7,228 research outputs found
Surface flux pinning in superconducting amorphous (Mo0.6Ru0.4)B18
Superconducting critical current density was measured as a function of a perpendicular applied magnetic field in glassy (Mo0.6Ru0.4)82B18. The pinning force density was observed to depend linearly on 1/w, where w is the sample width measured perpendicular to both the current and field. This dependence is attributed to pinning by the sample edges. The bulk pinning contribution can be separated from the edge pinning contribution by extrapolation of the Fp vs 1/w curve. The edge contribution of the flux pinning was nearly eliminated by electrolytically polishing the sample. The contribution of the flux pinning profile due to edge pinning is analyzed in terms of the dynamic pinning model modified for edge pinning
Ion mixing to produce amorphous Mo-Ru superconducting films
Amorphous Mo55Ru45 alloy films were formed by ion mixing of multilayered samples. The ion mixed films, which contain no metalloid element, show excellent superconducting properties. The measured properties are correlated with the microstructure obtained by both x-ray diffraction and transmission electron microscopy
Asset Location for Retirement Savers
This paper uses data on actual returns on taxable bonds, tax-exempt bonds, and a small sample of equity mutual funds over the 1962-1998 period to compare two asset location strategies for retirement savers. The first strategy gives priority to holding equities, through equity mutual funds, in a saver's tax-deferred account, while the second strategy gives priority to holding fixed-income investments in the tax-deferred account. We consider high-income taxable individual investors who saved in each year and invested in one of actively-managed funds in our sample. Over the thirty-seven year span that we consider, such savers would have accumulated a larger stock of wealth if they had held their equity mutual fund in their tax-deferred account than if they had held the fund in a conventional taxable form. The explanation for this apparent contradiction of the often-stated bonds in the tax-deferred account' prescription has two parts. First, many equity mutual funds impose substantial tax burdens on their investors. This raises the effective tax rate on investing in equities through mutual funds rather than in a buy-and-hold personal portfolio. Second, taxable investors who wish to hold fixed income assets can do so by holding tax-exempt bonds as well as by holding taxable bonds. The interest rate differential between taxable and tax-exempt bonds suggests that the effective tax rate on fixed income investments may be lower than the statutory tax rate for high-income investors.
Critical Casimir forces in colloidal suspensions on chemically patterned surfaces
We investigate the behavior of colloidal particles immersed in a binary
liquid mixture of water and 2,6-lutidine in the presence of a chemically
patterned substrate. Close to the critical point of the mixture, the particles
are subjected to critical Casimir interactions with force components normal and
parallel to the surface. Because the strength and sign of these interactions
can be tuned by variations in the surface properties and the mixtures
temperature, critical Casimir forces allow the formation of highly ordered
monolayers but also extend the use of colloids as model systems.Comment: 4 papges, 4 figures, accepted at Phys. Rev. Let
Ages and Abundances of Red Sequence Galaxies as a Function of LINER Emission Line Strength
Although the spectrum of a prototypical early-type galaxy is assumed to lack
emission lines, a substantial fraction (likely as high as 30%) of nearby red
sequence galaxy spectra contain emission lines with line ratios characteristic
of low ionization nuclear emission-line regions (LINERs). We use spectra of
~6000 galaxies from the Sloan Digital Sky Survey (SDSS) in a narrow redshift
slice (0.06 < z < 0.08) to compare the stellar populations of red sequence
galaxies with and without LINER-like emission. The spectra are binned by
internal velocity dispersion and by emission properties to produce high S/N
stacked spectra. The recent stellar population models of R. Schiavon (2007)
make it possible to measure ages, [Fe/H], and individual elemental abundance
ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We
find that red sequence galaxies with strong LINER-like emission are
systematically 2-3.5 Gyr (10-40%) younger than their emission-free counterparts
at the same velocity dispersion. This suggests a connection between the
mechanism powering the emission (whether AGN, post-AGB stars, shocks, or
cooling flows) and more recent star formation in the galaxy. We find that mean
stellar age and [Fe/H] increase with velocity dispersion for all galaxies.
Elemental abundance [Mg/Fe] increases modestly with velocity dispersion in
agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly
with velocity dispersion than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar
for all galaxies. At fixed velocity dispersion, galaxies with fainter r-band
luminosities have lower [Fe/H] and older ages but similar abundance ratios
compared to brighter galaxies.Comment: 25 pages, 17 figures, Accepted for publication in ApJ as of 16 July
2007; acceptance status updated, paper unchange
The history of star formation and mass assembly in early-type galaxies
We define a volume limited sample of over 14,000 early-type galaxies (ETGs)
selected from data release six of the Sloan Digital Sky Survey. The density of
environment of each galaxy is robustly measured. By comparing narrow band
spectral line indices with recent models of simple stellar populations (SSPs)
we investigate trends in the star formation history as a function of galaxy
mass (velocity dispersion), density of environment and galactic radius. We find
that age, metallicity and alpha-enhancement all increase with galaxy mass and
that field ETGs are younger than their cluster counterparts by ~2 Gyr. We find
negative radial metallicity gradients for all masses and environments, and
positive radial age gradients for ETGs with velocity dispersion over 180 km/s.
Our results are qualitatively consistent with a relatively simple picture for
ETG evolution in which the low-mass halos accreted by a proto-ETG contained not
only gas but also a stellar population. This fossil population is
preferentially found at large radii in massive ETGs because the stellar
accretions were dissipationless. We estimate that the typical, massive ETG
should have been assembled at z < 3.5. The process is similar in the cluster
and the field but occurred earlier in dense environments.Comment: 5 pages, 2 figures. Submitted to MNRA
Fortnightly Fluctuations in the O-C Diagram of CS 1246
Dominated by a single, large-amplitude pulsation mode, the rapidly-pulsating
hot subdwarf B star CS 1246 is a prime candidate for a long-term O-C diagram
study. We collected nearly 400 hours of photometry with the PROMPT telescopes
over a time span of 14 months to begin looking for secular variations in the
pulse timings. Interestingly, the O-C diagram is dominated by a strong
sinusoidal pattern with a period of 14.1 days and an amplitude of 10.7
light-seconds. Underneath this sine wave is a secular trend implying a decrease
in the 371.7-s pulsational period of Pdot = -1.9 x 10^-11, which we attribute
to the evolution of the star through the H-R diagram. The sinusoidal variation
could be produced by the presence of a low-mass companion, with m sin i ~ 0.12
Msun, orbiting the subdwarf B star at a distance of 20 Rsun. An analysis of the
combined light curve reveals the presence of a low-amplitude first harmonic to
the main pulsation mode.Comment: Accepted for publication in MNRAS. 11 pages, 8 figures, 5 table
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