14,811 research outputs found
Continuous and discrete models of cooperation in complex bacterial colonies
We study the effect of discreteness on various models for patterning in
bacterial colonies. In a bacterial colony with branching pattern, there are
discrete entities - bacteria - which are only two orders of magnitude smaller
than the elements of the macroscopic pattern. We present two types of models.
The first is the Communicating Walkers model, a hybrid model composed of both
continuous fields and discrete entities - walkers, which are coarse-graining of
the bacteria. Models of the second type are systems of reaction diffusion
equations, where the branching of the pattern is due to non-constant diffusion
coefficient of the bacterial field. The diffusion coefficient represents the
effect of self-generated lubrication fluid on the bacterial movement. We
implement the discreteness of the biological system by introducing a cutoff in
the growth term at low bacterial densities. We demonstrate that the cutoff does
not improve the models in any way. Its only effect is to decrease the effective
surface tension of the front, making it more sensitive to anisotropy. We
compare the models by introducing food chemotaxis and repulsive chemotactic
signaling into the models. We find that the growth dynamics of the
Communication Walkers model and the growth dynamics of the Non-Linear diffusion
model are affected in the same manner. From such similarities and from the
insensitivity of the Communication Walkers model to implicit anisotropy we
conclude that the increased discreteness, introduced be the coarse-graining of
the walkers, is small enough to be neglected.Comment: 16 pages, 10 figures in 13 gif files, to be published in proceeding
of CMDS
Reversal and Termination of Current-Induced Domain Wall Motion via Magnonic Spin-Transfer Torque
We investigate the domain wall dynamics of a ferromagnetic wire under the
combined influence of a spin-polarized current and magnonic spin-transfer
torque generated by an external field, taking also into account Rashba
spin-orbit coupling interactions. It is demonstrated that current-induced
motion of the domain wall may be completely reversed in an oscillatory fashion
by applying a magnonic spin-transfer torque as long as the spin-wave velocity
is sufficiently high. Moreover, we show that the motion of the domain wall may
be fully terminated by means of the generation of spin-waves, suggesting the
possibility to pin the domain-walls to predetermined locations. We also discuss
how strong spin-orbit interactions modify these results.Comment: Accepted for publication in Phys. Rev.
Cubic Dresselhaus Spin-Orbit Coupling in 2D Electron Quantum Dots
We study effects of the oft-neglected cubic Dresselhaus spin-orbit coupling
(i.e., ) in GaAs/AlGaAs quantum dots. Using a semiclassical
billiard model, we estimate the magnitude of the spin-orbit induced avoided
crossings in a closed quantum dot in a Zeeman field. Using these results,
together with previous analyses based on random matrix theory, we calculate
corresponding effects on the conductance through an open quantum dot. Combining
our results with an experiment on conductance through an 8 um^2 quantum dot [D
M Zumbuhl et al., Phys. Rev. B 72, 081305 (2005)] suggests that 1) the GaAs
Dresselhaus coupling constant, , is approximately 9 eVA^3,
significantly less than the commonly cited value of 27.5 eVA^3 and 2) the
majority of the spin-flip component of spin-orbit coupling can come from the
cubic Dresselhaus term.Comment: 4 pages plus supplementary tabl
Nonradiative lifetimes in intermediate band materials - absence of lifetime recovery
Intermediate band photovoltaics hold the promise of being highly efficient
and cost effective photovoltaic cells. Intermediate states in the band gap,
however, are known to facilitate nonradiative recombination. Much effort has
been dedicated to producing metallic intermediate bands in hopes of producing
lifetime recovery -- an increase in carrier lifetime as doping levels increase.
We show that lifetime recovery induced by the insulator-to-metal transition
will not occur, because the metallic extended states will be localised by
phonons during the recombination process. Only trivial forms of lifetime
recovery, e.g., from an overall shift in intermediate levels, are possible.
Future work in intermediate band photovoltaics must focus on optimizing subgap
optical absorption and minimizing recombination, but not via lifetime recovery.Comment: 8 page
An instability criterion for a finite amplitude localized disturbance in a shear flow of electrically conducting fluids
The stability of shear flows of electrically conducting fluids, with respect
to finite amplitude three-dimensional localized disturbances is considered. The
time evolution of the fluid impulse integral, characterizing such disturbances,
for the case of low magnetic Reynolds number is obtained by integrating
analytically the vorticity equation. Analysis of the resulted equation reveals
a new instability criterion.Comment: 10 pages in LaTex, no figures, accepted in Phys. Fluid
Scaling and localization lengths of a topologically disordered system
We consider a noninteracting disordered system designed to model particle
diffusion, relaxation in glasses, and impurity bands of semiconductors.
Disorder originates in the random spatial distribution of sites. We find strong
numerical evidence that this model displays the same universal behavior as the
standard Anderson model. We use finite-size-scaling to find the localization
length as a function of energy and density, including localized states away
from the delocalization transition. Results at many energies all fit onto the
same universal scaling curve.Comment: 5+ page
In situ XRF and gamma ray spectrometer for Mars sample return mission
A combined in situ X-ray fluorescence (XRF) and passive gamma ray spectrometer instrument is proposed for the chemical elemental analysis of various Martian surfaces and samples. The combined instrument can be carried on board a rover. The passive gamma ray or the neutron excited gamma ray system would be used to determine the elemental composition of the Martian surface while the rover is in motion. The XRF system would be used to perform analysis either on the Martian surface or on collected samples when the rover is stationary. The latter function is important both in cataloging the collected samples and in the selection of samples to be returned to earth. For both systems, data accumulation time would be on the order of 30 minutes. No sample preparation would be necessary
Topological Transitions in Metamaterials
The ideas of mathematical topology play an important role in many aspects of
modern physics - from phase transitions to field theory to nonlinear dynamics
(Nakahara M (2003) in Geometry, Topology and Physics, ed Brewer DF (IOP
Publishing Ltd, Bristol and Philadelphia), Monastryskiy M (1987) in Riemann
Topology and Physics, (Birkhauser Verlag AG)). An important example of this is
the Lifshitz transition (Lifshitz IM (1960) Anomalies of electron
characteristics of a metal in the high-pressure region, Sov Phys JETP 11:
1130-1135), where the transformation of the Fermi surface of a metal from a
closed to an open geometry (due to e.g. external pressure) leads to a dramatic
effect on the electron magneto-transport (Kosevich AM (2004) Topology and
solid-state physics. Low Temp Phys 30: 97-118). Here, we present the optical
equivalent of the Lifshitz transition in strongly anisotropic metamaterials.
When one of the components of the dielectric permittivity tensor of such a
composite changes sign, the corresponding iso-frequency surface transforms from
an ellipsoid to a hyperboloid. Since the photonic density of states can be
related to the volume enclosed by the iso-frequency surface, such a topological
transition in a metamaterial leads to a dramatic change in the photonic density
of states, with a resulting effect on every single physical parameter related
to the metamaterial - from thermodynamic quantities such as its equilibrium
electromagnetic energy to the nonlinear optical response to
quantum-electrodynamic effects such as spontaneous emission. In the present
paper, we demonstrate the modification of spontaneous light emission from
quantum dots placed near the surface of the metamaterial undergoing the
topological Lifshitz transition, and present the theoretical description of the
effect
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