14,600 research outputs found
Validity of the Law of Mass Action in Three-Dimensional Coagulation Processes
Diffusion-limited reactions are studied in detail on the classical coalescing process. We demonstrate how, with the aid of a recent renormalization group approach, fluctuations can be integrated systematically. We thereby obtain an exact relation between the microscopic physics (lattice structure and particle shape and size) and the macroscopic decay rate in the law of mass action. Moreover, we find a strong violation of the law of mass action. The corresponding term in the kinetic equations originates in longwavelength fluctuations and is a universal function of the macroscopic decay rate
Current reversal and exclusion processes with history-dependent random walks
A class of exclusion processes in which particles perform history-dependent
random walks is introduced, stimulated by dynamic phenomena in some biological
and artificial systems. The particles locally interact with the underlying
substrate by breaking and reforming lattice bonds. We determine the
steady-state current on a ring, and find current-reversal as a function of
particle density. This phenomenon is attributed to the non-local interaction
between the walkers through their trails, which originates from strong
correlations between the dynamics of the particles and the lattice. We
rationalize our findings within an effective description in terms of
quasi-particles which we call front barriers. Our analytical results are
complemented by stochastic simulations.Comment: 5 pages, 6 figure
Physical limitations to the spatial resolution of solid-state detectors
In this paper we explore the effect of -ray emission, fluctuations in
th e signal deposition on the detection of charged particles in silicon-based
detec tors. We show that these two effects ultimately limit the resolution that
can be achieved by interpolation of the signal in finely segmented
position-sensitive solid-state devices.Comment: 5 page
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
Elasticity of Stiff Polymer Networks
We study the elasticity of a two-dimensional random network of rigid rods
(``Mikado model''). The essential features incorporated into the model are the
anisotropic elasticity of the rods and the random geometry of the network. We
show that there are three distinct scaling regimes, characterized by two
distinct length scales on the elastic backbone. In addition to a critical
rigidiy percolation region and a homogeneously elastic regime we find a novel
intermediate scaling regime, where elasticity is dominated by bending
deformations.Comment: 4 pages, 4 figure
Effects of different 3D QED vertex ansaetze on critical coupling
We study the semi-metal/insulator phase transition in graphene using a
Schwinger-Dyson approach. We consider various forms of vertex ansaetze to
truncate the hierarchy of Schwinger-Dyson equations. We define a Ball-Chiu type
vertex that truncates the equations without violating gauge invariance. We show
that there is a family of these vertices, parametrized by a continuous
parameter that we call a, all of which satisfy the Ward identity. We have
calculated the critical coupling of the phase transition using different values
of a. We have also tested a common approximation in which only the first term
in the Ball-Chiu ansatz is included. This vertex is independent of a, and,
although it is not gauge invariant, it has been used many times in the
literature because of the numerical simplifications it provides. We have found
that, with a one-loop photon polarization tensor, the results obtained for the
critical coupling from the truncated vertex and the full vertex with a = 1
agree very well, but other values of a give significantly different results. We
have also done a fully self-consistent calculation, in which the photons are
backcoupled to the fermion degrees of freedom, for one choice a = 1. Our
results show that when photon dynamics are correctly taken into account, it is
no longer true that the truncated vertex and the full Ball-Chiu vertex with a =
1 agree well. The conclusion is that traditional vertex truncations do not
really make sense in a system that does not respect Lorentz invariance, like
graphene, and the need to include vertex contributions self-consistently is
likely inescapable
Scaling and universality in coupled driven diffusive models
Inspired by the physics of magnetohydrodynamics (MHD) a simplified coupled
Burgers-like model in one dimension (1d), a generalization of the Burgers model
to coupled degrees of freedom, is proposed to describe 1dMHD. In addition to
MHD, this model serves as a 1d reduced model for driven binary fluid mixtures.
Here we have performed a comprehensive study of the universal properties of the
generalized d-dimensional version of the reduced model. We employ both
analytical and numerical approaches. In particular, we determine the scaling
exponents and the amplitude-ratios of the relevant two-point time-dependent
correlation functions in the model. We demonstrate that these quantities vary
continuously with the amplitude of the noise cross-correlation. Further our
numerical studies corroborate the continuous dependence of long wavelength and
long time-scale physics of the model on the amplitude of the noise
cross-correlations, as found in our analytical studies. We construct and
simulate lattice-gas models of coupled degrees of freedom in 1d, belonging to
the universality class of our coupled Burgers-like model, which display similar
behavior. We use a variety of numerical (Monte-Carlo and Pseudospectral
methods) and analytical (Dynamic Renormalization Group, Self-Consistent
Mode-Coupling Theory and Functional Renormalization Group) approaches for our
work. The results from our different approaches complement one another.
Possible realizations of our results in various nonequilibrium models are
discussed.Comment: To appear in JSTAT (2009); 52 pages in JSTAT format. Some figure
files have been replace
Entwicklung und Validierung des Bedürfnisinventars bei Gedächtnisstörungen (BIG-65): Krankheitskorrelierte Bedürfnisse bei Menschen mit Hirnleistungsstörungen und Demenz
Zusammenfassung: Hintergrund: Der Bedarf an einer systematischen Erfassung krankheitskorrelierter Patientenbedürfnisse bei Hirnleistungsstörungen und Demenz zur Auswahl optimaler Behandlungsoptionen steigt zunehmend. Es fehlt jedoch an validen Messinstrumenten, die zur Erfassung krankheitskorrelierter Bedürfnisse auch bei Menschen mit Demenz eingesetzt werden können. Methode: Die Studie umfasst die Konstruktion und Validierung des Bedürfnisinventars bei Gedächtnisstörungen (BIG-65) zur Erfassung krankheitskorrelierter Bedürfnisse. Der BIG-65 wurde theoriegeleitet entwickelt und basiert auf einer systematischen Literaturrecherche. Er wurde in einer Abklärungsstation, nach umfassender Untersuchung und Diagnosestellung, hinsichtlich seiner psychometrischen Eigenschaften an einer Gelegenheitsstichprobe (n = 83) validiert. Ergebnisse: Der BIG-65 hat 66Items und bietet neben einer breiten Auswahl an biopsychosozialen und umweltbezogenen Bedürfnissen eine geeignete Struktur zur Erfassung krankheitskorrelierter Bedürfnisse bei Menschen mit Hirnleistungsstörungen. Er verfügt über eine besonders hohe Augenscheinvalidität und eine sehr hohe Test-Retest-Reliabilität (rtt = 0,916). Im Mittel wurden 3,5 (SD = 3,7) unabgedeckte Bedürfnisse angegeben. Am häufigsten genannt wurden: "weniger vergessen" (50%), "bessere Konzentration" (23,2%), "Informationen zur Krankheit" (20,7%), "Informationen über Behandlungen" (17,1%) sowie "sich weniger Sorgen machen", "weniger gereizt sein", "Verbesserung der Stimmung", "Verbesserung der Orientierung" (alle 13,4%). Bedürfnisprofile unterscheiden sich zwischen verschiedenen Patientengruppen mit präklinischen (subjektive und milde kognitive Beeinträchtigung) und klinischen (Demenz) Hirnleistungsstörungen. Schlussfolgerungen: Krankheitskorrelierte Bedürfnisse können mit dem BIG-65 bis zu einer mittelschweren Demenz reliabel erfasst werden. Mit zunehmendem Demenzschweregrad oder einem Mini-Mental-Status <20Punkten sollte die Erfassung, z.B. mit zusätzlichen Beobachtungsmethoden des emotionalen Ausdrucks, ergänzt werden. Die Ergebnisse zeigen, dass Menschen mit Hirnleistungsstörungen individuelle Strategien zur Stabilisierung von Lebensqualität verfolgen. Neben einem objektiven Assessment von Krankheitssymptomen kann die Priorisierung optimaler Behandlungsmaßnahmen von der systematischen Erfassung krankheitskorrelierter Patientenbedürfnisse profitiere
Unfolding cross-linkers as rheology regulators in F-actin networks
We report on the nonlinear mechanical properties of a statistically
homogeneous, isotropic semiflexible network cross-linked by polymers containing
numerous small unfolding domains, such as the ubiquitous F-actin cross-linker
Filamin.
We show that the inclusion of such proteins has a dramatic effect on the
large strain behavior of the network. Beyond a strain threshold, which depends
on network density, the unfolding of protein domains leads to bulk shear
softening. Past this critical strain, the network spontaneously organizes
itself so that an appreciable fraction of the Filamin cross-linkers are at the
threshold of domain unfolding. We discuss via a simple mean-field model the
cause of this network organization and suggest that it may be the source of
power-law relaxation observed in in vitro and in intracellular microrheology
experiments. We present data which fully justifies our model for a simplified
network architecture.Comment: 11 pages, 4 figures. to appear in Physical Review
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