1,622 research outputs found
Deep far infrared ISOPHOT survey in "Selected Area 57", I. Observations and source counts
We present here the results of a deep survey in a 0.4 sq.deg. blank field in
Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared
Space Observatory (ISO) at both 60 um and 90 um. The resulting sky maps have a
spatial resolution of 15 x 23 sq.arcsec. per pixel which is much higher than
the 90 x 90 sq.arcsec. pixels of the IRAS All Sky Survey. We describe the main
instrumental effects encountered in our data, outline our data reduction and
analysis scheme and present astrometry and photometry of the detected point
sources. With a formal signal to noise ratio of 6.75 we have source detection
limits of 90 mJy at 60 um and 50 mJy at 90 um. To these limits we find
cumulated number densities of 5+-3.5 per sq.deg. at 60 um and 14.8+-5.0 per
sq.deg.at 90 um. These number densities of sources are found to be lower than
previously reported results from ISO but the data do not allow us to
discriminate between no-evolution scenarios and various evolutionary models.Comment: 15 pages, 11 figures, accepted by Astronomy & Astrophysic
Noncyclic covers of knot complements
Hempel has shown that the fundamental groups of knot complements are
residually finite. This implies that every nontrivial knot must have a
finite-sheeted, noncyclic cover. We give an explicit bound, , such
that if is a nontrivial knot in the three-sphere with a diagram with
crossings and a particularly simple JSJ decomposition then the complement of
has a finite-sheeted, noncyclic cover with at most sheets.Comment: 29 pages, 8 figures, from Ph.D. thesis at Columbia University;
Acknowledgments added; Content correcte
Andreev Reflection and Spin Injection into and wave Superconductors
We study the effect of spin injection into and wave superconductors,
with an emphasis on the interplay between boundary and bulk spin transport
properties. The quantities of interest include the amount of non-equilibrium
magnetization (), as well as the induced spin-dependent current () and
boundary voltage (). In general, the Andreev reflection makes each of the
three quantities depend on a different combination of the boundary and bulk
contributions. The situation simplifies either for half-metallic ferromagnets
or in the strong barrier limit, where both and depend solely on the
bulk spin transport/relaxation properties. The implications of our results for
the on-going spin injection experiments in high cuprates are discussed.Comment: 4 pages, REVTEX, 1 figure included; typos correcte
Directed self-organization of graphene nanoribbons on SiC
Realization of post-CMOS graphene electronics requires production of
semiconducting graphene, which has been a labor-intensive process. We present
tailoring of silicon carbide crystals via conventional photolithography and
microelectronics processing to enable templated graphene growth on
4H-SiC{1-10n} (n = 8) crystal facets rather than the customary {0001} planes.
This allows self-organized growth of graphene nanoribbons with dimensions
defined by those of the facet. Preferential growth is confirmed by Raman
spectroscopy and high-resolution transmission electron microscopy (HRTEM)
measurements, and electrical characterization of prototypic graphene devices is
presented. Fabrication of > 10,000 top-gated graphene transistors on a 0.24 cm2
SiC chip demonstrates scalability of this process and represents the highest
density of graphene devices reported to date.Comment: 13 pages, 5 figure
The Optical Design and Characterization of the Microwave Anisotropy Probe
The primary goal of the MAP satellite, now in orbit, is to make high fidelity
polarization sensitive maps of the full sky in five frequency bands between 20
and 100 GHz. From these maps we will characterize the properties of the cosmic
microwave background (CMB) anisotropy and Galactic and extragalactic emission
on angular scales ranging from the effective beam size, <0.23 degree, to the
full sky. MAP is a differential microwave radiometer. Two back-to-back shaped
offset Gregorian telescopes feed two mirror symmetric arrays of ten corrugated
feeds. We describe the prelaunch design and characterization of the optical
system, compare the optical models to the measurements, and consider multiple
possible sources of systematic error.Comment: ApJ in press; 22 pages with 11 low resolution figures; paper is
available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
A New Precision Measurement of the 7Be(p,gamma)8B Cross section with an Implanted 7Be Target
The 7Be(p,gamma)8B reaction plays a central role in the evaluation of solar
neutrino fluxes. We report on a new precision measurement of the cross section
of this reaction, following our previous experiment with an implanted 7Be
target, a raster scanned beam and the elimination of the backscattering loss.
The new measurement incorporates a more abundant 7Be target and a number of
improvements in design and procedure. The point at Elab=991 keV was measured
several times under varying conditions, yielding a value of S17(Ec.m. =850 keV)
= 24.0(5) eV b. Measurements were carried out at lower energies as well. Due to
the precise knowledge of the implanted 7Be density profile it was possible to
reconstitute both the off- and on resonance parts of the cross section and to
obtain from the entire set of measurements an extrapolated value of
S17(0)=21.2(7) eV b.Comment: 4 Pages, 3 Figure
First-Principles Calculations of Hyperfine Interactions in La_2CuO_4
We present the results of first-principles cluster calculations of the
electronic structure of La_2CuO_4. Several clusters containing up to nine
copper atoms embedded in a background potential were investigated.
Spin-polarized calculations were performed both at the Hartree-Fock level and
with density functional methods with generalized gradient corrections to the
local density approximation. The distinct results for the electronic structure
obtained with these two methods are discussed. The dependence of the
electric-field gradients at the Cu and the O sites on the cluster size is
studied and the results are compared to experiments. The magnetic hyperfine
coupling parameters are carefully examined. Special attention is given to a
quantitative determination of on-site and transferred hyperfine fields. We
provide a detailed analysis that compares the hyperfine fields obtained for
various cluster sizes with results from additional calculations of spin states
with different multiplicities. From this we conclude that hyperfine couplings
are mainly transferred from nearest neighbor Cu^{2+} ions and that
contributions from further distant neighbors are marginal. The mechanisms
giving rise to transfer of spin density are worked out. Assuming conventional
values for the spin-orbit coupling, the total calculated hyperfine interaction
parameters are compared to informations from experiments.Comment: 23 pages, 9 figure
Experimental evidence of strong phonon scattering in isotopical disordered systems: The case of LiH_xD_{1-x} crystals
The observation of the local - mode vibration, the two - mode behavior of the
LO phonons at large isotope concentration, as well as large line broadening in
LIH - D mixed crystals directly evidence strong additional phonon scattering
due to the isotope - induced disorder.Comment: 9 pages, 4 figure
Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2
Raman spectra were measured for mono-, bi- and trilayer graphene grown on SiC
by solid state graphitization, whereby the number of layers was pre-assigned by
angle-resolved ultraviolet photoemission spectroscopy. It was found that the
only unambiguous fingerprint in Raman spectroscopy to identify the number of
layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The
Raman spectra of epitaxial graphene show significant differences as compared to
micromechanically cleaved graphene obtained from highly oriented pyrolytic
graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not
exhibit any obvious shoulder structures but it is much broader and almost
resembles a single-peak even for multilayers. Flakes of epitaxial graphene were
transferred from SiC onto SiO2 for further Raman studies. A comparison of the
Raman data obtained for graphene on SiC with data for epitaxial graphene
transferred to SiO2 reveals that the G peak blue-shift is clearly due to the
SiC substrate. The broadened 2D peak however stems from the graphene structure
itself and not from the substrate.Comment: 27 pages, 8 figure
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