11,324 research outputs found
Brayton-cycle radioisotope heat-source design study. Phase II /preliminary design/ report
Brayton cycle radioisotope heat source desig
Supporting the active learning of collaborative database browsing techniques
We describe the implications of a study of database browsing behaviour for the development of a system to support more effective browsing. In particular we consider the importance of collaborative working, both in learning browsing skills and in co‐operating on a shared information‐retrieval task. From our study, we believe that an interface to support collaboration should promote the awareness of the activities of others, better visualization of the information data structures being browsed, and effective communication of the browsing process
Origin of electron cyclotron maser-induced radio emissions at ultra-cool dwarfs: magnetosphere-ionosphere coupling currents
A number of ultra-cool dwarfs emit circularly polarised radio waves generated
by the electron cyclotron maser instability. In the solar system such radio is
emitted from regions of strong auroral magnetic field-aligned currents. We thus
apply ideas developed for Jupiter's magnetosphere, being a well-studied
rotationally-dominated analogue in our solar system, to the case of
fast-rotating UCDs. We explain the properties of the radio emission from UCDs
by showing that it would arise from the electric currents resulting from an
angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by
an extremely powerful analogue of the process which causes Jupiter's auroras.
Such a velocity gradient indicates that these bodies interact significantly
with their space environment, resulting in intense auroral emissions. These
results strongly suggest that auroras occur on bodies outside our solar system.Comment: Accepted for publication in the Astrophysical Journa
Electron-Phonon Interactions in C-derived Molecular Solids
We present {\it ab initio} density-functional calculations of molecular
solids formed from C-derived closed-shell fullerenes. Solid
CH is found to bind weakly and exhibits many of the electronic
structure features of solid C with an enhanced electron-phonon
interaction potential. We show that chemical doping of this structure is
feasible, albeit more restrictive than its C counterpart, with an
estimated superconducting transition temperature exceeding those of the
alkali-doped C solids.Comment: Lower quality postscript file for Figure 1 is used in the manuscript
in order to meet submission quota for pre-print server. Higher quality
postscript file available from author: [email protected] This article has
been updated to reflect changes incorporated during the peer review process.
It is published in PRB 70, 140504(R) 200
Performance of differenced range data types in Voyager navigation
Voyager radio navigation made use of a differenced rage data type for both Saturn encounters because of the low declination singularity of Doppler data. Nearly simultaneous two-way range from two-station baselines was explicitly differenced to produce this data type. Concurrently, a differential VLBI data type (DDOR), utilizing doubly differenced quasar-spacecraft delays, with potentially higher precision was demonstrated. Performance of these data types is investigated on the Jupiter-to-Saturn leg of Voyager 2. The statistics of performance are presented in terms of actual data noise comparisons and sample orbit estimates. Use of DDOR as a primary data type for navigation to Uranus is discussed
Statistical limitations in functional neuroimaging I: Non-inferential methods and statistical models
Magnetic Field Effect in a Two-dimensional Array of Short Josephson Junctions
We study analytically the effect of a constant magnetic field on the dynamics
of a two dimensional Josephson array. The magnetic field induces spatially
dependent states and coupling between rows, even in the absence of an external
load. Numerical simulations support these conclusions
Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
Spin Transport in a Mott Insulator of Ultracold Fermions
Strongly correlated materials are expected to feature unconventional
transport properties, such that charge, spin, and heat conduction are
potentially independent probes of the dynamics. In contrast to charge
transport, the measurement of spin transport in such materials is highly
challenging. We observed spin conduction and diffusion in a system of ultracold
fermionic atoms that realizes the half-filled Fermi-Hubbard model. For strong
interactions, spin diffusion is driven by super-exchange and
doublon-hole-assisted tunneling, and strongly violates the quantum limit of
charge diffusion. The technique developed in this work can be extended to
finite doping, which can shed light on the complex interplay between spin and
charge in the Hubbard model.Comment: 16 pages, 10 figure
Brayton-cycle radioisotope heat source design study. Phase I - /Conceptual design/ report
Conceptual designs for radioisotope heat source systems to provide 25 kW thermal power to Brayton cycle power conversion system for space application
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