47,905 research outputs found
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
ORGANIZING THE ECONOMICS ACADEMY: THE EVOLUTION OF PROFESSIONAL ECONOMICS ASSOCIATIONS, 1777-2000
Scholarly societies in economics (and many other professions) are clubs that provide members with a range of club goods, many of which have broader and economically significant spillover consequences for society at large. Yet surprisingly little is known about the historical evolution or current composition of these associations. This analysis of the development of professional economics societies worldwide provides perspectives on the evolution of the economics research industry they serve. Although the origins of current economic associations can be traced at least as far back as 1777, almost all of the growth in professional economics associations has been concentrated in the past 125 years and especially between 1945 and 2000. At the beginning of the 20th century almost all economic associations were general economics societies. The fractionalization of the profession, leading to a proliferation of associations with sub-disciplinary focus began in 1920 and accelerated after 1960. By 2000, almost two thirds of all economic associations served sub-disciplines ranging from law and economics through fisheries economics to public choice and game theory. There are comparatively few economic associations in the poorest parts of the world that are often most in need of the public goods economists can provide.Professional associations, club goods, economic societies, knowledge, Teaching/Communication/Extension/Profession,
The Evolution of Economics Clubs: 1777-2000
Replaced with revised version of paper 01/30/06.Professional associations, club goods, economic societies, knowledge, Teaching/Communication/Extension/Profession, A11, A12, D71, N011,
Efimov Physics in 6Li Atoms
A new narrow 3-atom loss resonance associated with an Efimov trimer crossing
the 3-atom threshold has recently been discovered in a many-body system of
ultracold 6Li atoms in the three lowest hyperfine spin states at a magnetic
field near 895 G. O'Hara and coworkers have used measurements of the 3-body
recombination rate in this region to determine the complex 3-body parameter
associated with Efimov physics. Using this parameter as the input, we calculate
the universal predictions for the spectrum of Efimov states and for the 3-body
recombination rate in the universal region above 600 G where all three
scattering lengths are large. We predict an atom-dimer loss resonance at (672
+/- 2) G associated with an Efimov trimer disappearing through an atom-dimer
threshold. We also predict an interference minimum in the 3-body recombination
rate at (759 +/- 1) G where the 3-spin mixture may be sufficiently stable to
allow experimental study of the many-body system.Comment: 27 pages, 9 figures, REVTeX4, published versio
Three-body Recombination of Lithium-6 Atoms with Large Negative Scattering Lengths
The 3-body recombination rate at threshold for distinguishable atoms with
large negative pair scattering lengths is calculated in the zero-range
approximation. The only parameters in this limit are the 3 scattering lengths
and the Efimov parameter, which can be complex valued. We provide semi-analytic
expressions for the cases of 2 or 3 equal scattering lengths and we obtain
numerical results for the general case of 3 different scattering lengths. Our
general result is applied to the three lowest hyperfine states of Lithium-6
atoms. Comparisons with recent experiments provide indications of loss features
associated with Efimov trimers near the 3-atom threshold.Comment: 4 pages, 4 figures, agrees with published versio
Planar Superconductor-Normal-Superconductor Josephson Junctions in MgB2
Since the discovery of superconductivity in MgB2 considerable progress has
been made in determining the physical properties of the material, which are
promising for bulk conductors. Tunneling studies show that the material is
reasonably isotropic and has a well-developed s-wave energy gap (∆),
implying that electronic devices based on MgB2 could operate close to 30K.
Although a number of groups have reported the formation of thin films by
post-reaction of precursors, heterostructure growth is likely to require
considerable technological development, making single-layer device structures
of most immediate interest. MgB2 is unlike the cuprate superconductors in that
grain boundaries do not form good Josephson junctions, and although a SQUID
based on MgB2 nanobridges has been fabricated, the nanobridges themselves do
not show junction-like properties. Here we report the successful creation of
planar MgB2 junctions by localised ion damage in thin films. The critical
current (IC) of these devices is strongly modulated by applied microwave
radiation and magnetic field. The product of the critical current and normal
state resistance (ICRN) is remarkably high, implying a potential for very high
frequency applications.Comment: 7 pages including 4 figure
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