153 research outputs found
Nonlinear Response of HTSC Thin Film Microwave Resonators in an Applied DC Magnetic Field
The non-linear microwave surface impedance of patterned YBCO thin films, was
measured using a suspended line resonator in the presence of a perpendicular DC
magnetic field of magnitude comparable to that of the microwave field.
Signature of the virgin state was found to be absent even for relatively low
microwave power levels. The microwave loss was initially found to decrease for
small applied DC field before increasing again. Also, non-linearities inherent
in the sample were found to be substantially suppressed at low powers at these
applied fields. These two features together can lead to significant improvement
in device performance.Comment: 4 pages, LaTeX type, Uses IEEE style files, 600 dpi PostScript file
with color figures available at http://sagar.physics.neu.edu/preprints.html
Submitted to IEEE Transactions on Applied Superconductivit
Comments on "Vortex Glass and Lattice Melting Transitions in a YNi_2B_2C Single Crystal"
Recently, Mun et.al. (Phys. Rev. Lett., 76, 2790 (1996)) have published their
results on single crystal YNi_2B_2C, claiming that their experimental
observations can be explained in terms of formation of Vortex Glass and Lattice
melting. Our experiments, carried out on samples obtained from the SAME source,
reveal a much richer phase diagram and span wider regions of experimental
parameter space than Mun et. al. that encompasses most of their observations.
We speculate that this material has anomalous intrinsic properties and the
results cannot be explained by simple models about the flux lattice.Comment: 1 page, LaTeX type, 1 PostScript figure, Uses PRABIB.STY file, 600
dpi PS file available at http://sagar.physics.neu.edu/preprints.html To
appear in Physical Review Letter
Composition of a new chemotype of Senecio chrysanthemoides DC.
1,10β-epoxy-6-oxo-furanoeremophilane 1 (21.8%) along with furanoligularenone 2, germacrene D 3, selin-11-en-4α-ol 4 and caryophyllene oxide 5 have been identified in Senecio chrysanthemoides DC. of the Garhwal region of Himalaya at an altitude of 3600 m. 1,10β-epoxy-6-oxo-furanoeremophilane 1 has not been reported in Senecio chrysanthemoides DC. but reported in the other species of the genus Senecio. Presence of compounds 1-5 and absence of previously reported β-thujone and 6-hydroxy-p-menth-4(5)-en-3-one makes it a new chemotype within the genus Senecio.
The Peculiar Type Ic Supernova 1997ef: Another Hypernova
SN 1997ef has been recognized as a peculiar supernova from its light curve
and spectral properties. The object was classified as a Type Ic supernova (SN
Ic) because its spectra are dominated by broad absorption lines of oxygen and
iron, lacking any clear signs of hydrogen or helium line features. The light
curve is very different from that of previously known SNe Ic, showing a very
broad peak and a slow tail. The strikingly broad line features in the spectra
of SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest the
interesting possibility that SN 1997ef may also be a hypernova. The light curve
and spectra of SN 1997ef were modeled first with a standard SN~Ic model
assuming an ordinary kinetic energy of explosion erg. The
explosion of a CO star of mass gives a
reasonably good fit to the light curve but clearly fails to reproduce the broad
spectral features. Then, models with larger masses and energies were explored.
Both the light curve and the spectra of SN 1997ef are much better reproduced by
a C+O star model with 8 \e{51} erg and .
Therefore, we conclude that SN 1997ef is very likely a hypernova on the basis
of its kinetic energy of explosion. Finally, implications for the deviation
from spherical symmetry are discussed in an effort to improve the light curve
and spectral fits.Comment: "To appear in the Astrophysical Journal, Vol.534 (2000)
Composition of a new chemotype of Senecio chrysanthemoides DC.
1745-17481,10β-epoxy-6-oxo-furanoeremophilane 1 (21.8%) along with furanoligularenone 2, germacrene D 3, selin-11-en-4α-ol 4 and caryophyllene oxide 5 have been identified in Senecio chrysanthemoides DC. of the Garhwal region of Himalaya at an altitude of 3600 m. 1,10β-epoxy-6-oxo-furanoeremophilane 1 has not been reported in Senecio chrysanthemoides DC. but reported in the other species of the genus Senecio. Presence of compounds 1-5 and absence of previously reported β-thujone and 6-hydroxy-p-menth-4(5)-en-3-one makes it a new chemotype within the genus Senecio
New features in the microwave response of YBCO crystals: Evidence for a multi-component order parameter
New features are reported in precision measurements of the complex microwave
conductivity of high quality crystals grown in
crucibles. A third peak in the normal conductivity,
, at around , and enhanced pair conductivity
below are observed. The data are inconsistent with a
single order parameter, and instead are indicative of multi-component
superconductivity. Overall, these results point to the presence of multiple
pairing interactions in and also provide a natural
explanation to account for the low temperature conductivity peak observed
in all crystals.Comment: 4 pages 4 figures, Revtex, multicols [to appear in Phys. Rev. B
(Rapid Comm)]. Our preprints also available at
http://sagar.physics.neu.edu/preprints.htm
Evidence for a colour dependence in the size distribution of main belt asteroids
We present the results of a project to detect small (~1 km) main-belt
asteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observed
in 2 filters (MegaPrime g' and r') in order to compare the results in each
band. Owing to the observational cadence we did not observe the same asteroids
through each filter and thus do not have true colour information. However
strong differences in the size distributions as seen in the two filters point
to a colour-dependence at these sizes, perhaps to be expected in this regime
where asteroid cohesiveness begins to be dominated by physical strength and
composition rather than by gravity. The best fit slopes of the cumulative size
distributions (CSDs) in both filters tend towards lower values for smaller
asteroids, consistent with the results of previous studies. In addition to this
trend, the size distributions seen in the two filters are distinctly different,
with steeper slopes in r' than in g'. Breaking our sample up according to
semimajor axis, the difference between the filters in the inner belt is found
to be somewhat less pronounced than in the middle and outer belt, but the CSD
of those asteroids seen in the r' filter is consistently and significantly
steeper than in g' throughout. The CSD slopes also show variations with
semimajor axis within a given filter, particularly in r'. We conclude that the
size distribution of main belt asteroids is likely to be colour dependent at
kilometer sizes and that this dependence may vary across the belt.Comment: 28 pages, 5 figures, submitted to the Astronomical Journa
SiFTO: An Empirical Method for Fitting SNe Ia Light Curves
We present SiFTO, a new empirical method for modeling type Ia supernovae (SNe
Ia) light curves by manipulating a spectral template. We make use of
high-redshift SN observations when training the model, allowing us to extend it
bluer than rest frame U. This increases the utility of our high-redshift SN
observations by allowing us to use more of the available data. We find that
when the shape of the light curve is described using a stretch prescription,
applying the same stretch at all wavelengths is not an adequate description.
SiFTO therefore uses a generalization of stretch which applies different
stretch factors as a function of both the wavelength of the observed filter and
the stretch in the rest-frame B band. We compare SiFTO to other published
light-curve models by applying them to the same set of SN photometry, and
demonstrate that SiFTO and SALT2 perform better than the alternatives when
judged by the scatter around the best fit luminosity distance relationship. We
further demonstrate that when SiFTO and SALT2 are trained on the same data set
the cosmological results agree.Comment: Modified to better match published version in Ap
Supernova Shock Breakout from a Red Supergiant
Massive stars undergo a violent death when the supply of nuclear fuel in
their cores is exhausted, resulting in a catastrophic "core-collapse"
supernova. Such events are usually only detected at least a few days after the
star has exploded. Observations of the supernova SNLS-04D2dc with the Galaxy
Evolution Explorer space telescope reveal a radiative precursor from the
supernova shock before the shock reached the surface of the star and show the
initial expansion of the star at the beginning of the explosion. Theoretical
models of the ultraviolet light curve confirm that the progenitor was a red
supergiant, as expected for this type of supernova. These observations provide
a way to probe the physics of core-collapse supernovae and the internal
structures of their progenitor starsComment: Science, in press. 32 pages, 7 figure
Photometric selection of high-redshift type Ia supernovae
We present a method for selecting high-redshift type Ia supernovae (SNe Ia)
located via rolling SN searches. The technique, using both color and magnitude
information of events from only 2-3 epochs of multi-band real-time photometry,
is able to discriminate between SNe Ia and core collapse SNe. Furthermore, for
the SNe Ia, the method accurately predicts the redshift, phase and light-curve
parameterization of these events based only on pre-maximum-light data. We
demonstrate the effectiveness of the technique on a simulated survey of SNe Ia
and core-collapse SNe, where the selection method effectively rejects most
core-collapse SNe while retaining SNe Ia. We also apply the selection code to
real-time data acquired as part of the Canada-France-Hawaii Telescope Supernova
Legacy Survey (SNLS). During the period May 2004 to January 2005 in the SNLS,
440 SN candidates were discovered of which 70 were confirmed spectroscopically
as SNe Ia and 15 as core-collapse events. For this test dataset, the selection
technique correctly identifies 100% of the identified SNe II as non-SNe Ia with
only a 1-2% false rejection rate. The predicted parameterization of the SNe Ia
has a precision of |delta_z|/(1+z_spec)<0.09 in redshift, and +/- 2-3
rest-frame days in phase, providing invaluable information for planning
spectroscopic follow-up observations. We also investigate any bias introduced
by this selection method on the ability of surveys such as SNLS to measure
cosmological parameters (e.g., w and omega matter), and find any effect to be
negligible.Comment: Accepted for publication in A
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