13,759 research outputs found
Superconductivity in Mg10Ir19B16
Mg10Ir19B16, a previously unreported compound in the Mg-Ir-B chemical system,
is found to be superconducting at temperatures near 5 K. The fact that the
compound exhibits a range of superconducting temperatures between 4 and 5 K
suggests that a range of stoichiometries is allowed, though no structural
evidence for this is observed. The compound has a large, noncentrosymmetric,
body centered cubic unit cell with a = 10.568 Angstrom, displaying a structure
type for which no previous superconductors have been reported.Comment: submitted to PR
Magnetic field dependence of vortex activation energy: a comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors
The dissipative mechanism at low current density is compared in three
different classes of superconductors. This is achieved by measurement of
resistance as a function of temperature and magnetic field in clean
polycrystalline samples of NbSe2, MgB2 and Bi2Sr2Ca2Cu3O10 superconductors.
Thermally activated flux flow behavior is clearly identified in bulk MgB2.
While the activation energy at low fields for MgB2 is comparable to
Bi2Sr2Ca2Cu3O10, its field dependence follows a parabolic behavior unlike a
power law dependence seen in Bi2Sr2Ca2Cu3O10. We analyze our results based on
the Kramer's scaling for grain boundary pinning in MgB2and NbSe2
The Antarctic Slope Current in a Changing Climate
The Antarctic Slope Current (ASC) is a coherent circulation feature that rings the Antarctic continental shelf and regulates the flow of water towards the Antarctic coastline. The structure and variability of the ASC influences key processes near the Antarctic coastline that have global implications, such as the melting of Antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation. Recent theoretical, modeling, and observational advances have revealed new dynamical properties of the ASC, making it timely to review. Earlier reviews of the ASC focused largely on local classifications of water properties of the ASC's primary front. Here, we instead provide a classification of the current's frontal structure based on the dynamical mechanisms that govern both the alongâslope and crossâslope circulation; these two modes of circulation are strongly coupled, similar to the Antarctic Circumpolar Current. Highly variable motions, such as dense overflows, tides, and eddies are shown to be critical components of crossâslope and crossâshelf exchange, but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations. Results linking the ASC to larger modes of climate variability, such as El Niño, show that the ASC is an integral part of global climate. An improved dynamical understanding of the ASC is still needed to accurately model and predict future Antarctic sea ice extent, the stability of the Antarctic ice sheets, and the Southern Ocean's contribution to the global carbon cycle
Momentum-Dependent Mean Field Based Upon the Dirac-Brueckner Approach for Nuclear Matter
A momentum-dependent mean field potential, suitable for application in the
transport-model description of nucleus-nucleus collisions, is derived in a
microscopic way. The derivation is based upon the Bonn meson-exchange model for
the nucleon-nucleon interaction and the Dirac-Brueckner approach for nuclear
matter. The properties of the microscopic mean field are examined and compared
with phenomenological parametrizations which are commonly used in
transport-model calculations.Comment: 15 pages text (RevTex) and 4 figures (postscript in a separate
uuencoded file), UI-NTH-930
Corona of Magnetars
We develop a theoretical model that explains the formation of hot coronae
around strongly magnetized neutron stars -- magnetars. The starquakes of a
magnetar shear its external magnetic field, which becomes non-potential and is
threaded by an electric current. Once twisted, the magnetosphere cannot untwist
immediately because of its self-induction. The self-induction electric field
lifts particles from the stellar surface, accelerates them, and initiates
avalanches of pair creation in the magnetosphere. The created plasma corona
maintains the electric current demanded by curl(B) and regulates the
self-induction e.m.f. by screening. This corona persists in dynamic
equilibrium: it is continually lost to the stellar surface on the
light-crossing time of 10^{-4} s and replenished with new particles. In
essence, the twisted magnetosphere acts as an accelerator that converts the
toroidal field energy to particle kinetic energy. Using a direct numerical
experiment, we show that the corona self-organizes quickly (on a millisecond
timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic
lines. The heating rate of the corona is ~10^{36} erg/s, in agreement with the
observed persistent, high-energy output of magnetars. We deduce that a static
twist that is suddenly implanted into the magnetosphere will decay on a
timescale of 1-10 yrs. The particles accelerated in the corona impact the solid
crust, knock out protons, and regulate the column density of the hydrostatic
atmosphere of the star. The transition layer between the atmosphere and the
corona is the likely source of the observed 100-keV emission from magnetars.
The corona emits curvature radiation and can supply the observed IR-optical
luminosity. (Abridged)Comment: 70 pages, 14 figures, accepted to Ap
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