3,305 research outputs found
Co-evolution of strategy and structure in complex networks with dynamical linking
Here we introduce a model in which individuals differ in the rate at which
they seek new interactions with others, making rational decisions modeled as
general symmetric two-player games. Once a link between two individuals has
formed, the productivity of this link is evaluated. Links can be broken off at
different rates. We provide analytic results for the limiting cases where
linking dynamics is much faster than evolutionary dynamics and vice-versa, and
show how the individual capacity of forming new links or severing inconvenient
ones maps into the problem of strategy evolution in a well-mixed population
under a different game. For intermediate ranges, we investigate numerically the
detailed interplay determined by these two time-scales and show that the scope
of validity of the analytical results extends to a much wider ratio of time
scales than expected
Similarity based cooperation and spatial segregation
We analyze a cooperative game, where the cooperative act is not based on the
previous behaviour of the co-player, but on the similarity between the players.
This system has been studied in a mean-field description recently [A. Traulsen
and H. G. Schuster, Phys. Rev. E 68, 046129 (2003)]. Here, the spatial
extension to a two-dimensional lattice is studied, where each player interacts
with eight players in a Moore neighborhood. The system shows a strong
segregation independent on parameters. The introduction of a local conversion
mechanism towards tolerance allows for four-state cycles and the emergence of
spiral waves in the spatial game. In the case of asymmetric costs of
cooperation a rich variety of complex behavior is observed depending on both
cooperation costs. Finally, we study the stabilization of a cooperative fixed
point of a forecast rule in the symmetric game, which corresponds to
cooperation across segregation borders. This fixed point becomes unstable for
high cooperation costs, but can be stabilized by a linear feedback mechanism.Comment: 7 pages, 9 figure
Current-induced magnetization dynamics in disordered itinerant ferromagnets
Current-driven magnetization dynamics in ferromagnetic metals are studied in
a self-consistent adiabatic local-density approximation in the presence of
spin-conserving and spin-dephasing impurity scattering. Based on a quantum
kinetic equation, we derive Gilbert damping and spin-transfer torques entering
the Landau-Lifshitz equation to linear order in frequency and wave vector.
Gilbert damping and a current-driven dissipative torque scale identically and
compete, with the result that a steady current-driven domain-wall motion is
insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform
magnetization is found to be much more stable against spin torques in the
itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic
spin-transfer torque reminiscent of the spin pumping in multilayers is
identified and shown to govern the current-induced domain-wall distortion
Future geodesic completeness of some spatially homogeneous solutions of the vacuum Einstein equations in higher dimensions
It is known that all spatially homogeneous solutions of the vacuum Einstein
equations in four dimensions which exist for an infinite proper time towards
the future are future geodesically complete. This paper investigates whether
the analogous statement holds in higher dimensions. A positive answer to this
question is obtained for a large class of models which can be studied with the
help of Kaluza-Klein reduction to solutions of the Einstein-scalar field
equations in four dimensions. The proof of this result makes use of a criterion
for geodesic completeness which is applicable to more general spatially
homogeneous models.Comment: 18 page
Dynamic exchange coupling and Gilbert damping in magnetic multilayers
We theoretically study dynamic properties of thin ferromagnetic films in
contact with normal metals. Moving magnetizations cause a flow of spins into
adjacent conductors, which relax by spin flip, scatter back into the
ferromagnet, or are absorbed by another ferromagnet. Relaxation of spins
outside the moving magnetization enhances the overall damping of the
magnetization dynamics in accordance with the Gilbert phenomenology. Transfer
of spins between different ferromagnets by these nonequilibrium spin currents
leads to a long-ranged dynamic exchange interaction and novel collective
excitation modes. Our predictions agree well with recent
ferromagnetic-resonance experiments on ultrathin magnetic films.Comment: 15 pages, 3 figures, for MMM'02 proceeding
XMM-Newton and optical follow-up observations of three new polars from the Sloan Digital Sky Survey
We report follow-up XMM-Newton and optical observations of three new polars
found in the Sloan Digital Sky Survey. Simple modeling of the X-ray spectra,
and consideration of the details of the X-ray and optical lightcurves
corroborate the polar nature of these three systems and provide further
insights into their accretion characteristics. During the XMM-Newton
observation of SDSS J072910.68+365838.3, X-rays are undetected apart from a
probable flare event, during which we find both the typical hard X-ray
bremsstrahlung component and a very strong line O VII (E=0.57 keV), but no
evidence of a soft blackbody contribution. In SDSS J075240.45+362823.2 we
identify an X-ray eclipse at the beginning of the observation, roughly in phase
with the primary minimum of the optical broad band curve. The X-ray spectra
require the presence of both hard and soft X-ray components, with their
luminosity ratio consistent with that found in other recent XMM-Newton results
on polars. Lastly, SDSS J170053.30+400357.6 appears optically as a very typical
polar, however its large amplitude optical modulation is 180 degrees out of
phase with the variation in our short X-ray lightcurve.Comment: 9 pages, 9 figures, accepted for publication in the ApJ (January
2005
Reliability and correlation analysis of computed methods to convert conventional 2D radiological hindfoot measurements to a 3D setting using weightbearing CT
Parallax and Luminosity Measurements of an L Subdwarf
We present the first parallax and luminosity measurements for an L subdwarf,
the sdL7 2MASS J05325346+8246465. Observations conducted over three years by
the USNO infrared astrometry program yield an astrometric distance of
26.7+/-1.2 pc and a proper motion of 2.6241+/-0.0018"/yr. Combined with
broadband spectral and photometric measurements, we determine a luminosity of
log(Lbol/Lsun) = -4.24+/-0.06 and Teff = 1730+/-90 K (the latter assuming an
age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the
luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models
indicates that its mass is just below the sustained hydrogen burning limit, and
is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde
Galactic orbit which is both eccentric (3 <~ R <~ 8.5 kpc) and extends well
away from the plane (Delta_Z = +/-2 kpc), consistent with membership in the
inner halo population. The relatively bright J-band magnitude of 2MASS
J05325346+8246465 implies significantly reduced opacity in the 1.2 micron
region, consistent with inhibited condensate formation as previously proposed.
Its as yet unknown subsolar metallicity remains the primary limitation in
constraining its mass; determination of both parameters would provide a
powerful test of interior and evolutionary models for low-mass stars and brown
dwarfs.Comment: Accepted to ApJ 10 September 2007; 13 pages, 5 figures, 3 tables,
formatted in emulateapj styl
Inhibition of ammonia monooxygenase from ammonia oxidising archaea by linear and aromatic alkynes
Ammonia monooxygenase (AMO) is a key nitrogen-transforming enzyme belonging to the same copper-dependent membrane monooxygenase family (CuMMO) as the particulate methane monooxygenase (pMMO). The AMO from ammonia-oxidizing archaea (AOA) is very divergent from both the AMO of ammonia-oxidizing bacteria (AOB) and the pMMO from methanotrophs, and little is known about the structure or substrate range of the archaeal AMO. This study compares inhibition by C 2 to C 8 linear 1-alkynes of AMO from two phylogenetically distinct strains of AOA, " Candidatus Nitrosocosmicus franklandus" C13 and " Candidatus Nitrosotalea sinensis" Nd2, with AMO from Nitrosomonas europaea and pMMO from Methylococcus capsulatus (Bath). An increased sensitivity of the archaeal AMO to short-chain-length alkynes (≤C 5) appeared to be conserved across AOA lineages. Similarities in C 2 to C 8 alkyne inhibition profiles between AMO from AOA and pMMO from M. capsulatus suggested that the archaeal AMO has a narrower substrate range than N. europaea AMO. Inhibition of AMO from " Ca Nitrosocosmicus franklandus" and N. europaea by the aromatic alkyne phenylacetylene was also investigated. Kinetic data revealed that the mechanisms by which phenylacetylene inhibits " Ca Nitrosocosmicus franklandus" and N. europaea are different, indicating differences in the AMO active site between AOA and AOB. Phenylacetylene was found to be a specific and irreversible inhibitor of AMO from " Ca Nitrosocosmicus franklandus," and it does not compete with NH 3 for binding at the active site. IMPORTANCE Archaeal and bacterial ammonia oxidizers (AOA and AOB, respectively) initiate nitrification by oxidizing ammonia to hydroxylamine, a reaction catalyzed by ammonia monooxygenase (AMO). AMO enzyme is difficult to purify in its active form, and its structure and biochemistry remain largely unexplored. The bacterial AMO and the closely related particulate methane monooxygenase (pMMO) have a broad range of hydrocarbon cooxidation substrates. This study provides insights into the AMO of previously unstudied archaeal genera, by comparing the response of the archaeal AMO, a bacterial AMO, and pMMO to inhibition by linear 1-alkynes and the aromatic alkyne, phenylacetylene. Reduced sensitivity to inhibition by larger alkynes suggests that the archaeal AMO has a narrower hydrocarbon substrate range than the bacterial AMO, as previously reported for other genera of AOA. Phenylacetylene inhibited the archaeal and bacterial AMOs at different thresholds and by different mechanisms of inhibition, highlighting structural differences between the two forms of monooxygenase
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