706 research outputs found
Materials and techniques for spacecraft static charge control
An overview of the design, development, fabrication, and testing of transparent conductive coatings and conductive lattices deposited or formed on high resistivity spacecraft dielectric materials to obtain control static charge buildup on spacecraft external surfaces is presented. Fabrication techniques for the deposition of indium/tin oxide coatings and copper grid networks on Kapton and FEP Teflon films and special frit coatings for OSR and solar cell cover glasses are discussed. The techniques include sputtering, photoetching, silkscreening, and mechanical processes. A facility designed and built to simulate the electron plasma at geosynchronous altitudes is described along with test procedures. The results of material characterizations as well as electron irradiation aging effects in this facility for spacecraft polymers treated to control static charge are presented. The data presents results for electron beam energies up to 30 kV and electron current densities of 30 nA/cm squared. Parameters measured include secondary emission, surface leakage, and through the sample currents as a function of primary beam energy and voltage
Spatial patterns in the evolution of Cenozoic dynamic topography and its influence on the Antarctic continent
Our knowledge of dynamic topography in Antarctica remains in an infancy stage compared to other continents. We assess the space-time variability in dynamic topography in Antarctica by analysing grids of global dynamic topography in the Cenozoic (and late Cretaceous) based on the tomographic model S40RTS. Our model reveals that the Gamburtsev Province and Dronning Maud Land, two of the major nucleation sites for the East Antarctic Ice Sheet (EAIS) were ~500 m higher 60 Ma ago. The increased elevation may have facilitated ephemeral ice cap development in the early Cenozoic. Between ca 25 and 50 Ma the northern Wilkes Subglacial Basin was ca 200 m higher than today and a major increase in regional elevation (>600 m) occurred over the last 20-15 Ma over the northern and southern Victoria Land in the Transantarctic Mountains (TAM). The most prominent signal is observed over the Ross Sea Rift (RSR) where predicted Neogene dynamic topography exceeds 1,000 m. The flow of warm mantle from the West Antarctic Rift System (WARS)may have driven these dynamic topography effects over the TAM and RSR. However, we found that these effects are comparatively less significant over the Marie Byrd Land Dome and the interior of the WARS. If these contrasting dynamic topography effects are included, then the predicted elevations of the Ross Sea Embayment ca 20 Ma ago are more similar to the interior of the WARS, with significant implications for the early development of the West Antarctic Ice Sheet
Polaron and bipolaron dispersion curves in one dimension for intermediate coupling
Bipolaron energies are calculated as a function of wave vector by a
variational method of Gurari appropriate for weak or intermediate coupling
strengths, for a model with electron-phonon interactions independent of phonon
wave vectors and a short-ranged Coulomb repulsion. It is assumed that the bare
electrons have a constant effective mass. A two-parameter trial function is
taken for the relative motion of the two electrons in the bipolaron. Energies
of bipolarons are compared with those of two single polarons as a function of
wave vector for various parameter values. Results for effective masses at the
zone center are also obtained. Comparison is made with data of other authors
for bipolarons in the Hubbard-Holstein model, which differs mainly from the
present model in that it has a tight-binding band structure for the bare
electrons.Comment: 11 pages including six figures. Physical Review B, to be publishe
Invasive Signal Crayfish in the UK: Survey Methods to Inform Evidence-based Management
With invasive crayfish becoming increasingly widespread, evidence-based management is crucial to
protect freshwater ecosystems. Knowledge of the structure and function of invasive crayfish
populations allows for an effective evaluation of management efforts. Recent methodological
developments have enabled the first truly quantitative studies of UK invasive crayfish populations in
the field. This was achieved by the ‘triple drawdown’ (TDD) survey approach. In this article, we
explore current survey approaches and their limitations, and we introduce the TDD method with its
implications for crayfish survey, policy development and management
Optical conductivity of polaronic charge carriers
The optical conductivity of charge carriers coupled to quantum phonons is
studied in the framework of the one-dimensional spinless Holstein model. For
one electron, variational diagonalisation yields exact results in the
thermodynamic limit, whereas at finite carrier density analytical
approximations based on previous work on single-particle spectral functions are
obtained. Particular emphasis is put on deviations from weak-coupling,
small-polaron or one-electron theories occurring at intermediate coupling
and/or finite carrier density. The analytical results are in surprisingly good
agreement with exact data, and exhibit the characteristic polaronic excitations
observed in experiments on manganites.Comment: 23 pages, 11 figure
Many-body large polaron optical conductivity in SrTiNbO
Recent experimental data on the optical conductivity of niobium doped
SrTiO are interpreted in terms of a gas of large polarons with effective
coupling constant . The {theoretical approach takes into
account} many-body effects, the electron-phonon interaction with multiple
LO-phonon branches, and the degeneracy and the anisotropy of the Ti t
conduction band. {Based on the Fr\"{o}hlich interaction, the many-body
large-polaron theory} provides an interpretation for the essential
characteristics, except -- interestingly -- for the unexpectedly large
intensity of a peak at meV, of the observed optical conductivity
spectra of SrTiNbO \textit{without} any adjustment of
material parameters.Comment: to appear in Phys. Rev.
Effective theory for the Goldstone field in the BCS-BEC crossover at T=0
We perform a detailed study of the effective Lagrangian for the Goldstone
mode of a superfluid Fermi gas at zero temperature in the whole BCS-BEC
crossover. By using a derivative expansion of the response functions, we derive
the most general form of this Lagrangian at the next to leading order in the
momentum expansion in terms of four coefficient functions. This involves the
elimination of all the higher order time derivatives by careful use of the
leading order field equations. In the infinite scattering length limit where
conformal invariance is realized, we show that the effective Lagrangian must
contain an unnoticed invariant combination of higher spatial gradients of the
Goldstone mode, while explicit couplings to spatial gradients of the trapping
potential are absent. Across the whole crossover, we determine all the
coefficient functions at the one-loop level, taking into account the dependence
of the gap parameter on the chemical potential in the mean-field approximation.
These results are analytically expressed in terms of elliptic integrals of the
first and second kind. We discuss the form of these coefficients in the extreme
BCS and BEC regimes and around the unitary limit, and compare with recent work
by other authors.Comment: 27 pages. 4 references added, typos corrected, expanded Section III
Intraplate strike-slip faulting in East Antarctica: new geophysical views from the Rennick Graben and Wilkes Subglacial Basin
Intraplate strike-slip faulting can occur in association with different geodynamic settings, ranging from subduction-related to collision and extension. Geological and geophysical research in Northern Victoria Land (NVL) in East Antarctica, has led to the interpretation that major fault systems that were active during the early Paleozoic Ross Orogen were reactivated much later as right-lateral intraplate strike-slip fault systems from ca 48 Ma, and that these faults may have accomodated differential shear along evolving oceanic transform faults located between southeastern Australia and Tasmania. One of the main structures in NVL that has been inferred to relate to this unusual geodynamic process is the Rennick Graben (RG), but its age, extent and kinematics have remained both poorly constrained and controversial. Even less well-understood are the potential tectonic linkages between the RG and the deep sub-basins that lie within the much broader Wilkes Subglacial Basin (WSB), in the hinterland of the Transantarctic Mountains.
Here, we present new interpretations of enhanced potential field images derived from aeromagnetic and airborne and land-gravity observations to help constrain the extent and architecture of the RG and the sub-basins within the WSB. We show that the RG is a composite pull-part basin that extends from the Oates Coast towards the margin of the Ross Sea Rift, part of the West Antarctic Rift System. We suggest that the more cratonic WSB region was also affected by extensional and transtensional processes, the latter potentially linked to an evolving and distributed left-lateral Paleogene(?) strike-slip plate boundary between East Antarctica and Australia
Camera calibration in sport event scenarios
The main goal of this paper is the design of a novel and robust methodology for calibrating cameras from a single image in sport scenarios, such as a soccer field, or a basketball or tennis court. In these sport scenarios, the only references we use to calibrate the camera are the lines and circles delimiting the different regions. The first problem we address is the extraction of image primitives, including the challenging problems of shaded regions and lens distortion. From these primitives, we automatically recognise the location of the sport court in the scene by estimating the homography which matches the
actual court with its projection onto the image. This is achieved even when only a few primitives are available. Finally, from this homography, we recover the camera calibration parameters. In particular, we estimate the focal length as well as the position and orientation in the 3D space. We present some experiments on models and real courts which illustrate the accuracy of the proposed methodology
Mobile small polaron
Extending the Froehlich polaron problem to a discrete ionic lattice we study
a polaronic state with a small radius of the wave function but a large size of
the lattice distortion. We calculate the energy dispersion and the effective
mass of the polaron with the 1/\lambda perturbation theory and with the exact
Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The
``small'' Froehlich polaron is found to be lighter than the small Holstein
polaron by one or more orders of magnitude.Comment: 4 pages, 4 figures, published versio
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