7,497 research outputs found
Magnesium Diboride Flexible Flat Cables for Cryogenic Electronics
Magnesium diboride (MgB2) thin films are a potential alternative to
low-temperature superconductors (LTS) due to a higher critical temperature (Tc)
of approximately 39 K. The reactive evaporation deposition technique also
affords relatively simple growth of MgB2 films on flexible substrates compared
to high-temperature superconductors (HTS). We have designed and fabricated a
cable architecture consisting of MgB2 traces on flexible yttria-stabilized
zirconia (YSZ) compatible with commercially available connectors or direct
wirebonds. Key performance metrics such as critical current density (Jc) and Tc
are measured and compared. We discuss thermal conductivity and passivation
schemes for these cables.Comment: 4 pages with 4 figures. Presented at ASC 2010 - submitted to IEEE
Transactions on Superconductivit
On fractional GJMS operators
We describe a new interpretation of the fractional GJMS operators as
generalized Dirichlet-to-Neumann operators associated to weighted GJMS
operators on naturally associated smooth metric measure spaces. This gives a
geometric interpretation of the Caffarelli--Silvestre extension for
when , and both a geometric interpretation
and a curved analogue of the higher order extension found by R. Yang for
when . We give three applications of this
correspondence. First, we exhibit some energy identities for the fractional
GJMS operators in terms of energies in the compactified Poincar\'e--Einstein
manifold, including an interpretation as a renormalized energy. Second, for
, we show that if the scalar curvature and the fractional
-curvature of the boundary are nonnegative, then the
fractional GJMS operator is nonnegative. Third, by assuming
additionally that is not identically zero, we show that
satisfies a strong maximum principle.Comment: 38 pages. Final version, to appear in Communications on Pure and
Applied Mathematic
Rates of solar angles for two-axis concentrators
The Sun's position by the azimuth and elevation angles and its rate of change at any time of day are determined to design 2 axis tracking mechanisms of solar concentrators. The Sun's angles and their rates for selected months of the year (March, June, September and December) and for seven selected atitudes (0, + or - 30, + or - 60, + or - 90) covering both the northern and southern hemispheres were studied. The development of the angle and angle rate analytical expressions for any month, hour of day, and latitude provides the solar concentrator designer with a quantitative determination of the limiting Sun's position and angle rates for an accurate automatic tracking mechanism
Confinement and Localization on Domain Walls
We continue the studies of localization of the U(1) gauge fields on domain
walls. Depending on dynamics of the bulk theory the gauge field localized on
the domain wall can be either in the Coulomb phase or squeezed into flux tubes
implying (Abelian) confinement of probe charges on the wall along the wall
surface. First, we consider a simple toy model with one flavor in the bulk at
weak coupling (a minimal model) realizing the latter scenario. We then suggest
a model presenting an extension of the Seiberg--Witten theory which is at
strong coupling, but all theoretical constructions are under full control if we
base our analysis on a dual effective action. Finally, we compare our findings
with the wall in a "nonminimal" theory with two distinct quark flavors that had
been studied previously. In this case the U(1) gauge field trapped on the wall
is exactly massless because it is the Goldstone boson of a U(1) symmetry in the
bulk spontaneously broken on the wall. The theory on the wall is in the Coulomb
phase. We explain why the mechanism of confinement discussed in the first part
of the paper does not work in this case, and strings are not formed on the
walls.Comment: 55 pp; v2: several remarks adde
Graphene Spin Transistor
Graphitic nanostructures, e.g. carbon nanotubes (CNT) and graphene, have been
proposed as ideal materials for spin conduction[1-7]; they have long electronic
mean free paths[8] and small spin-orbit coupling[9], hence are expected to have
very long spin-scattering times. In addition, spin injection and detection in
graphene opens new opportunities to study exotic electronic states such as the
quantum Hall[10,11] and quantum spin Hall[9] states, and spin-polarized edge
states[12] in graphene ribbons. Here we perform the first non-local four-probe
experiments[13] on graphene contacted by ferromagnetic Permalloy electrodes. We
observe sharp switching and often sign-reversal of the non-local resistance at
the coercive field of the electrodes, indicating definitively the presence of a
spin current between injector and detector. The non-local resistance changes
magnitude and sign quasi-periodically with back-gate voltage, and
Fabry-Perot-like oscillations[6,14,15] are observed, consistent with
quantum-coherent transport. The non-local resistance signal can be observed up
to at least T = 300 K
Use of high-resolution measurements for the retrieval of temperature and gas-concentration profiles from outgoing infrared spectra in the presence of cirrus clouds
We explore ways in which high-spectral-resolution measurements can aid in the retrieval of atmospheric temperature and gas-concentration profiles from outgoing infrared spectra when optically thin cirrus clouds are present. Simulated outgoing spectra that contain cirrus are fitted with spectra that do not contain cirrus, and the residuals are examined. For those lines with weighting functions that peak near the same altitude as the thin cirrus, unique features are observed in the residuals. These unique features are highly sensitive to the resolution of the instrumental line shape. For thin cirrus these residual features are narrow (≤0.1 cm-1), so high spectral resolution is required for unambiguous observation. The magnitudes of these unique features are larger than the noise of modern instruments. The sensitivities of these features to cloud height and cloud optical depth are also discussed. Our sensitivity studies show that, when the errors in the estimation of temperature profiles are not large, the dominant contribution to the residuals is the misinterpretation of cirrus. An analysis that focuses on information content is also presented. An understanding of the magnitude of the effect and of its dependence on spectral resolution as well as on spectral region is important for retrieving spacecraft data and for the design of future infrared instruments for forecasting weather and monitoring greenhouse gases
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