1,168 research outputs found
Stiften gehen : »Wir möchten gestalten.« – »… ein bisschen mehr Gleichheit schaffen …«
First passage behaviour of fractional Brownian motion in two-dimensional wedge domains
We study the survival probability and the corresponding first passage time
density of fractional Brownian motion confined to a two-dimensional open wedge
domain with absorbing boundaries. By analytical arguments and numerical
simulation we show that in the long time limit the first passage time density
scales as t**{-1+pi*(2H-2)/(2*Theta)} in terms of the Hurst exponent H and the
wedge angle Theta. We discuss this scaling behaviour in connection with the
reaction kinetics of FBM particles in a one-dimensional domain.Comment: 6 pages, 4 figure
Photon assisted Levy flights of minority carriers in n-InP
We study the photoluminescence spectra of n-doped InP bulk wafers, both in
the reflection and the transmission geometries relative to the excitation beam.
From the observed spectra we estimate the spatial distribution of minority
carriers allowing for the spectral filtering due to re-absorption of
luminescence in the wafer. This distribution unambiguously demonstrates a
non-exponential drop-off with distance from the excitation region. Such a
behavior evidences an anomalous photon-assisted transport of minority carriers
enhanced owing to the high quantum efficiency of emission. It is shown that the
transport conforms very well to the so-called Levy-flights process
corresponding to a peculiar random walk that does not reduce to diffusion. The
index gamma of the Levy flights distribution is found to be in the range gamma
= 0.64 to 0.79, depending on the doping. Thus, we propose the high-efficiency
direct-gap semiconductors as a remarkable laboratory system for studying the
anomalous transport.Comment: 12 pages, 9 figure
Emil von Behring (1854 - 1917) an Moritz Schmidt-Metzler (1838 - 1907) : Frankfurt, 4. III. [18]96
Emil von Behring (1854 - 1917) an Moritz Schmidt-Metzler (1838 - 1907) : Wiesbaden, 24. VIII, [18]99
Postkarte Emil von Behrings an Moritz Schmidt-Metzler mit Fragen zum Frankfurter Oberbürgermeister Franz Adickes
First passages in bounded domains: When is the mean first passage time meaningful?
We study the first passage statistics to adsorbing boundaries of a Brownian
motion in bounded two-dimensional domains of different shapes and
configurations of the adsorbing and reflecting boundaries. From extensive
numerical analysis we obtain the probability P(\omega) distribution of the
random variable \omega=\tau_1/(\tau_1+\tau_2), which is a measure for how
similar the first passage times \tau_1 and \tau_2 are of two independent
realisations of a Brownian walk starting at the same location. We construct a
chart for each domain, determining whether P(\omega) represents a unimodal,
bell-shaped form, or a bimodal, M-shaped behaviour. While in the former case
the mean first passage time (MFPT) is a valid characteristic of the first
passage behaviour, in the latter case it is an insufficient measure for the
process. Strikingly we find a distinct turnover between the two modes of
P(\omega), characteristic for the domain shape and the respective location of
absorbing and reflective boundaries. Our results demonstrate that large
fluctuations of the first passage times may occur frequently in two-dimensional
domains, rendering quite vague the general use of the MFPT as a robust measure
of the actual behaviour even in bounded domains, in which all moments of the
first passage distribution exist.Comment: 9 pages, 6 figure
Subdiffusion and weak ergodicity breaking in the presence of a reactive boundary
We derive the boundary condition for a subdiffusive particle interacting with
a reactive boundary with finite reaction rate. Molecular crowding conditions,
that are found to cause subdiffusion of larger molecules in biological cells,
are shown to effect long-tailed distributions with identical exponent for both
the unbinding times from the boundary to the bulk and the rebinding times from
the bulk. This causes a weak ergodicity breaking: typically, an individual
particle either stays bound or remains in the bulk for very long times. We
discuss why this may be beneficial for in vivo gene regulation by DNA-binding
proteins, whose typical concentrations are nanomolarComment: 4 pages, 1 figure, REVTeX4, accepted to Phys Rev Lett, some typos
correcte
Origin and pathogenesis of nodular lymphocyte–predominant Hodgkin lymphoma as revealed by global gene expression analysis
The pathogenesis of nodular lymphocyte–predominant Hodgkin lymphoma (NLPHL) and its relationship to other lymphomas are largely unknown. This is partly because of the technical challenge of analyzing its rare neoplastic lymphocytic and histiocytic (L&H) cells, which are dispersed in an abundant nonneoplastic cellular microenvironment. We performed a genome-wide expression study of microdissected L&H lymphoma cells in comparison to normal and other malignant B cells that indicated a relationship of L&H cells to and/or that they originate from germinal center B cells at the transition to memory B cells. L&H cells show a surprisingly high similarity to the tumor cells of T cell–rich B cell lymphoma and classical Hodgkin lymphoma, a partial loss of their B cell phenotype, and deregulation of many apoptosis regulators and putative oncogenes. Importantly, L&H cells are characterized by constitutive nuclear factor {kappa}B activity and aberrant extracellular signal-regulated kinase signaling. Thus, these findings shed new light on the nature of L&H cells, reveal several novel pathogenetic mechanisms in NLPHL, and may help in differential diagnosis and lead to novel therapeutic strategies
Exact solution of a linear molecular motor model driven by two-step fluctuations and subject to protein friction
We investigate by analytical means the stochastic equations of motion of a
linear molecular motor model based on the concept of protein friction. Solving
the coupled Langevin equations originally proposed by Mogilner et al. (A.
Mogilner et al., Phys. Lett. {\bf 237}, 297 (1998)), and averaging over both
the two-step internal conformational fluctuations and the thermal noise, we
present explicit, analytical expressions for the average motion and the
velocity-force relationship. Our results allow for a direct interpretation of
details of this motor model which are not readily accessible from numerical
solutions. In particular, we find that the model is able to predict
physiologically reasonable values for the load-free motor velocity and the
motor mobility.Comment: 12 pages revtex, 6 eps-figure
Bulk-mediated surface diffusion on a cylinder: propagators and crossovers
We consider the effective surface motion of a particle that freely diffuses
in the bulk and intermittently binds to that surface. From an exact approach we
derive various regimes of the effective surface motion characterized by
physical rates for binding/unbinding and the bulk diffusivity. We obtain a
transient regime of superdiffusion and, in particular, a saturation regime
characteristic for the cylindrical geometry. This saturation, however, in a
finite system is not terminal but eventually turns over to normal surface
diffusion. The first passage behavior of particles to the cylinder surface is
derived. Consequences for actual systems are discussed.Comment: 4 pages REVTeX4, 2 figure
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