12,677 research outputs found
The effect of internal and global modes on the radial distribution function of confined semiflexible polymers
The constraints imposed by nano- and microscale confinement on the
conformational degrees of freedom of thermally fluctuating biopolymers are
utilized in contemporary nano-devices to specifically elongate and manipulate
single chains. A thorough theoretical understanding and quantification of the
statistical conformations of confined polymer chains is thus a central concern
in polymer physics. We present an analytical calculation of the radial
distribution function of harmonically confined semiflexible polymers in the
weakly bending limit. Special emphasis has been put on a proper treatment of
global modes, i.e. the possibility of the chain to perform global movements
within the channel. We show that the effect of these global modes significantly
impacts the chain statistics in cases of weak and intermediate confinement.
Comparing our analytical model to numerical data from Monte Carlo simulations
we find excellent agreement over a broad range of parameters.Comment: 6 pages, 3 figures typo corrected, slightly revised line of
reasoning, results unchange
High-resolution radio imaging of two luminous quasars beyond redshift 4.5
Context. Radio-loud active galactic nuclei in the early Universe are rare.
The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out
from the known sample with their compact emission on milliarcsecond (mas)
angular scale with high (0.1-Jy level) flux densities measured at GHz radio
frequencies. This makes them ideal targets for very long baseline
interferometry (VLBI) observations. Aims. By means of VLBI imaging we can
reveal the inner radio structure of quasars and model their brightness
distribution to better understand the geometry of the jet and the physics of
the sources. Methods. We present sensitive high-resolution VLBI images of
J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The
data were taken in an astrometric observing programme involving a global
five-element radio telescope array. We combined the data from five different
epochs from 2017 February to August. Results. For one of the highest redshift
blazars known, J0906+6930, we present the first-ever VLBI image obtained at a
frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that
this source has a sharply bent helical inner jet structure within ~3 mas from
the core. The quasar J2102+6015 shows an elongated radio structure in the
east-west direction within the innermost ~2 mas that can be described with a
symmetric three-component brightness distribution model at 8.6 GHz. Because of
their non-pointlike mas-scale structure, these sources are not ideal as
astrometric reference objects. Our results demonstrate that VLBI observing
programmes conducted primarily with astrometric or geodetic goals can be
utilized for astrophysical purposes as well.Comment: 8 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Shape functions of dipolar ferromagnets at the Curie point
We present a complete mode coupling theory for the critical dynamics of ferromagnets above the Curie point with both short range exchange and long range dipolar interaction. This theory allows us to determine the full Kubo relaxation functions at the critical point. In particular, we are able to explain recent spin echo measurements
Nonaffine rubber elasticity for stiff polymer networks
We present a theory for the elasticity of cross-linked stiff polymer
networks. Stiff polymers, unlike their flexible counterparts, are highly
anisotropic elastic objects. Similar to mechanical beams stiff polymers easily
deform in bending, while they are much stiffer with respect to tensile forces
(``stretching''). Unlike in previous approaches, where network elasticity is
derived from the stretching mode, our theory properly accounts for the soft
bending response. A self-consistent effective medium approach is used to
calculate the macroscopic elastic moduli starting from a microscopic
characterization of the deformation field in terms of ``floppy modes'' --
low-energy bending excitations that retain a high degree of non-affinity. The
length-scale characterizing the emergent non-affinity is given by the ``fiber
length'' , defined as the scale over which the polymers remain straight.
The calculated scaling properties for the shear modulus are in excellent
agreement with the results of recent simulations obtained in two-dimensional
model networks. Furthermore, our theory can be applied to rationalize bulk
rheological data in reconstituted actin networks.Comment: 12 pages, 10 figures, revised Section II
Scaling function for the noisy Burgers equation in the soliton approximation
We derive the scaling function for the one dimensional noisy Burgers equation
in the two-soliton approximation within the weak noise canonical phase space
approach. The result is in agreement with an earlier heuristic expression and
exhibits the correct scaling properties. The calculation presents the first
step in a many body treatment of the correlations in the Burgers equation.Comment: Replacement: Several corrections, 4 pages, Revtex file, 3 figures. To
appear in Europhysics Letter
Entropic forces generated by grafted semiflexible polymers
The entropic force exerted by the Brownian fluctuations of a grafted
semiflexible polymer upon a rigid smooth wall are calculated both analytically
and by Monte Carlo simulations. Such forces are thought to play an important
role for several cellular phenomena, in particular, the physics of
actin-polymerization-driven cell motility and movement of bacteria like
Listeria. In the stiff limit, where the persistence length of the polymer is
larger than its contour length, we find that the entropic force shows scaling
behavior. We identify the characteristic length scales and the explicit form of
the scaling functions. In certain asymptotic regimes we give simple analytical
expressions which describe the full results to a very high numerical accuracy.
Depending on the constraints imposed on the transverse fluctuations of the
filament there are characteristic differences in the functional form of the
entropic forces; in a two-dimensional geometry the entropic force exhibits a
marked peak.Comment: 21 pages, 18 figures, minor misprints correcte
Solitons in the noisy Burgers equation
We investigate numerically the coupled diffusion-advective type field
equations originating from the canonical phase space approach to the noisy
Burgers equation or the equivalent Kardar-Parisi-Zhang equation in one spatial
dimension. The equations support stable right hand and left hand solitons and
in the low viscosity limit a long-lived soliton pair excitation. We find that
two identical pair excitations scatter transparently subject to a size
dependent phase shift and that identical solitons scatter on a static soliton
transparently without a phase shift. The soliton pair excitation and the
scattering configurations are interpreted in terms of growing step and
nucleation events in the interface growth profile. In the asymmetrical case the
soliton scattering modes are unstable presumably toward multi soliton
production and extended diffusive modes, signalling the general
non-integrability of the coupled field equations. Finally, we have shown that
growing steps perform anomalous random walk with dynamic exponent z=3/2 and
that the nucleation of a tip is stochastically suppressed with respect to
plateau formation.Comment: 11 pages Revtex file, including 15 postscript-figure
Evaluation of electrode shape and nondestructive evaluation method for welded solar cell interconnects
Resistance welds of solar cell interconnect tabs were evaluated. Both copper-silver and silver-silver welds were made with various heat inputs and weld durations. Parallel gap and annular gap weld electrode designs were used. The welds were analyzed by light microscope, electron microprobe and scanning laser acoustic microscope. These analyses showed the size and shape of the weld, the relationship between the acoustic micrographs, the visible electrode footprint, and the effect of electrode misalignment. The effect of weld heat input on weld microstructure was also shown
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