25,631 research outputs found
Satellite altimeters after Skylab and GEOS-C - should they utilize a single transmitter or an array of pulsed amplifiers?
An attempt was made to determine if future satellite altimeters, operating at K-band, should use a single transmitter or an array of pulsed amplifiers. The two systems were compared as a function of reliability, power, service life, system requirements, and weight. Preliminary results indicate the modular system to be superior in service life and reliability although it requires more power and is substantially heavier than the single system. No concrete conclusions were reached as to which system should be used
Laser-assisted electron-argon scattering at small angles
Electron-argon scattering in the presence of a linearly polarized, low
frequency laser field is studied theoretically. The scattering geometries of
interest are small angles where momentum transfer is nearly perpendicular to
the field, which is where the Kroll-Watson approximation has the potential to
break down. The Floquet R matrix method solves the velocity gauge Schr\"odinger
equation, using a larger reaction volume than previous treatments in order to
carefully assess the importance of the long range polarization potential to the
cross section. A comparison of the cross sections calculated with the target
potential fully included inside 20 and 100 a.u. shows no appreciable
differences, which demonstrates that the long range interaction can not account
for the high cross sections measured in experiments.Comment: 2 figure
Near threshold rotational excitation of molecular ions by electron-impact
New cross sections for the rotational excitation of H by electrons are
calculated {\it ab initio} at low impact energies. The validity of the
adiabatic-nuclei-rotation (ANR) approximation, combined with -matrix
wavefunctions, is assessed by comparison with rovibrational quantum defect
theory calculations based on the treatment of Kokoouline and Greene ({\it Phys.
Rev. A} {\bf 68} 012703 2003). Pure ANR excitation cross sections are shown to
be accurate down to threshold, except in the presence of large oscillating
Rydberg resonances. These resonances occur for transitions with
and are caused by closed channel effects. A simple analytic formula is derived
for averaging the rotational probabilities over such resonances in a 3-channel
problem. In accord with the Wigner law for an attractive Coulomb field,
rotational excitation cross sections are shown to be large and finite at
threshold, with a significant but moderate contribution from closed channels.Comment: 3 figures, a5 page
Renner-Teller effects in HCO+ dissociative recombination
A theoretical description of the dissociative recombination process for the
HCO+ ion suggests that the nonadiabatic Renner-Teller coupling between
electronic and vibrational degrees of freedom plays an important role. This
finding is consistent with a recent study of this process for another
closed-shell molecule, the H3+ ion, where Jahn-Teller coupling was shown to
generate a relatively high rate. The cross section obtained here for the
dissociative recombination of HCO+exhibits encouraging agreement with a
merged-beam experiment.Comment: 11 page
Evidence for Antiferromagnetic Order in LaCeCuO from Angular Magnetoresistance Measurements
We investigated the in-plane angular magnetoresistivity (AMR) of -phase LaCeCuO (LCCO) thin films () fabricated by a pulsed laser deposition technique. The in-plane
AMR with shows a twofold symmetry instead of the
fourfold behavior found in other electron-doped cuprates such as PrCeCuO and NdCeCuO. The twofold AMR
disappears above a certain temperature, . The is well above
for ( K), and decreases with increasing doping,
until it is no longer observed above at . This twofold AMR
below is suggested to originate from an antiferromagnetic or spin
density wave order.Comment: to be published in Phys. Rev. B, Vol. 80 (2009
Andreev Reflection in Heavy-Fermion Superconductors and Order Parameter Symmetry in CeCoIn_5
Differential conductance spectra are obtained from nanoscale junctions on the
heavy-fermion superconductor CeCoIn along three major crystallographic
orientations. Consistency and reproducibility of characteristic features among
the junctions ensure their spectroscopic nature. All junctions show a similar
conductance asymmetry and Andreev reflection-like conductance with reduced
signal (~ 10%-13%), both commonly observed in heavy-fermion superconductor
junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates
that our data provide the first spectroscopic evidence for
symmetry. To quantify our conductance spectra, we propose a model by
considering the general phenomenology in heavy fermions, the two-fluid
behavior, and an energy-dependent density of states. Our model fits to the
experimental data remarkably well and should invigorate further investigations.Comment: 4 pages, 4 figures; Phys. Rev. Lett., published versio
Nonclassical paths in the recurrence spectrum of diamagnetic atoms
Using time-independent scattering matrices, we study how the effects of
nonclassical paths on the recurrence spectra of diamagnetic atoms can be
extracted from purely quantal calculations. This study reveals an intimate
relationship between two types of nonclassical paths: exotic ghost orbits and
diffractive orbits. This relationship proves to be a previously unrecognized
reason for the success of semiclassical theories, like closed-orbit theory, and
permits a comprehensive reformulation of the semiclassical theory that
elucidates its convergence properties.Comment: 5 pages, 4 figure
A Spatial Quantile Regression Hedonic Model of Agricultural Land Prices
Abstract Land price studies typically employ hedonic analysis to identify the impact of land characteristics on price. Owing to the spatial fixity of land, however, the question of possible spatial dependence in agricultural land prices arises. The presence of spatial dependence in agricultural land prices can have serious consequences for the hedonic model analysis. Ignoring spatial autocorrelation can lead to biased estimates in land price hedonic models. We propose using a flexible quantile regression-based estimation of the spatial lag hedonic model allowing for varying effects of the characteristics and, more importantly, varying degrees of spatial autocorrelation. In applying this approach to a sample of agricultural land sales in Northern Ireland we find that the market effectively consists of two relatively separate segments. The larger of these two segments conforms to the conventional hedonic model with no spatial lag dependence, while the smaller, much thinner market segment exhibits considerable spatial lag dependence. Un mod�le h�donique � r�gression quantile spatiale des prix des terrains agricoles R�sum� Les �tudes sur le prix des terrains font g�n�ralement usage d'une analyse h�donique pour identifier l'impact des caract�ristiques des terrains sur le prix. Toutefois, du fait de la fixit� spatiale des terrains, la question d'une �ventuelle d�pendance spatiale sur la valeur des terrains agricoles se pose. L'existence d'une d�pendance spatiale dans le prix des terrains agricoles peut avoir des cons�quences importantes sur l'analyse du mod�le h�donique. En ignorant cette corr�lation s�rielle, on s'expose au risque d'�valuations biais�es des mod�les h�doniques du prix des terrains. Nous proposons l'emploi d'une estimation � base de r�gression flexible du mod�le h�donique � d�calage spatial, tenant compte de diff�rents effets des caract�ristiques, et surtout de diff�rents degr�s de corr�lations s�rielles spatiales. En appliquant ce principe � un �chantillon de ventes de terrains agricoles en Irlande du Nord, nous d�couvrons que le march� se compose de deux segments relativement distincts. Le plus important de ces deux segments est conforme au mod�le h�donique traditionnel, sans d�pendance du d�calage spatial, tandis que le deuxi�me segment du march�, plus petit et beaucoup plus �troit, pr�sente une d�pendance consid�rable du d�calage spatial. Un modelo hed�nico de regresi�n cuantil espacial de los precios del terreno agr�cola Resumen T�picamente, los estudios del precio de la tierra emplean un an�lisis hed�nico para identificar el impacto de las caracter�sticas de la tierra sobre el precio. No obstante, debido a la fijeza espacial de la tierra, surge la cuesti�n de una posible dependencia espacial en los precios del terreno agr�cola. La presencia de dependencia espacial en los precios del terreno agr�cola puede tener consecuencias graves para el modelo de an�lisis hed�nico. Ignorar la autocorrelaci�n espacial puede conducir a estimados parciales en los modelos hed�nicos del precio de la tierra. Proponemos el uso de una valoraci�n basada en una regresi�n cuantil flexible del modelo hed�nico del lapso espacial que tenga en cuenta los diversos efectos de las caracter�sticas y, particularmente, los diversos grados de autocorrelaci�n espacial. Al aplicar este planteamiento a una muestra de ventas de terreno agr�cola en Irlanda del Norte, descubrimos que el mercado consiste efectivamente de dos segmento relativamente separados. El m�s grande de estos dos segmentos se ajusta al modelo hed�nico convencional sin dependencia del lapso espacial, mientras que el segmento m�s peque�o, y mucho m�s fino, muestra una dependencia considerable del lapso espacial.Spatial lag, quantile regression, hedonic model, C13, C14, C21, Q24,
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