41,968 research outputs found
Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors
The build-up of exciton populations in resonantly laser excited
semiconductors is studied microscopically. For excitation at the -exciton
resonance, it is shown that polarization with a strict -type radial symmetry
can be efficiently converted into an incoherent -type population. As a
consequence, inversion between the and exciton states can be obtained
leading to the appearance of significant terahertz gain.Comment: 4 pages, 3 figure
Event-by-event fluctuations and the QGP
We discuss the physics underlying event-by-event fluctuations in relativistic
heavy ion collisions. We will emphasize how the fluctuations of particle ratios
can be utilized to explore the properties of the matter created in these
collisions. In particular, we will argue that the fluctutions of the ratio of
positively over negatively charged particles may serve as a unique signature
for the Quark Gluon Plasma.Comment: Proceedings Quark Matter 2001, Stony Brook, NY January 200
Agent Teams and Evolutionary Computation: Optimizing Semi- Parametric Spatial Autoregressive Models
Classical spatial autoregressive models share the same weakness as the classical linear regression models, namely it is not possible to estimate non-linear relationships between the dependent and independent variables. In the case of classical linear regression a semi-parametric approach can be used to address this issue. Therefore an advanced semi- parametric modelling approach for spatial autoregressive models is introduced. Advanced semi-parametric modelling requires determining the best configuration of independent variable vectors, number of spline-knots and their positions. To solve this combinatorial optimization problem an asynchronous multi-agent system based on genetic-algorithms is utilized. Three teams of agents work each on a subset of the problem and cooperate through sharing their most optimal solutions. Through this system more complex relationships between the dependent and independent variables can be derived. These could be better suited for the possibly non-linear real-world problems faced by applied spatial econometricians.
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