2,026 research outputs found
Reaction Brownian Dynamics and the effect of spatial fluctuations on the gain of a push-pull network
Brownian Dynamics algorithms are widely used for simulating soft-matter and
biochemical systems. In recent times, their application has been extended to
the simulation of coarse-grained models of cellular networks in simple
organisms. In these models, components move by diffusion, and can react with
one another upon contact. However, when reactions are incorporated into a
Brownian Dynamics algorithm, attention must be paid to avoid violations of the
detailed-balance rule, and therefore introducing systematic errors in the
simulation. We present a Brownian Dynamics algorithm for reaction-diffusion
systems that rigorously obeys detailed balance for equilibrium reactions. By
comparing the simulation results to exact analytical results for a bimolecular
reaction, we show that the algorithm correctly reproduces both equilibrium and
dynamical quantities. We apply our scheme to a ``push-pull'' network in which
two antagonistic enzymes covalently modify a substrate. Our results highlight
that the diffusive behaviour of the reacting species can reduce the gain of the
response curve of this network.Comment: 25 pages, 7 figures, submitted to Journal of Chemical Physic
Selective Predation on the Seeds of Woody Plants
Selective predation on the seeds of woody plants. J. Torrey Bot. Soc. 124: 67-70). 1997.-Seed predation may be an important factor influencing the structure of successional plant communities. We used a cafeteria-style experiment, placed in an old field and an early successional forest, to determine predator preferences for seeds of nine species of woody plants. Intensity of seed predation was equivalent in both sites. Seed predators preferred Acer saccharum, flex vertic illata, and Viburnum dentatum, but this was not related to seed mass. Predation intensity was more variable in the old field than in the forest, possibly related to the higher ground-layer heterogeneity of the old field site. We conclude that predator choice will allow some species to escape seed predation, potentially altering future plant community composition
Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures
Quantum mechanical calculations based on an extended Heisenberg model are
compared with ferromagnetic resonance (FMR) experiments on prototype trilayer
systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first
time quantitatively the sources of the temperature dependence of interlayer
exchange coupling. Magnon excitations are responsible for about 75% of the
reduction of the coupling strength from zero to room temperature. The remaining
25% are due to temperature effects in the effective quantum well and the
spacer/magnet interfaces.Comment: accepted for publication in PR
Selective Predation on the Seeds of Woody Plants
Selective predation on the seeds of woody plants. J. Torrey Bot. Soc. 124: 67-70). 1997.-Seed predation may be an important factor influencing the structure of successional plant communities. We used a cafeteria-style experiment, placed in an old field and an early successional forest, to determine predator preferences for seeds of nine species of woody plants. Intensity of seed predation was equivalent in both sites. Seed predators preferred Acer saccharum, flex vertic illata, and Viburnum dentatum, but this was not related to seed mass. Predation intensity was more variable in the old field than in the forest, possibly related to the higher ground-layer heterogeneity of the old field site. We conclude that predator choice will allow some species to escape seed predation, potentially altering future plant community composition
Synchronization of spin-torque driven nanooscillators for point contacts on a quasi-1D nanowire: Micromagnetic simulations
In this paper we present detailed numerical simulation studies on the
synchronization of two spin-torque nanooscillators (STNO) in the quasi-1D
geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a
spin polarized current injected via square-shaped CoFe nanomagnets on the top
of this stripe. In a sufficiently large out-of-plane field, a propagating
oscillation mode appears in such a system. Due to the absence of the
geometrically caused wave decay in 1D systems, this mode is expected to enable
a long-distance synchronization between STNOs. Indeed, our simulations predict
that synchronization of two STNOs on a nanowire is possible up to the
intercontact distance 3 mkm (for the nanowire width 50 nm). However, we have
also found several qualitatively new features of the synchronization behaviour
for this system, which make the achievement of a stable synchronization in this
geometry to a highly non-trivial task. In particular, there exist a minimal
distance between the nanocontacts, below which a synchronization of STNOs can
not be achieved. Further, when the current value in the first contact is kept
constant, the amplitude of synchronized oscillations depends non-monotonously
on the current value in the second contact. Finally, for one and the same
currents values through the contacts there might exist several synchronized
states (with different frequencies), depending on the initial conditions.Comment: 13 pages with 4 figurews, recently submitted to PR
Spin Motion in Electron Transmission through Ultrathin Ferromagnetic Films Accessed by Photoelectron Spectroscopy
Ab initio and model calculations demonstrate that the spin motion of
electrons transmitted through ferromagnetic films can be analyzed in detail by
means of angle- and spin-resolved core-level photoelectron spectroscopy. The
spin motion appears as precession of the photoelectron spin polarization around
and as relaxation towards the magnetization direction. In a systematic study
for ultrathin Fe films on Pd(001) we elucidate its dependence on the Fe film
thickness and on the Fe electronic structure. In addition to elastic and
inelastic scattering, the effect of band gaps on the spin motion is addressed
in particular.Comment: 4 pages, 5 figure
Anatomy of Spin-Transfer Torque
Spin-transfer torques occur in magnetic heterostructures because the
transverse component of a spin current that flows from a non-magnet into a
ferromagnet is absorbed at the interface. We demonstrate this fact explicitly
using free electron models and first principles electronic structure
calculations for real material interfaces. Three distinct processes contribute
to the absorption: (1) spin-dependent reflection and transmission; (2) rotation
of reflected and transmitted spins; and (3) spatial precession of spins in the
ferromagnet. When summed over all Fermi surface electrons, these processes
reduce the transverse component of the transmitted and reflected spin currents
to nearly zero for most systems of interest. Therefore, to a good
approximation, the torque on the magnetization is proportional to the
transverse piece of the incoming spin current.Comment: 16 pages, 8 figures, submitted to Phys. Rev.
Adiabatic Domain Wall Motion and Landau-Lifshitz Damping
Recent theory and measurements of the velocity of current-driven domain walls
in magnetic nanowires have re-opened the unresolved question of whether
Landau-Lifshitz damping or Gilbert damping provides the more natural
description of dissipative magnetization dynamics. In this paper, we argue that
(as in the past) experiment cannot distinguish the two, but that
Landau-Lifshitz damping nevertheless provides the most physically sensible
interpretation of the equation of motion. From this perspective, (i) adiabatic
spin-transfer torque dominates the dynamics with small corrections from
non-adiabatic effects; (ii) the damping always decreases the magnetic free
energy, and (iii) microscopic calculations of damping become consistent with
general statistical and thermodynamic considerations
Spin Transfer Torque for Continuously Variable Magnetization
We report quantum and semi-classical calculations of spin current and
spin-transfer torque in a free-electron Stoner model for systems where the
magnetization varies continuously in one dimension.Analytic results are
obtained for an infinite spin spiral and numerical results are obtained for
realistic domain wall profiles. The adiabatic limit describes conduction
electron spins that follow the sum of the exchange field and an effective,
velocity-dependent field produced by the gradient of the magnetization in the
wall. Non-adiabatic effects arise for short domain walls but their magnitude
decreases exponentially as the wall width increases. Our results cast doubt on
the existence of a recently proposed non-adiabatic contribution to the
spin-transfer torque due to spin flip scattering.Comment: 11 pages, 9 figure
A selfconsistent theory of current-induced switching of magnetization
A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
, which is in good agreement with experiment.Comment: 23 pages, 16 figure
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