1,317 research outputs found
Superconducting properties of RuSr2GdCu2O8 studied by SQUID magnetometry
For polycrystalline RuSr2GdCu2O8 (Ru-1212), distinct peaks have been reported
in d.c. magnetization in the superconducting state of the sample. Sr2GdRuO6
(Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in
the same temperature regime. Based on measurements performed on both bulk and
powdered samples of Ru-1212 and Sr-2116, we exclude the possibility, that the
observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities.
The effect is related to the superconductivity of Ru-1212, but it is not an
intrinsic property of this compound. We provide evidence that the observation
of magnetization peaks in the superconducting state of Ru-1212 is due to flux
motion generated by the movement of the sample in an inhomogeneous field,
during the measurement in the SQUID magnetometer. We propose several tests,
that help to decide, whether the features observed in a SQUID magnetization
measurement of Ru-1212 represent a property of the compound or not.Comment: 22 pages, 9 figure
Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models
Measurements of atmospheric O2/N2 ratios and CO2 concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O2/N2 + CO2) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996-2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O2 and CO2 flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere. Copyright 2006 by the American Geophysical Union
A provenance task abstraction framework
Visual analytics tools integrate provenance recording to externalize analytic processes or user insights. Provenance can be captured on varying levels of detail, and in turn activities can be characterized from different granularities. However, current approaches do not support inferring activities that can only be characterized across multiple levels of provenance. We propose a task abstraction framework that consists of a three stage approach, composed of (1) initializing a provenance task hierarchy, (2) parsing the provenance hierarchy by using an abstraction mapping mechanism, and (3) leveraging the task hierarchy in an analytical tool. Furthermore, we identify implications to accommodate iterative refinement, context, variability, and uncertainty during all stages of the framework. A use case describes exemplifies our abstraction framework, demonstrating how context can influence the provenance hierarchy to support analysis. The paper concludes with an agenda, raising and discussing challenges that need to be considered for successfully implementing such a framework
Unraveling DNA tori under tension
Motivated by recent experiments, we develop a model for DNA toroids under
external tension. We find that tori are the equilibrium states for our model up
to a critical tension, above which they become only metstable. Above this
tension, we find a cascade of transitions between discrete toroid states that
successively lowers the winding number, until the ground state (rod) is
reached. In this process, this model predicts a nearly constant force plateau
as a function of extension, in agreement with experiment.Comment: 9 pages, 11 figure
Mercury in the snow and firn at Summit Station, Central Greenland, and implications for the study of past atmospheric mercury levels
Gaseous Elemental Mercury (Hg° or GEM) was investigated at Summit Station, Greenland, in the interstitial air extracted from the perennial snowpack (firn) at depths ranging from the surface to 30 m, during summer 2005 and spring 2006. Photolytic production and destruction of Hg° were observed close to the snow surface during summer 2005 and spring 2006, and we observed dark oxidation of GEM up to 270 cm depth in June 2006. Photochemical transformation of gaseous elemental mercury resulted in diel variations in the concentrations of this gas in the near-surface interstitial air, but destruction of Hg° was predominant in June, and production was the main process in July. This seasonal evolution of the chemical mechanisms involving gaseous elemental mercury produces a signal that propagates downward through the firn air, but is unobservably small below 15 m in depth. As a consequence, multi-annual averaged records of GEM concentration should be well preserved in deep firn air at depths below 15 m, and available for the reconstruction of the past atmospheric history of GEM over the last decades
A novel approach to task abstraction to make better sense of provenance data
Working Group Report in 'Provenance and Logging for Sense Making' report from Dagstuhl Seminar 18462: Provenance and Logging for Sense Making, Dagstuhl Reports, Volume 8, Issue 1
The Robinson Gravitational Wave Background Telescope (BICEP): a bolometric large angular scale CMB polarimeter
The Robinson Telescope (BICEP) is a ground-based millimeter-wave bolometric
array designed to study the polarization of the cosmic microwave background
radiation (CMB) and galactic foreground emission. Such measurements probe the
energy scale of the inflationary epoch, tighten constraints on cosmological
parameters, and verify our current understanding of CMB physics. Robinson
consists of a 250-mm aperture refractive telescope that provides an
instantaneous field-of-view of 17 degrees with angular resolution of 55 and 37
arcminutes at 100 GHz and 150 GHz, respectively. Forty-nine pair of
polarization-sensitive bolometers are cooled to 250 mK using a 4He/3He/3He
sorption fridge system, and coupled to incoming radiation via corrugated feed
horns. The all-refractive optics is cooled to 4 K to minimize polarization
systematics and instrument loading. The fully steerable 3-axis mount is capable
of continuous boresight rotation or azimuth scanning at speeds up to 5 deg/s.
Robinson has begun its first season of observation at the South Pole. Given the
measured performance of the instrument along with the excellent observing
environment, Robinson will measure the E-mode polarization with high
sensitivity, and probe for the B-modes to unprecedented depths. In this paper
we discuss aspects of the instrument design and their scientific motivations,
scanning and operational strategies, and the results of initial testing and
observations.Comment: 18 pages, 11 figures. To appear in Millimeter and Submillimeter
Detectors and Instrumentation for Astronomy III, Proceedings of SPIE, 6275,
200
Properties of the ferrimagnetic double-perovskite A_{2}FeReO_{6} (A=Ba and Ca)
Ceramics of A_{2}FeReO_{6} double-perovskite have been prepared and studied
for A=Ba and Ca. Ba_{2}FeReO_{6} has a cubic structure (Fm3m) with 8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with
and
. The barium compound is metallic from 5 K to 385
K, i.e. no metal-insulator transition has been seen up to 385 K, and the
calcium compound is semiconducting from 5 K to 385 K. Magnetization
measurements show a ferrimagnetic behavior for both materials, with T_{c}=315 K
for Ba_{2}FeReO_{6} and above 385 K for Ca_{2}FeReO_{6}. A specific heat
measurement on the barium compound gave an electron density of states at the
Fermi level, N(E_{F}) equal to 6.1. At 5 K, we
observed a negative magnetoresistance of 10 % in a magnetic field of 5 T, but
only for Ba_{2}FeReO_{6}. Electrical, thermal and magnetic properties are
discussed and compared to the analogous compounds Sr_{2}Fe(Mo,Re)O_{6}.Comment: 5 pages REVTeX, 7 figures included, submitted to PR
Orbital Structure and Magnetic Ordering in Layered Manganites: Universal Correlation and Its Mechanism
Correlation between orbital structure and magnetic ordering in bilayered
manganites is examined. A level separation between the and
orbitals in a Mn ion is calculated in the ionic model for a
large number of the compounds. It is found that the relative stability of the
orbitals dominates the magnetic transition temperatures as well as the magnetic
structures. A mechanism of the correlation between orbital and magnetism is
investigated based on the theoretical model with the two orbitals under
strong electron correlation.Comment: 4 pages, 4 figure
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