22,980 research outputs found
Skew critical problems
Skew critical problems occur in continuous and discrete nonholonomic
Lagrangian systems. They are analogues of constrained optimization problems,
where the objective is differentiated in directions given by an apriori
distribution, instead of tangent directions to the constraint. We show
semiglobal existence and uniqueness for nondegenerate skew critical problems,
and show that the solutions of two skew critical problems have the same contact
as the problems themselves. Also, we develop some infrastructure that is
necessary to compute with contact order geometrically, directly on manifolds
Crystal structure, thermodynamics, magnetics and disorder properties of Be-Fe-Al intermetallics
The elastic and magnetic properties, thermodynamical stability, deviation
from stoichiometry and order/disorder transformations of phases that are
relevant to Be alloys were investigated using density functional theory
simulations coupled with phonon density of states calculations to capture
temperature effects. A novel structure and composition were identified for the
Be-Fe binary {\epsilon} phase. In absence of Al, FeBe_5 is predicted to form at
equilibrium above ~ 1250 K, while the {\epsilon} phase is stable only below ~
1650 K, and FeBe_2 is stable at all temperatures below melting. Small additions
of Al are found to stabilise FeBe_5 over FeBe_2 and {\epsilon}, while at high
Al content, AlFeBe_4 is predicted to form. Deviations from stoichiometric
compositions are also considered and found to be important in the case of
FeBe_5 and {\epsilon}. The propensity for disordered vs ordered structures is
also important for AlFeBe_4 (which exhibits complete Al-Fe disordered at all
temperatures) and FeBe_5 (which exhibits an order-disorder transition at ~ 950
K).Comment: 14 pages, 10 figures, accepted for publication in J. Alloy Compd. on
14 March 201
Linear magnetoresistance in metals: guiding center diffusion in a smooth random potential
We predict that guiding center (GC) diffusion yields a linear and
non-saturating (transverse) magnetoresistance in 3D metals. Our theory is
semi-classical and applies in the regime where the transport time is much
greater than the cyclotron period, and for weak disorder potentials which are
slowly varying on a length scale much greater than the cyclotron radius. Under
these conditions, orbits with small momenta along magnetic field are
squeezed and dominate the transverse conductivity. When disorder potentials are
stronger than the Debye frequency, linear magnetoresistance is predicted to
survive up to room temperature and beyond. We argue that magnetoresistance from
GC diffusion explains the recently observed giant linear magnetoresistance in
3D Dirac materials
Neglected Dimensions of Global Security: The Global Health Risk Framework Commission
The world has experienced global health crises ranging from novel influenzas (H5N1 and H1N1) and coronaviruses (SARS and MERS) to the Ebola and Zika viruses. In each case, governments and international organizations seemed unable to react quickly and decisively. Health crises have unmasked critical vulnerabilities— weak health systems, failures of leadership, and political overreaction and underreaction. The Global Health Risk Framework Commission, for which the National Academy of Medicine served as the secretariat, recently set out a comprehensive strategy to safeguard human and economic security from pandemic threats
Red Eyes on Wolf-Rayet Stars: 60 New Discoveries via Infrared Color Selection
We have spectroscopically identified 60 Galactic Wolf-Rayet (WR) stars,
including 38 nitrogen types (WN) and 22 carbon types (WC). Using photometry
from the Spitzer/GLIMPSE and 2MASS databases, the WRs were selected via a
method we have established that exploits their unique infrared colors, which is
mainly the result of excess radiation from free-free scattering within their
dense ionized winds. The selection criteria has been refined since our last
report, and now yields WRs at a rate of ~20% in spectroscopic follow-up of
candidates that comprise a broad color space defined by the color distribution
of all known WRs having B>14 mag. However, there are subregions within the
broad color space which yield WRs at a rate of >50%. Cross-correlation of WR
candidates with archival X-ray point-source catalogs increases the WR detection
rate of the broad color space to ~40%; ten new WR X-ray sources have been
found, in addition to a previously unrecognized X-ray counterpart to a known
WR. The extinction values, distances, and galactocentric radii of all new WRs
are calculated using the method of spectroscopic parallax. Although the
majority of the new WRs have no obvious association with stellar clusters, two
WC8 stars reside in a previously unknown massive-star cluster that lies near
the intersection of the Scutum-Centaurus Arm and the Galaxy's bar, in which
five OB supergiants were also identified. In addition, two WC and four WN stars
were identified in association with the stellar clusters Danks 1 and 2. A WN9
star has also been associated with the cluster [DBS2003] 179. This work brings
the total number of known Galactic WRs to 476, or ~7-8% of the total
empirically estimated population. An examination of their Galactic distribution
reveals a tracing of spiral arms and an enhanced WR surface density toward
several massive-star formation sites (abridged).Comment: Accepted to the Astronomical Journal on May 20, 2011. Document is 39
pages, including 20 figures and 8 table
Robustness of Majorana Fermion induced Fractional Josephson Effect
It is shown in previous works that the coupling between two Majorana end
states in superconducting quantum wires leads to fractional Josephson effect.
However, in realistic experimental conditions, multiple bands of the wires are
occupied and the Majorana end states are accompanied by other fermionic end
states. This raises the question concerning the robustness of fractional
Josephson effect in these situations. In this work, we show that the absence of
the avoided energy crossing which gives rise to the fractional Josephson effect
is robust, even when the Majorana fermions are coupled with arbitrary strengths
to other fermions. Moreover, we calculate the temperature dependence of the
fractional Josephson current and show that it is suppressed by thermal
excitations to the other fermion bound states.Comment: 4+ pages, 3 figure
Intercomparisons of GOES-derived cloud parameters and surface observations over San Nicolas Island
The spatial sampling limitations of surface measurement systems necessitate the use of satellite data for the investigation of large-scale cloud processes. Understanding the information contained in the satellite-observed radiances, however, requires a connection between the remotely sensed cloud properties and those more directly observed within the troposphere. Surface measurements taken during the First ISCCP Regional Experiment (FIRE) Marine Stratocumulus Intensive Field Observations (IFO) are compared here to cloud properties determined from Geostationary Operational Environmental Satellite (GOES) data in order to determine how well the island measurements represent larger areas and to verify some of the satellite-measured parameters
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