25,234 research outputs found
Transitions in turbulent rotating Rayleigh-B\'enard convection
Numerical simulations of rotating Rayleigh-B\'enard convection are presented
for both no slip and free slip boundaries. The goal is to find a criterion
distinguishing convective flows dominated by the Coriolis force from those
nearly unaffected by rotation. If one uses heat transport as an indicator of
which regime the flow is in, one finds that the transition between the flow
regimes always occurs at the same value of a certain combination of Reynolds,
Prandtl and Ekman numbers for both boundary conditions. If on the other hand
one uses the helicity of the velocity field to identify flows nearly
independent of rotation, one finds the transition at a different location in
parameter space
STATE TRADING ENTERPRISES AND REVENUE GAINS FROM MARKET POWER: THE CASE OF BARLEY MARKETING AND THE CANADIAN WHEAT BOARD
According to the U.S. General Accounting Office, the Canadian Wheat Board (CWB) is the largest state trading enterprise reporting to the World Trade Organization under article XVII requirements. This study estimates the market power exerted by the CWB in international barley markets. The analysis incorporates international price discrimination across markets for similar types of barley, the intertwining relationships between feed and malting barley markets, and producer behavior in the absence of the CWB. The CWB was able to capture an annual average of $72 million in additional revenue beyond the amount that would have been generated by purely competitive multiple sellers of Canadian barley during the period 1985-94.Marketing,
Pure Gravity Mediation and Spontaneous B-L Breaking from Strong Dynamics
In pure gravity mediation (PGM), the most minimal scheme for the mediation of
supersymmetry (SUSY) breaking to the visible sector, soft masses for the
standard model gauginos are generated at one loop rather than via direct
couplings to the SUSY-breaking field. In any concrete implementation of PGM,
the SUSY-breaking field is therefore required to carry nonzero charge under
some global or local symmetry. As we point out in this note, a prime candidate
for such a symmetry might be B-L, the Abelian gauge symmetry associated with
the difference between baryon number B and lepton number L. The F-term of the
SUSY-breaking field then not only breaks SUSY, but also B-L, which relates the
respective spontaneous breaking of SUSY and B-L at a fundamental level. As a
particularly interesting consequence, we find that the heavy Majorana neutrino
mass scale ends up being tied to the gravitino mass, Lambda_N ~ m_3/2. Assuming
nonthermal leptogenesis to be responsible for the generation of the baryon
asymmetry of the universe, this connection may then explain why SUSY
necessarily needs to be broken at a rather high energy scale, so that m_3/2 >~
1000 TeV in accord with the concept of PGM. We illustrate our idea by means of
a minimal model of dynamical SUSY breaking, in which B-L is identified as a
weakly gauged flavor symmetry. We also discuss the effect of the B-L gauge
dynamics on the superparticle mass spectrum as well as the resulting
constraints on the parameter space of our model. In particular, we comment on
the role of the B-L D-term.Comment: 25 pages, 1 figur
Space reactor/Stirling cycle systems for high power lunar application
An analysis is performed to mathematically model a 550 kWe lunar base power supply which uses a SP-100 reactor coupled with Stirling converters. The reactor is placed in an excavation to keep activated coolant in the hole and to allow maintenance of the components outside the hole. Two technology levels are considered. They are 1050 and 1300 K heater head Stirling converts. It is found that for a 1050 K converter the total mass which provided 1000 volts DC at 250 m is 14,366 kg while the 1300 K system mass is 12,104 kg. The radiation area of the 1050 and 1300 K systems are 641 and 356 sq m respectively. Comparisons are made with Brayton and thermionic systems with both near term and advanced technology considered
Recoverable Information and Emergent Conservation Laws in Fracton Stabilizer Codes
We introduce a new quantity, that we term recoverable information, defined
for stabilizer Hamiltonians. For such models, the recoverable information
provides a measure of the topological information, as well as a physical
interpretation, which is complementary to topological entanglement entropy. We
discuss three different ways to calculate the recoverable information, and
prove their equivalence. To demonstrate its utility, we compute recoverable
information for fracton models using all three methods where appropriate. From
the recoverable information, we deduce the existence of emergent
Gauss-law type constraints, which in turn imply emergent conservation
laws for point-like quasiparticle excitations of an underlying topologically
ordered phase.Comment: Added additional cluster model calculation (SPT example) and a new
section discussing the general benefits of recoverable informatio
A Direct Elliptic Solver Based on Hierarchically Low-rank Schur Complements
A parallel fast direct solver for rank-compressible block tridiagonal linear
systems is presented. Algorithmic synergies between Cyclic Reduction and
Hierarchical matrix arithmetic operations result in a solver with arithmetic complexity and memory footprint. We provide a
baseline for performance and applicability by comparing with well known
implementations of the -LU factorization and algebraic multigrid
with a parallel implementation that leverages the concurrency features of the
method. Numerical experiments reveal that this method is comparable with other
fast direct solvers based on Hierarchical Matrices such as -LU and
that it can tackle problems where algebraic multigrid fails to converge
Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers
We present a comprehensive study of the exchange bias effect in a model
system. Through numerical analysis of the exchange bias and coercive fields as
a function of the antiferromagnetic layer thickness we deduce the absolute
value of the averaged anisotropy constant of the antiferromagnet. We show that
the anisotropy of IrMn exhibits a finite size effect as a function of
thickness. The interfacial spin disorder involved in the data analysis is
further supported by the observation of the dual behavior of the interfacial
uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we
have observed that the antiferromagnetic uncompensated spins are dominantly
frozen with nearly no rotating spins due to the chemical intermixing, which
correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure
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