gamma-Ray Sources",
Barcelona, Spain, July 2006; submitted to Astrophysics and Space Science. 10
pages, including 6 eps figures. Uses Springer's ApSS macro
Evidence of growing spatial correlations at the glass transition from nonlinear response experiments
The ac nonlinear dielectric response of glycerol was
measured close to its glass transition temperature to investigate the
prediction that supercooled liquids respond in an increasingly non-linear way
as the dynamics slows down (as spin-glasses do). We find that
indeed displays several non trivial features. It is peaked
as a function of the frequency and obeys scaling as a function of
, with the relaxation time of the liquid. The height
of the peak, proportional to the number of dynamically correlated molecules
, increases as the system becomes glassy, and decays as a
power-law of over several decades beyond the peak. These findings
confirm the collective nature of the glassy dynamics and provide the first
direct estimate of the dependence of .Comment: 22 pages, 6 figures. With respect to v1, a few new sentences were
added in the introduction and conclusion, references were updated, some typos
corrected
Composition, structure and stability of RuO_2(110) as a function of oxygen pressure
Using density-functional theory (DFT) we calculate the Gibbs free energy to
determine the lowest-energy structure of a RuO_2(110) surface in thermodynamic
equilibrium with an oxygen-rich environment. The traditionally assumed
stoichiometric termination is only found to be favorable at low oxygen chemical
potentials, i.e. low pressures and/or high temperatures. At realistic O
pressure, the surface is predicted to contain additional terminal O atoms.
Although this O excess defines a so-called polar surface, we show that the
prevalent ionic model, that dismisses such terminations on electrostatic
grounds, is of little validity for RuO_2(110). Together with analogous results
obtained previously at the (0001) surface of corundum-structured oxides, these
findings on (110) rutile indicate that the stability of non-stoichiometric
terminations is a more general phenomenon on transition metal oxide surfaces.Comment: 12 pages including 5 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
One-dimensional pair cascade emission in gamma-ray binaries
In gamma-ray binaries such as LS 5039 a large number of electron-positron
pairs are created by the annihilation of primary very high energy (VHE)
gamma-rays with photons from the massive star. The radiation from these
particles contributes to the total high energy gamma-ray flux and can initiate
a cascade, decreasing the effective gamma-ray opacity in the system. The aim of
this paper is to model the cascade emission and investigate if it can account
for the VHE gamma-ray flux detected by HESS from LS 5039 at superior
conjunction, where the primary gamma-rays are expected to be fully absorbed. A
one-dimensional cascade develops along the line-of-sight if the deflections of
pairs induced by the surrounding magnetic field can be neglected. A
semi-analytical approach can then be adopted, including the effects of the
anisotropic seed radiation field from the companion star. Cascade equations are
numerically solved, yielding the density of pairs and photons. In LS 5039, the
cascade contribution to the total flux is large and anti-correlated with the
orbital modulation of the primary VHE gamma-rays. The cascade emission
dominates close to superior conjunction but is too strong to be compatible with
HESS measurements. Positron annihilation does not produce detectable 511 keV
emission. This study provides an upper limit to cascade emission in gamma-ray
binaries at orbital phases where absorption is strong. The pairs are likely to
be deflected or isotropized by the ambient magnetic field, which will reduce
the resulting emission seen by the observer. Cascade emission remains a viable
explanation for the detected gamma-rays at superior conjunction in LS 5039.Comment: 8 pages, 7 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
New nonlinear dielectric materials: Linear electrorheological fluids under the influence of electrostriction
The usual approach to the development of new nonlinear dielectric materials
focuses on the search for materials in which the components possess an
inherently large nonlinear dielectric response. In contrast, based on
thermodynamics, we have presented a first-principles approach to obtain the
electrostriction-induced effective third-order nonlinear susceptibility for the
electrorheological (ER) fluids in which the components have inherent linear,
rather than nonlinear, responses. In detail, this kind of nonlinear
susceptibility is in general of about the same order of magnitude as the
compressibility of the linear ER fluid at constant pressure. Moreover, our
approach has been demonstrated in excellent agreement with a different
statistical method. Thus, such linear ER fluids can serve as a new nonlinear
dielectric material.Comment: 11 page
On the proper reconstruction of complex dynamical systems spoilt by strong measurement noise
This article reports on a new approach to properly analyze time series of
dynamical systems which are spoilt by the simultaneous presence of dynamical
noise and measurement noise. It is shown that even strong external measurement
noise as well as dynamical noise which is an intrinsic part of the dynamical
process can be quantified correctly, solely on the basis of measured times
series and proper data analysis. Finally real world data sets are presented
pointing out the relevance of the new approach
Dirac-screening stabilized surface-state transport in a topological insulator
We report magnetotransport studies on a gated strained HgTe device. This
material is a threedimensional topological insulator and exclusively shows
surface state transport. Remarkably, the Landau level dispersion and the
accuracy of the Hall quantization remain unchanged over a wide density range
(). This implies that
even at large carrier densities the transport is surface state dominated, where
bulk transport would have been expected to coexist already. Moreover, the
density dependence of the Dirac-type quantum Hall effect allows to identify the
contributions from the individual surfaces. A model can describe
the experiments, but only when assuming a steep band bending across the regions
where the topological surface states are contained. This steep potential
originates from the specific screening properties of Dirac systems and causes
the gate voltage to influence the position of the Dirac points rather than that
of the Fermi level.Comment: 12 pages 4 figure
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