71,770 research outputs found
The Force Exerted by a Molecular Motor
The stochastic driving force exerted by a single molecular motor (e.g., a
kinesin, or myosin) moving on a periodic molecular track (microtubule, actin
filament, etc.) is discussed from a general viewpoint open to experimental
test. An elementary "barometric" relation for the driving force is introduced
that (i) applies to a range of kinetic and stochastic models, (ii) is
consistent with more elaborate expressions entailing explicit representations
of externally applied loads and, (iii) sufficiently close to thermal
equilibrium, satisfies an Einstein-type relation in terms of the velocity and
diffusion coefficient of the (load-free) motor. Even in the simplest two-state
models, the velocity-vs.-load plots exhibit a variety of contrasting shapes
(including nonmonotonic behavior). Previously suggested bounds on the driving
force are shown to be inapplicable in general by analyzing discrete jump models
with waiting time distributions.Comment: submitted to PNA
Extended Kinetic Models with Waiting-Time Distributions: Exact Results
Inspired by the need for effective stochastic models to describe the complex
behavior of biological motor proteins that move on linear tracks exact results
are derived for the velocity and dispersion of simple linear sequential models
(or one-dimensional random walks) with general waiting-time distributions. The
concept of ``mechanicity'' is introduced in order to conveniently quantify
departures from simple ``chemical,'' kinetic rate processes, and its
significance is briefly indicated. The results are extended to more elaborate
models that have finite side-branches and include death processes (to represent
the detachment of a motor from the track).Comment: 17 pages, 2 figure
GHz Spin Noise Spectroscopy in n-Doped Bulk GaAs
We advance spin noise spectroscopy to an ultrafast tool to resolve high
frequency spin dynamics in semiconductors. The optical non-demolition
experiment reveals the genuine origin of the inhomogeneous spin dephasing in
n-doped GaAs wafers at densities at the metal-to-insulator transition. The
measurements prove in conjunction with depth resolved spin noise measurements
that the broadening of the spin dephasing rate does not result from thermal
fluctuations or spin-phonon interaction, as previously suggested, but from
surface electron depletion
Moment free energies for polydisperse systems
A polydisperse system contains particles with at least one attribute
(such as particle size in colloids or chain length in polymers) which takes
values in a continuous range. It therefore has an infinite number of conserved
densities, described by a density {\em distribution} . The free
energy depends on all details of , making the analysis of phase
equilibria in such systems intractable. However, in many (especially
mean-field) models the {\em excess} free energy only depends on a finite number
of (generalized) moments of ; we call these models truncatable.
We show, for these models, how to derive approximate expressions for the {\em
total} free energy which only depend on such moment densities. Our treatment
unifies and explores in detail two recent separate proposals by the authors for
the construction of such moment free energies. We show that even though the
moment free energy only depends on a finite number of density variables, it
gives the same spinodals and critical points as the original free energy and
also correctly locates the onset of phase coexistence. Results from the moment
free energy for the coexistence of two or more phases occupying comparable
volumes are only approximate, but can be refined arbitrarily by retaining
additional moment densities. Applications to Flory-Huggins theory for
length-polydisperse homopolymers, and for chemically polydisperse copolymers,
show that the moment free energy approach is computationally robust and gives
new geometrical insights into the thermodynamics of polydispersity.Comment: RevTeX, 43 pages including figure
PROJECTED COSTS AND RETURNS - SUGARCANE, LOUISIANA, 2001.
This report presents estimates of costs and returns associated with sugarcane production practices in Louisiana for 2001. It is part of a continuing effort to provide farmers, researchers, extension personnel, lending agencies and others working in agriculture and/or agribusiness timely planning information. Sugarcane production is unique in that it is a perennial crop grown in a rotation; processing, storage and marketing services are provided by a single entity and payments for said services are "in kind." Further, the large majority of growers are tenants, paying approximately 20 percent of the "after milling crop proceeds" (12.2% of gross production) for land. Returns shown in Table 1A-3C and in the whole farm analysis in Appendix A reflect returns to management and risk. No charges for family living expenses or management are included as a cost in this analysis.Farm Management,
Radiation environment and shielding for early manned Mars missions
The problem of shielding a crew during early manned Mars missions is discussed. Requirements for shielding are presented in the context of current astronaut exposure limits, natural ionizing radiation sources, and shielding inherent in a particular Mars vehicle configuration. An estimated range for shielding weight is presented based on the worst solar flare dose, mission duration, and inherent vehicle shielding
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