1,520 research outputs found
Radial Distribution Function for Semiflexible Polymers Confined in Microchannels
An analytic expression is derived for the distribution of the
end-to-end distance of semiflexible polymers in external potentials
to elucidate the effect of confinement on the mechanical and statistical
properties of biomolecules. For parabolic confinement the result is exact
whereas for realistic potentials a self-consistent ansatz is developed, so that
is given explicitly even for hard wall confinement. The
theoretical result is in excellent quantitative agreement with fluorescence
microscopy data for actin filaments confined in rectangularly shaped
microchannels. This allows an unambiguous determination of persistence length
and the dependence of statistical properties such as Odijk's deflection
length on the channel width . It is shown that neglecting the
effect of confinement leads to a significant overestimation of bending
rigidities for filaments
Local functional models of critical correlations in thin-films
Recent work on local functional theories of critical inhomogeneous fluids and
Ising-like magnets has shown them to be a potentially exact, or near exact,
description of universal finite-size effects associated with the excess
free-energy and scaling of one-point functions in critical thin films. This
approach is extended to predict the two-point correlation function G in
critical thin-films with symmetric surface fields in arbitrary dimension d. In
d=2 we show there is exact agreement with the predictions of conformal
invariance for the complete spectrum of correlation lengths as well as the
detailed position dependence of the asymptotic decay of G. In d=3 and d>=4 we
present new numerical predictions for the universal finite-size correlation
length and scaling functions determining the structure of G across the
thin-film. Highly accurate analytical closed form expressions for these
universal properties are derived in arbitrary dimension.Comment: 4 pages, 1 postscript figure. Submitted to Phys Rev Let
Droplet shapes on structured substrates and conformal invariance
We consider the finite-size scaling of equilibrium droplet shapes for fluid
adsorption (at bulk two-phase co-existence) on heterogeneous substrates and
also in wedge geometries in which only a finite domain of the
substrate is completely wet. For three-dimensional systems with short-ranged
forces we use renormalization group ideas to establish that both the shape of
the droplet height and the height-height correlations can be understood from
the conformal invariance of an appropriate operator. This allows us to predict
the explicit scaling form of the droplet height for a number of different
domain shapes. For systems with long-ranged forces, conformal invariance is not
obeyed but the droplet shape is still shown to exhibit strong scaling
behaviour. We argue that droplet formation in heterogeneous wedge geometries
also shows a number of different scaling regimes depending on the range of the
forces. The conformal invariance of the wedge droplet shape for short-ranged
forces is shown explicitly.Comment: 20 pages, 7 figures. (Submitted to J.Phys.:Cond.Mat.
QCD radiative and power corrections and Generalized GDH sum rules
We extend the earlier suggested QCD-motivated model for the -dependence
of the generalized Gerasimov-Drell-Hearn (GDH) sum rule which assumes the
smooth dependence of the structure function , while the sharp dependence
is due to the contribution and is described by the elastic part of the
Burkhardt-Cottingham sum rule. The model successfully predicts the low crossing
point for the proton GDH integral, but is at variance with the recent very
accurate JLAB data. We show that, at this level of accuracy, one should include
the previously neglected radiative and power QCD corrections, as boundary
values for the model. We stress that the GDH integral, when measured with such
a high accuracy achieved by the recent JLAB data, is very sensitive to QCD
power corrections. We estimate the value of these power corrections from the
JLAB data at . The inclusion of all QCD corrections leads
to a good description of proton, neutron and deuteron data at all .Comment: 10 pages, 4 figures (to be published in Physical Review D
Prä- und posttherapeutische Larynxbildgebung
Zusammenfassung: Sowohl CT als auch MRT und neuerdings die PET-CT sind unentbehrliche Zusatzuntersuchungen zur Diagnostik und Stadieneinteilung von Tumoren des Larynx. Sie sind der klinischen Untersuchung (einschließlich endoskopischer Biopsie) beigeordnet und ergänzen diese komplementär. Eine sehr genaue Kenntnis der submukösen Tumorausbreitungswege, der diagnostischen Zeichen der Tumorinfiltration und deren Konsequenzen für Stadieneinteilung und Therapie sind unentbehrlich für die Interpretation von CT-, MRT- und PET-CT-Bildern. Sowohl CT als auch MRT sind hochsensitive Untersuchungen zum Nachweis der neoplastischen Infiltration des präepi- und paraglottischen Raums, der Subglottis und des Knorpels. Die Spezifität ist jedoch mit beiden Methoden weniger hoch als zunächst erwartet, wodurch eine Tendenz zum Überschätzen der Tumorausbreitung resultiert. Neuere Untersuchungen haben jedoch gezeigt, dass die Spezifität der MRT mittels Anwendung neuer diagnostischer Kriterien signifikant verbessert werden kann, da eine Unterscheidung zwischen Tumor und peritumoraler Entzündung in vielen Fällen möglich ist. Der sehr hohe negative Vorhersagewert der beiden Schnittbildverfahren ist aus klinischer Sicht wichtig, da er es ermöglicht, die neoplastische Knorpelinfiltration auszuschließen. Beide Methoden verbessern signifikant die prätherapeutische Stagingtreffsicherheit, wenn sie zusätzlich zur Endoskopie eingesetzt werden. Bei submukösen Tumoren liefern sowohl CT als auch MRT wertvolle Hinweise auf eine mögliche Ätiologie, auf das Ausmaß des submukösen Wachstums und die geeignete Biopsiestelle. Sie spielen auch eine wichtige Rolle bei der Diagnose von Laryngozelen, der Abklärung von N.-laryngeus-recurrens-Paresen und Larynxfrakture
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
First passage time exponent for higher-order random walks:Using Levy flights
We present a heuristic derivation of the first passage time exponent for the
integral of a random walk [Y. G. Sinai, Theor. Math. Phys. {\bf 90}, 219
(1992)]. Building on this derivation, we construct an estimation scheme to
understand the first passage time exponent for the integral of the integral of
a random walk, which is numerically observed to be . We discuss
the implications of this estimation scheme for the integral of a
random walk. For completeness, we also address the case. Finally, we
explore an application of these processes to an extended, elastic object being
pulled through a random potential by a uniform applied force. In so doing, we
demonstrate a time reparameterization freedom in the Langevin equation that
maps nonlinear stochastic processes into linear ones.Comment: 4 figures, submitted to PR
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