2,354 research outputs found
SETI science working group report
This report covers the initial activities and deliberations of a continuing working group asked to assist the SETI Program Office at NASA. Seven chapters present the group's consensus on objectives, strategies, and plans for instrumental R&D and for a microwave search for extraterrestrial in intelligence (SETI) projected for the end of this decade. Thirteen appendixes reflect the views of their individual authors. Included are discussions of the 8-million-channel spectrum analyzer architecture and the proof-of-concept device under development; signal detection, recognition, and identification on-line in the presence of noise and radio interference; the 1-10 GHz sky survey and the 1-3 GHz targeted search envisaged; and the mutual interests of SETI and radio astronomy. The report ends with a selective, annotated SETI reading list of pro and contra SETI publications
Shear Modulus of an Elastic Solid under External Pressure as a function of Temperature: The case of Helium
The energy of a dislocation loop in a continuum elastic solid under pressure
is considered within the framework of classical mechanics. For a circular loop,
this is a function with a maximum at pressures that are well within reach of
experimental conditions for solid helium suggesting, in this case, that
dislocation loops can be generated by a pressure-assisted thermally activated
process. It is also pointed out that pinned dislocations segments can alter the
shear response of solid helium, by an amount consistent with current
measurements, without any unpinning.Comment: 5 pages, 3 figure
Query processing of spatial objects: Complexity versus Redundancy
The management of complex spatial objects in applications, such as geography and cartography,
imposes stringent new requirements on spatial database systems, in particular on efficient
query processing. As shown before, the performance of spatial query processing can be improved
by decomposing complex spatial objects into simple components. Up to now, only decomposition
techniques generating a linear number of very simple components, e.g. triangles or trapezoids, have
been considered. In this paper, we will investigate the natural trade-off between the complexity of
the components and the redundancy, i.e. the number of components, with respect to its effect on
efficient query processing. In particular, we present two new decomposition methods generating
a better balance between the complexity and the number of components than previously known
techniques. We compare these new decomposition methods to the traditional undecomposed representation
as well as to the well-known decomposition into convex polygons with respect to their
performance in spatial query processing. This comparison points out that for a wide range of query
selectivity the new decomposition techniques clearly outperform both the undecomposed representation
and the convex decomposition method. More important than the absolute gain in performance
by a factor of up to an order of magnitude is the robust performance of our new decomposition
techniques over the whole range of query selectivity
Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films
We establish the dominant effect of anisotropic weak localization (WL) in
three dimensions associated with a propagative Fermi surface, on the
conductivity correction in heavily nitrogen doped ultrananocrystalline diamond
(UNCD) films based on magneto-resistance studies at low temperatures. Also, low
temperature electrical conductivity can show weakly localized transport in 3D
combined with the effect of electron-electron interactions in these materials,
which is remarkably different from the conductivity in 2DWL or strong
localization regime. The corresponding dephasing time of electronic
wavefunctions in these systems described as ~ T^-p with p < 1, follows a
relatively weak temperature dependence compared to the generally expected
nature for bulk dirty metals having . The temperature dependence of
Hall (electron) mobility together with an enhanced electron density has been
used to interpret the unusual magneto-transport features and show delocalized
electronic transport in these n-type UNCD films, which can be described as
low-dimensional superlattice structures.Comment: 27 pages, 6 figures, To be published in Physical Review
Ascending pharyngeal artery collateral circulation simulating internal carotid artery hypoplasia
Complete occlusion of the cervical segment of the internal carotid artery may result in a collateral circuit between an enlarged ascending pharyngeal artery and the intracranial segment of the internal carotid artery. This anastomosis may simulate a severely stenotic or hypoplastic internal carotid artery. Differentiation between these entities is particularly important if carotid endarterectomy for relief of stenosis is contemplated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46658/1/234_2004_Article_BF00327693.pd
Implementation of the Backlund transformations for the Ablowitz-Ladik hierarchy
The derivation of the Backlund transformations (BTs) is a standard problem of
the theory of the integrable systems. Here, I discuss the equations describing
the BTs for the Ablowitz-Ladik hierarchy (ALH), which have been already
obtained by several authors. The main aim of this work is to solve these
equations. This can be done in the framework of the so-called functional
representation of the ALH, when an infinite number of the evolutionary
equations are replaced, using the Miwa's shifts, with a few equations linking
tau-functions with different arguments. It is shown that starting from these
equations it is possible to obtain explicit solutions of the BT equations. In
other words, the main result of this work is a presentation of the discrete BTs
as a superposition of an infinite number of evolutionary flows of the
hierarchy. These results are used to derive the superposition formulae for the
BTs as well as pure soliton solutions.Comment: 20 page
Electron localization by a magnetic vortex
We study the problem of an electron in two dimensions in the presence of a
magnetic vortex with a step-like profile. Dependending on the values of the
effective mass and gyromagnetic factor of the electron, it may be trapped by
the vortex. The bound state spectrum is obtained numerically, and some limiting
cases are treated analytically.Comment: 8 pages, latex, 4 figure
Statistical Mechanics of Kinks in (1+1)-Dimensions: Numerical Simulations and Double Gaussian Approximation
We investigate the thermal equilibrium properties of kinks in a classical
\F^4 field theory in dimensions. From large scale Langevin simulations
we identify the temperature below which a dilute gas description of kinks is
valid. The standard dilute gas/WKB description is shown to be remarkably
accurate below this temperature. At higher, ``intermediate'' temperatures,
where kinks still exist, this description breaks down. By introducing a double
Gaussian variational ansatz for the eigenfunctions of the statistical transfer
operator for the system, we are able to study this region analytically. In
particular, our predictions for the number of kinks and the correlation length
are in agreement with the simulations. The double Gaussian prediction for the
characteristic temperature at which the kink description ultimately breaks down
is also in accord with the simulations. We also analytically calculate the
internal energy and demonstrate that the peak in the specific heat near the
kink characteristic temperature is indeed due to kinks. In the neighborhood of
this temperature there appears to be an intricate energy sharing mechanism
operating between nonlinear phonons and kinks.Comment: 28 pages (8 Figures not included, hard-copies available), Latex,
LA-UR-93-276
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