2,761 research outputs found
Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model
A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined
theoretical and experimental study. Despite a dominant intradimer coupling J1,
sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K.
The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural
beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in
alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a
honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in
the coupling regime of these isostructural compounds is governed by the nature
of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a
Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the
superexchange is realized via O-O edges of the tetrahedron. Implications for a
broad range of systems are discussed.Comment: 8 pages, 5 figures, 1 table; discussion extende
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
Microflow valve control system design
A design synthesis for a microflow control system is presented based on the interrogation of an analytical model, testing, and observation. The key issues relating to controlling a microflow using a variable geometry flow channel are explored through the implementation and testing of open and closed-loop control systems. The reliance of closed-loop systems on accurate flow measurement and the need for an open-loop strategy are covered. A valve and control system capable of accurately controlling flowrates between 0.09 and 400 ml/h and with a range of 900:1 is demonstrated
Evaluation of the soil moisture prediction accuracy of a space radar using simulation techniques
Image simulation techniques were employed to generate synthetic aperture radar images of a 17.7 km x 19.3 km test site located east of Lawrence, Kansas. The simulations were performed for a space SAR at an orbital altitude of 600 km, with the following sensor parameters: frequency = 4.75 GHz, polarization = HH, and angle of incidence range = 7 deg to 22 deg from nadir. Three sets of images were produced corresponding to three different spatial resolutions; 20 m x 20 m with 12 looks, 100 m x 100 m with 23 looks, and 1 km x 1 km with 1000 looks. Each set consisted of images for four different soil moisture distributions across the test site. Results indicate that, for the agricultural portion of the test site, the soil moisture in about 90% of the pixels can be predicted with an accuracy of = + or - 20% of field capacity. Among the three spatial resolutions, the 1 km x 1 km resolution gave the best results for most cases, however, for very dry soil conditions, the 100 m x 100 m resolution was slightly superior
Adaptive filtering of radar images for autofocus applications
Autofocus techniques are being designed at the Jet Propulsion Laboratory to automatically choose the filter parameters (i.e., the focus) for the digital synthetic aperture radar correlator; currently, processing relies upon interaction with a human operator who uses his subjective assessment of the quality of the processed SAR data. Algorithms were devised applying image cross-correlation to aid in the choice of filter parameters, but this method also has its drawbacks in that the cross-correlation result may not be readily interpretable. Enhanced performance of the cross-correlation techniques of JPL was hypothesized given that the images to be cross-correlated were first filtered to improve the signal-to-noise ratio for the pair of scenes. The results of experiments are described and images are shown
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.
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
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
Electrical expression of spin accumulation in ferromagnet/semiconductor structures
We treat the spin injection and extraction via a ferromagnetic
metal/semiconductor Schottky barrier as a quantum scattering problem. This
enables the theory to explain a number of phenomena involving spin-dependent
current through the Schottky barrier, especially the counter-intuitive spin
polarization direction in the semiconductor due to current extraction seen in
recent experiments. A possible explanation of this phenomenon involves taking
into account the spin-dependent inelastic scattering via the bound states in
the interface region. The quantum-mechanical treatment of spin transport
through the interface is coupled with the semiclassical description of
transport in the adjoining media, in which we take into account the in-plane
spin diffusion along the interface in the planar geometry used in experiments.
The theory forms the basis of the calculation of spin-dependent current flow in
multi-terminal systems, consisting of a semiconductor channel with many
ferromagnetic contacts attached, in which the spin accumulation created by spin
injection/extraction can be efficiently sensed by electrical means. A
three-terminal system can be used as a magnetic memory cell with the bit of
information encoded in the magnetization of one of the contacts. Using five
terminals we construct a reprogrammable logic gate, in which the logic inputs
and the functionality are encoded in magnetizations of the four terminals,
while the current out of the fifth one gives a result of the operation.Comment: A review to appear in Mod. Phys. Lett.
Increased gravitational force reveals the mechanical, resonant nature of physiological tremor
Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head-to-foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.This work was funded by a BBSRC Industry Interchange Award
to J.P.R.S. and R.F.R. C.J.O. was funded by BBSRC grant
BB/I00579X/1. C.A.V. was funded by A∗Midex (Aix-Marseille
Initiative of Excellence
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