2,467 research outputs found
Space Station RT and E Utilization Study
Descriptive information on a set of 241 mission concepts was reviewed to establish preliminary Space Station outfitting needs for technology development missions. The missions studied covered the full range of in-space technology development activities envisioned for early Space Station operations and included both pressurized volume and attached payload requirements. Equipment needs were compared with outfitting plans for the life sciences and microgravity user communities, and a number of potential outfitting additions were identified. Outfitting implementation was addressed by selecting a strawman mission complement for each of seven technical themes, by organizing the missions into flight scenarios, and by assessing the associated outfitting buildup for planning impacts
Investigation of direct solar-to-microwave energy conversion techniques
Identification of alternative methods of producing microwave energy from solar radiation for purposes of directing power to the Earth from space is investigated. Specifically, methods of conversion of optical radiation into microwave radiation by the most direct means are investigated. Approaches based on demonstrated device functioning and basic phenomenologies are developed. There is no system concept developed, that is competitive with current baseline concepts. The most direct methods of conversion appear to require an initial step of production of coherent laser radiation. Other methods generally require production of electron streams for use in solid-state or cavity-oscillator systems. Further development is suggested to be worthwhile for suggested devices and on concepts utilizing a free-electron stream for the intraspace station power transport mechanism
Dynamical polarization, screening, and plasmons in gapped graphene
The one-loop polarization function of graphene has been calculated at zero
temperature for arbitrary wavevector, frequency, chemical potential (doping),
and band gap. The result is expressed in terms of elementary functions and is
used to find the dispersion of the plasmon mode and the static screening within
the random phase approximation. At long wavelengths the usual square root
behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The
presence of a small (compared to a chemical potential) gap leads to the
appearance of a new undamped plasmon mode. At greater values of the gap this
mode merges with the long-wavelength one, and vanishes when the Fermi level
enters the gap. The screening of charged impurities at large distances differs
from that in gapless graphene by slower decay of Friedel oscillations (
instead of ), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio
Coulomb Screening of 2D Massive Dirac Fermions
A model of 2D massive Dirac fermions, interacting with a instantaneous
Coulomb interaction, is presented to mimic the physics of gapped graphene. The
static polarization function is calculated explicitly to analyze screening
effect at the finite temperature and density. Results are compared with the
massless case . We also show that various other works can be reproduced within
our model in a straightforward and unified manner
Nonlinear electromagnetic response of graphene: Frequency multiplication and the self-consistent-field effects
Graphene is a recently discovered carbon based material with unique physical
properties. This is a monolayer of graphite, and the two-dimensional electrons
and holes in it are described by the effective Dirac equation with a vanishing
effective mass. As a consequence, electromagnetic response of graphene is
predicted to be strongly non-linear. We develop a quasi-classical kinetic
theory of the non-linear electromagnetic response of graphene, taking into
account the self-consistent-field effects. Response of the system to both
harmonic and pulse excitation is considered. The frequency multiplication
effect, resulting from the non-linearity of the electromagnetic response, is
studied under realistic experimental conditions. The frequency up-conversion
efficiency is analysed as a function of the applied electric field and
parameters of the samples. Possible applications of graphene in terahertz
electronics are discussed.Comment: 14 pages, 7 figures, invited paper written for a special issue of
JPCM "Terahertz emitters
Nagaoka ferromagnetism observed in a quantum dot plaquette
Engineered, highly-controllable quantum systems hold promise as simulators of
emergent physics beyond the capabilities of classical computers. An important
problem in many-body physics is itinerant magnetism, which originates purely
from long-range interactions of free electrons and whose existence in real
systems has been subject to debate for decades. Here we use a quantum simulator
consisting of a four-site square plaquette of quantum dots to demonstrate
Nagaoka ferromagnetism. This form of itinerant magnetism has been rigorously
studied theoretically but has remained unattainable in experiment. We load the
plaquette with three electrons and demonstrate the predicted emergence of
spontaneous ferromagnetic correlations through pairwise measurements of spin.
We find the ferromagnetic ground state is remarkably robust to engineered
disorder in the on-site potentials and can induce a transition to the low-spin
state by changing the plaquette topology to an open chain. This demonstration
of Nagaoka ferromagnetism highlights that quantum simulators can be used to
study physical phenomena that have not yet been observed in any system before.
The work also constitutes an important step towards large-scale quantum dot
simulators of correlated electron systems.Comment: This version: main (8 pages, 6 figures) + supplementary (15 pages, 8
figures
The effect of sublattice symmetry breaking on the electronic properties of a doped graphene
Motivated by a number of recent experimental studies, we have carried out the
microscopic calculation of the quasiparticle self-energy and spectral function
in a doped graphene when a symmetry breaking of the sublattices is occurred.
Our systematic study is based on the many-body GW approach that is
established on the random phase approximation and on graphene's massive Dirac
equation continuum model. We report extensive calculations of both the real and
imaginary parts of the quasiparticle self-energy in the presence of a gap
opening. We also present results for spectral function, renormalized Fermi
velocity and band gap renormalization of massive Dirac Fermions over a broad
range of electron densities. We further show that the mass generating in
graphene washes out the plasmaron peak in spectral weight.Comment: 22 Pages, 10 Figure
The Oceanic Variability Spectrum and Transport Trends
Oceanic meridional transports evaluated over the width of the Pacific Ocean from altimetric observations become incoherent surprisingly rapidly with meridional separation. Even
with 15 years of data, surface slopes show no significant coherence beyond 5◦ of latitude separation at any frequency. An analysis of the frequency/zonal-wavenumber spectral density
shows a broad continuum of motions at all time and space scales, with a significant excess of energy along a “non-dispersive” line extending between the simple barotropic and first baroclinic mode Rossby waves. It is speculated that much of that excess energy lies with coupled barotropic and first mode Rossby waves. The statistical significance of apparent oceanic transport trends depends upon the existence of a reliable frequency/wavenumber spectrum and for which only a few observational elements now exist.Jet Propulsion Laboratory (U.S.).United States. National Aeronautics and Space Administration (Jason-1 program)National Oceanographic Partnership Program (U.S.
A Simple Passive Scalar Advection-Diffusion Model
This paper presents a simple, one-dimensional model of a randomly advected
passive scalar. The model exhibits anomalous inertial range scaling for the
structure functions constructed from scalar differences. The model provides a
simple computational test for recent ideas regarding closure and scaling for
randomly advected passive scalars. Results suggest that high order structure
function scaling depends on the largest velocity eddy size, and hence scaling
exponents may be geometry-dependent and non-universal.Comment: 30 pages, 11 figure
Internal-tide driven tracer transport across the continental slope
The role of the internal tide in driving tracer transport across the continental slope is examined using simplified layered theory, channel model experiments, and observational diagnostics of near shelf-edge moorings. The effect of the internal tide is interpreted in terms of its Stokes' drift, which is separated into two distinct components: a bolus component, driven by the covariance of layer thickness and the velocity, and a shear component, driven by the velocity following the movement of an interface. For a three-layer ocean, in the model experiments and observations, the onshore propagation of an internal tide drives a Stokes' transport directed onshore in the surface and the bottom layers and directed offshore in the pycnocline. This reversing structure is due to the bolus component dominating near the boundaries, while the shear component dominates at the pycnocline. In the observational diagnostics, the Stokes' transport is not canceled by the Eulerian transport, which is mainly directed along bathymetric contours. The Stokes' drift of the internal tide then provides a systematic on shelf tracer transport if there is a tracer sink on the shelf, carried in the surface or bottom layers. Conversely, the tracer transport is directed offshore if there is a tracer source on the shelf with plumes of shelf tracer expected to be carried offshore along the pycnocline. This tracer transport as a result of the internal tide is diagnosed for heat, salt, and nitrate. The depth-integrated nitrate flux is directed onto the shelf supplying nutrients to the productive shelf seas
- …