2,948 research outputs found
Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly
We report on three different and complementary measurements, namely
magnetisation measurements, positron annihilation spectroscopy and NMR
measurements, which give evidence that the formation of oxygen vacancy clusters
is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of
YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the
optimally doped state where vacancies are present. We therefore conclude that
the single vacancies or point defects can not be responsible for this anomaly
but that clusters of oxygen vacancies are on its origin.Comment: 10 pages, 4 figures, submitted to PR
Observation of coherent Josephson response in the non-linear ab-plane microwave impedance of single crystals
We report novel non-linear phenomena in the -plane microwave impedance of
single crystals. The vs. data are well
described by the non-linear RSJ model : . The entire crystal behaves like a single Josephson junction. The
extraordinary coherence of the data suggests an intrinsic mechanism.Comment: 2 pages,1 figure, Submitted to Proc. of M^2SHTSC-V (Beijing), also
available at http://sagar.physics.neu.edu/preprint
Mineral industries of Illinois
"Directory of Illinois Mineral Operators" to accompany the map.Ope
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3
We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond
The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field
We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the
GOODS-N field. The galaxies were selected primarily because they appear to have
elongated morphologies, and slits were aligned with the major axes (as
determined from the rest-frame UV emission) of 11 of the 13. If the galaxies
are elongated because they are highly inclined, alignment of the slit and major
axis should maximize the observed velocity and reveal velocity shear, if
present. In spite of this alignment, we see spatially resolved velocity shear
in only two galaxies. We show that the seeing makes a large difference in the
observed velocity spread of a tilted emission line, and use this information to
place limits on the velocity spread of the ionized gas of the galaxies in the
sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the
velocity shear (half of the velocity range of a tilted emission line) that
would be observed under our best seeing conditions of ~0.5". When combined with
previous work, our data also indicate that aligning the slit along the major
axis does not increase the probability of observing a tilted emission line. We
then focus on the one-dimensional velocity dispersion \sigma, which is much
less affected by the seeing, and see that the elongated subsample exhibits a
significantly lower velocity dispersion than galaxies selected at random from
our total H-alpha sample, not higher as one might have expected. We also see
some evidence that the elongated galaxies are less reddened than those randomly
selected using only UV colors. Both of these results are counter to what would
be expected if the elongated galaxies were highly inclined disks. It is at
least as likely that the galaxies' elongated morphologies are due to merging
subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Evidence for Solar Metallicities in Massive Star-forming Galaxies at z>~2
We present results of near-IR spectroscopic measurements of 7 star-forming
galaxies at 2.1<z<2.5. Drawn from a large spectroscopic survey of galaxies
photometrically pre-selected by their U_nGR colors to lie at z~2, these
galaxies were chosen for their bright rest-frame optical luminosities
(K_s<=20.0). Most strikingly, the majority of the sample of 7 galaxies exhibit
[NII]/Ha nebular emission line ratios indicative of at least solar HII region
metallicities, at a lookback time of 10.5 Gyr. The broadband colors of the
K_s-bright sample indicate that most have been forming stars for more than a
Gyr at z~2, and have already formed stellar masses in excess of 10^11 Msun. The
descendants of these galaxies in the local universe are most likely metal-rich
and massive spiral and elliptical galaxies, while plausible progenitors for
them can be found among the population of z~3 Lyman Break Galaxies. While the
K_s-bright z~2 galaxies appear to be highly evolved systems, their large Ha
luminosities and uncorrected Ha star-formation rates of 24-60 Msun/yr indicate
that active star formation is still ongoing. The luminous UV-selected objects
presented here comprise more than half of the high-redshift (z>1.5) tails of
current K-band-selected samples such as the K20 and Gemini Deep Deep surveys.Comment: 15 pages including 5 figures. Accepted for publication in Ap
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