Coevolution of dynamical states and interactions in dynamic networks

We explore the coupled dynamics of the internal states of a set of interacting elements and the network of interactions among them. Interactions are modeled by a spatial game and the network of interaction links evolves adapting to the outcome of the game. As an example we consider a model of cooperation, where the adaptation is shown to facilitate the formation of a hierarchical interaction network that sustains a highly cooperative stationary state. The resulting network has the characteristics of a small world network when a mechanism of local neighbor selection is introduced in the adaptive network dynamics. The highly connected nodes in the hierarchical structure of the network play a leading role in the stability of the network. Perturbations acting on the state of these special nodes trigger global avalanches leading to complete network reorganization.Comment: 4 pages, 5 figures, for related material visit http:www.imedea.uib.es/physdept

Electron-Phonon Interaction in Embedded Semiconductor Nanostructures

The modification of acoustic phonons in semiconductor nanostructures embedded in a host crystal is investigated including corrections due to strain within continuum elasticity theory. Effective elastic constants are calculated employing {\em ab initio} density functional theory. For a spherical InAs quantum dot embedded in GaAs barrier material, the electron-phonon coupling is calculated. Its strength is shown to be suppressed compared to the assumption of bulk phonons

Saddle Points and Stark Ladders: Exact Calculations of Exciton Spectra in Superlattices

A new, exact method for calculating excitonic absorption in superlattices is described. It is used to obtain high resolution spectra showing the saddle point exciton feature near the top of the miniband. The evolution of this feature is followed through a series of structures with increasing miniband width. The Stark ladder of peaks produced by an axial electric field is investigated, and it is shown that for weak fields the line shapes are strongly modified by coupling to continuum states, taking the form of Fano resonances. The calculated spectra, when suitably broadened, are found to be in good agreement with experimental results.Comment: 9 pages Revtex v3.0, followed by 4 uuencoded postscript figures, SISSA-CM-94-00

Non-intrusive flow measurements on a reentry vehicle

This study evaluates the utility of various non-intrusive techniques for the measurement of the flow field on the windward side of the Space Shuttle or a similar re-entry vehicle. Included are linear (Rayleigh, Raman, Mie, Laser Doppler Velocimetry, Resonant Doppler Velocimetry) and nonlinear (Coherent Anti-Stokes Raman, Laser Induced Fluorescence) light scattering, electron beam fluorescence, thermal emission and mass spectroscopy. Flow field properties are taken from a nonequilibrium flow model by Shinn, Moss and Simmonds at NASA Langley. Conclusions are, when possible, based on quantitative scaling of known laboratory results to the conditions projected. Detailed discussion with researchers in the field contributed further to these conclusions and provided valuable insights regarding the experimental feasibility of each of the techniques

Formation Studies on the Nonaqueous Synthesis of Metal Oxide Nanoparticles in a 1.5 L Reactor System

In the last years, the nonaqueous synthesis has been demonstrated as a highly versatile method for the simple synthesis of highly crystalline metal oxide nanoparticles and nanomaterials. Thereby, we have presented the synthesis of a multitude of different metal oxides (e.g., TiO2, ZrO2, BaTiO3, Fe3O4). The mechanisms of particle formation as well as the influence of process parameters on the particle properties however remain largely unknown so far, as the molecular mechanisms are rather complex. In this paper, we show that the synthesis of metal oxide nanoparticles is feasible also on a multi-gram reactor scale on the example of anatase TiO2 nanoparticles. Using a reactor system equipped with a sampling system for with-drawal of samples at different stages of the reaction, the kinetics of particle formation could be determined and compared to the formation of organic side products and water. Additionally, insights into the influence of different process parameters on the particle properties are shown and can be utilized to tailor size and morphology of the product nanoparticles. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3489

Enhanced charge stripe order of superconducting La(2-x)Ba(x)CuO(4) in a magnetic field

The effect of a magnetic field on the charge stripe order in La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray diffraction for charge carrier concentrations ranging from strongly underdoped to optimally doped. We find that charge stripe order can be significantly enhanced by a magnetic field applied along the c-axis, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x=1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of a magnetic field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states.Comment: 6 pages, 4 figure

X-ray induced persistent photoconductivity in Si-doped Al0.35_{0.35}Ga0.65_{0.65}As

We demonstrate that X-ray irradiation can be used to induce an insulator-metal transition in Si-doped Al$_{0.35}$Ga$_{0.65}$As, a semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX} centers into their shallow donor state was revealed by studying the photoconductance along with fluorescence. The photoconductance as a function of incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying that core-hole excitation of Ga and As are efficient primary steps for the excitation of {\it DX} centers. A high quantum yield ($\gg 1$) suggests that the excitation is indirect and nonlocal, due to secondary electrons, holes, and fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we corrected i