13,301 research outputs found
Transfer molding of PMR-15 polyimide resin
Transfer molding is an economically viable method of producing small shapes of PMR-15 polyimide. It is shown that with regard to flexural, compressive, and tribological properties transfer-molded PMR-15 polyimide is essentially equivalent to PMR-15 polyimide produced by the more common method of compression molding. Minor variations in anisotropy are predictable effects of molding design and secondary finishing operations
Invariant expectations and vanishing of bounded cohomology for exact groups
We study exactness of groups and establish a characterization of exact groups
in terms of the existence of a continuous linear operator, called an invariant
expectation, whose properties make it a weak counterpart of an invariant mean
on a group. We apply this operator to show that exactness of a finitely
generated group implies the vanishing of the bounded cohomology of with
coefficients in a new class of modules, which are defined using the Hopf
algebra structure of .Comment: Final version, to appear in the Journal of Topology and Analysi
Relaxation-to-creep transition of domain-wall motion in two- dimensional random-field Ising model with ac driving field
With Monte Carlo simulations, we investigate the relaxation dynamics with a
domain wall for magnetic systems at the critical temperature. The dynamic
scaling behavior is carefully analyzed, and a dynamic roughening process is
observed. For comparison, similar analysis is applied to the relaxation
dynamics with a free or disordered surfaceComment: 5 pages, 5 figure
Laser induced magnetization switching in films with perpendicular anisotropy: a comparison between measurements and a multi-macrospin model
Thermally-assisted ultra-fast magnetization reversal in a DC magnetic field
for magnetic multilayer thin films with perpendicular anisotropy has been
investigated in the time domain using femtosecond laser heating. The experiment
is set-up as an optically pumped stroboscopic Time Resolved Magneto-Optical
Kerr Effect magnetometer. It is observed that a modest laser fluence of about
0.3 mJ/square-cm induces switching of the magnetization in an applied field
much less than the DC coercivity (0.8 T) on the sub-nanosecond time-scale. This
switching was thermally-assisted by the energy from the femtosecond pump-pulse.
The experimental results are compared with a model based on the Landau
Lifschitz Bloch equation. The comparison supports a description of the reversal
process as an ultra-fast demagnetization and partial recovery followed by
slower thermally activated switching due to the spin system remaining at an
elevated temperature after the heating pulse.Comment: 8 pages, 10 figures, to be submitted to PR
Impact of generalized benefit functions on the evolution of cooperation in spatial public goods games with continuous strategies
Cooperation and defection may be considered as two extreme responses to a
social dilemma. Yet the reality is much less clear-cut. Between the two
extremes lies an interval of ambivalent choices, which may be captured
theoretically by means of continuous strategies defining the extent of the
contributions of each individual player to the common pool. If strategies are
chosen from the unit interval, where 0 corresponds to pure defection and 1
corresponds to the maximal contribution, the question is what is the
characteristic level of individual investments to the common pool that emerges
if the evolution is guided by different benefit functions. Here we consider the
steepness and the threshold as two parameters defining an array of generalized
benefit functions, and we show that in a structured population there exist
intermediate values of both at which the collective contributions are maximal.
However, as the cost-to-benefit ratio of cooperation increases the
characteristic threshold decreases, while the corresponding steepness
increases. Our observations remain valid if more complex sigmoid functions are
used, thus reenforcing the importance of carefully adjusted benefits for high
levels of public cooperation.Comment: 8 two-column pages, 8 figures; accepted for publication in Physical
Review
Selected reliability studies for the NERVA program
An investigation was made into certain methods of reliability analysis that are particularly suitable for complex mechanisms or systems in which there are many interactions. The methods developed were intended to assist in the design of such mechanisms, especially for analysis of failure sensitivity to parameter variations and for estimating reliability where extensive and meaningful life testing is not feasible. The system is modeled by a network of interconnected nodes. Each node is a state or mode of operation, or is an input or output node, and the branches are interactions. The network, with its probabilistic and time-dependent paths is also analyzed for reliability and failure modes by a Monte Carlo, computerized simulation of system performance
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
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