17,683 research outputs found
S-Matrix Formulation of Mesoscopic Systems and Evanescent Modes
The Landauer-Butikker formalism is an important formalism to study mesoscopic
systems. Its validity for linear transport is well established theoretically as
well as experimentally. Akkermans et al [Phys. Rev. Lett. {\bf 66}, 76 (1991)]
had shown that the formalism can be extended to study thermodynamic properties
like persistent currents. It was earlier verified for simple one dimensional
systems. We study this formula very carefully and conclude that it requires
reinterpretation in quasi one dimension. This is essentially because of the
presence of evanescent modes in quasi one dimension.Comment: non
Length control of microtubules by depolymerizing motor proteins
In many intracellular processes, the length distribution of microtubules is
controlled by depolymerizing motor proteins. Experiments have shown that,
following non-specific binding to the surface of a microtubule, depolymerizers
are transported to the microtubule tip(s) by diffusion or directed walk and,
then, depolymerize the microtubule from the tip(s) after accumulating there. We
develop a quantitative model to study the depolymerizing action of such a
generic motor protein, and its possible effects on the length distribution of
microtubules. We show that, when the motor protein concentration in solution
exceeds a critical value, a steady state is reached where the length
distribution is, in general, non-monotonic with a single peak. However, for
highly processive motors and large motor densities, this distribution
effectively becomes an exponential decay. Our findings suggest that such motor
proteins may be selectively used by the cell to ensure precise control of MT
lengths. The model is also used to analyze experimental observations of
motor-induced depolymerization.Comment: Added section with figures and significantly expanded text, current
version to appear in Europhys. Let
Cluster formation and anomalous fundamental diagram in an ant trail model
A recently proposed stochastic cellular automaton model ({\it J. Phys. A 35,
L573 (2002)}), motivated by the motions of ants in a trail, is investigated in
detail in this paper. The flux of ants in this model is sensitive to the
probability of evaporation of pheromone, and the average speed of the ants
varies non-monotonically with their density. This remarkable property is
analyzed here using phenomenological and microscopic approximations thereby
elucidating the nature of the spatio-temporal organization of the ants. We find
that the observations can be understood by the formation of loose clusters,
i.e. space regions of enhanced, but not maximal, density.Comment: 11 pages, REVTEX, with 11 embedded EPS file
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