10,489 research outputs found
Transport coefficients for the shear dynamo problem at small Reynolds numbers
We build on the formulation developed in Sridhar & Singh (JFM, 664, 265,
2010), and present a theory of the \emph{shear dynamo problem} for small
magnetic and fluid Reynolds numbers, but for arbitrary values of the shear
parameter. Specializing to the case of a mean magnetic field that is slowly
varying in time, explicit expressions for the transport coefficients,
and , are derived. We prove that, when the velocity
field is non helical, the transport coefficient vanishes. We then
consider forced, stochastic dynamics for the incompressible velocity field at
low Reynolds number. An exact, explicit solution for the velocity field is
derived, and the velocity spectrum tensor is calculated in terms of the
Galilean--invariant forcing statistics. We consider forcing statistics that is
non helical, isotropic and delta-correlated-in-time, and specialize to the case
when the mean-field is a function only of the spatial coordinate and time
; this reduction is necessary for comparison with the numerical
experiments of Brandenburg, R{\"a}dler, Rheinhardt & K\"apyl\"a (ApJ, 676, 740,
2008). Explicit expressions are derived for all four components of the magnetic
diffusivity tensor, . These are used to prove that the
shear-current effect cannot be responsible for dynamo action at small \re and
\rem, but for all values of the shear parameter.Comment: 27 pages, 5 figures, Published in Physical Review
On cross-beam monitoring of atmospheric winds and turbulence with two orbiting telescopes
Crossed beam monitoring of atmospheric winds and turbulence with two orbiting astronomical telescopes mounted on single spacecraf
Advanced nickel-cadmium batteries for geosynchronous spacecraft
A nickel cadmium battery was developed that can be operated at 80 percent depth of discharge in excess of 10 years in a geosynchronous orbit application, and has about a 30 percent weight savings per spacecraft over present nickel cadmium batteries when used with a 1000 watts eclipse load. The approach used in the development was to replace nylon separators with inert polymer impregnated zirconia, use electrochemically deposited plates in place of conventional chemically precipitated ones, and use an additive to extend negative plate lifetime. The design has undergone extensive testing using both engineering and protoflight cell configurations
LAPS and SPIM Imaging Using ITO-Coated Glass as the Substrate Material
Light-addressable
potentiometric sensors (LAPS) and scanning photo-induced
impedance microscopy (SPIM) use photocurrent measurements for spatiotemporal
imaging of ion concentrations, electrical potentials, and impedance.
In this work, ITO-coated glass was confirmed to produce photocurrents
at anodic potentials with 405 nm diode laser illumination. Therefore,
it was developed as a low cost and robust substrate material for LAPS
and SPIM imaging compared to traditional expensive ultrathin Si substrates.
ITO showed good ac photocurrent and pH response without surface modification
and insulator. Local photocurrents were produced by scanning a focused
laser beam across the sample, which proved the light addressability
of ITO-coated glass. With a high-impedance PMMA dot deposited onto
the ITO as a model system, a lateral resolution of about 2.3 μm
was achieved
Measurement of the Permanent Electric Dipole Moment of the Xe Atom
We report on a new measurement of the CP-violating permanent Electric Dipole
Moment (EDM) of the neutral Xe atom. Our experimental approach is based
on the detection of the free precession of co-located nuclear spin-polarized
He and Xe samples. The EDM measurement sensitivity benefits
strongly from long spin coherence times of several hours achieved in diluted
gases and homogeneous weak magnetic fields of about 400~nT. A finite EDM is
indicated by a change in the precession frequency, as an electric field is
periodically reversed with respect to the magnetic guiding field. Our result,
ecm, is consistent with zero and is
used to place a new upper limit on the Xe EDM: ecm (95% C.L.). We also discuss the implications of this result for
various CP-violating observables as they relate to theories of physics beyond
the standard model
Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructure
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to
investigate ultrafast charge transfer in an epitaxial heterostructure made of
monolayer WS and graphene. This heterostructure combines the benefits of a
direct gap semiconductor with strong spin-orbit coupling and strong
light-matter interaction with those of a semimetal hosting massless carriers
with extremely high mobility and long spin lifetimes. We find that, after
photoexcitation at resonance to the A-exciton in WS, the photoexcited holes
rapidly transfer into the graphene layer while the photoexcited electrons
remain in the WS layer. The resulting charge transfer state is found to
have a lifetime of \,ps. We attribute our findings to differences in
scattering phase space caused by the relative alignment of WS and graphene
bands as revealed by high resolution ARPES. In combination with spin-selective
excitation using circularly polarized light the investigated WS/graphene
heterostructure might provide a new platform for efficient optical spin
injection into graphene.Comment: 28 pages, 14 figure
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
The emerging communication architecture in electrical energy supply and its implications
In the course of liberalisation of the electrical energy market, the pressure on the utilities to reduce their investment and maintenance costs is increasing. In order to lower these expenses and to be able to offer a more efficient supply with electrical energy, the utilities are increasingly using modern communication techniques. Control mechanisms that have been realized with a huge amount of hardware so far are more and more replaced by softwarebased solutions. Major points of concern in the near future are the standardisation of communication interfaces and protocols, as well as the implementation of autonomously acting entities performing vitally important actions like controlling protective systems
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