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 $\delta$-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