Circular polarization of the photoluminescence from a system of two-dimensional <Emphasis Type="Italic">A</Emphasis><Superscript>+</Superscript> centers in a magnetic field

A series of GaAs/AlGaAs quantum well structures with different concentrations of A ⁺ centers is studied experimentally by polarized photoluminescence spectroscopy. The spectral dependences of the circular polarization and the energy shifts in a magnetic field exhibit qualitatively different behavior for samples corresponding to the weak and strong localization conditions. The results obtained are explained in the context of a suggested model describing optical transitions between states of localized and free electrons and A ⁺ centers.     

Improving fast aspheric convex surface tests with dynamic null screens using LCDs

A method for testing fast aspheric convex surfaces with dynamic null screens using LCDs is shown. A flat null screen is designed and displayed on an LCD monitor with drop-shaped spots in such a way that the image, which is formed by reflection on the test surface, becomes an exactly square array of circular spots if the surface is perfect. Any departure from this geometry is indicative of defects on the surface. Here the whole surface is tested at once. The position of the spots on the LCD can be changed in a dynamic way, to perform point-shifting of the image spots. The proposed procedure improves the dynamic point-shifting method. As has been shown previously, this process reduces the numerical error during the integration procedure, thereby improving the sensitivity of the test. The positioning accuracy for the screen spots is related to the LCD's spatial resolution. Results of the evaluation of a parabolic convex surface with f/#=0.22 are shown.     

Multipath-channel estimation and application to ionospheric channels

In this paper, the theory of linear time-variant systems is applied to multipath channels and, in particular, to ionospheric channels. Some useful formulas in time and frequency domains are given for both deterministic and random linear time-variant systems and multipath channels. The fundamental parameters of a multipath channel are the delay and the gain coefficient of each path, and the number of significant paths. We propose a methodology for estimating path-delays and path-coefficients and its application to the ionospheric channel estimation. Also, if path coefficients are stationary random processes, the power spectral density of the coefficients can be estimated from measured data of real (actual) channels. Finally, exhaustive computer simulations have been realized for testing the algorithms and a sample of the results is provided in the paper; also, estimation results for an actual narrowband HF ionospheric channel are provided, where up to four significant paths (rays) are detected with a Doppler spread lower than 0.2 Hz.     

Interaction of heavy metal ions with an ion exchange resin obtained from a natural polyelectrolyte

An ion exchange resin was synthesized by using a natural polyelectrolyte, sodium alginate, and barium ion as a cross-linker reagent. Resin was characterized by TGA and SEM. Equilibrium and kinetic experiments of Pb²⁺, Hg²⁺, Ni²⁺, Co²⁺, Fe²⁺, and Fe³⁺ ions uptake by barium alginate beads were carried out in batch-type experiments under different values of pH. The removal efficiency increases with increasing pH. The uptake of metal ions occurs rapidly in the first hour. Maximum retention capacity was also determined being Fe(II) > Fe(III) > Co(II) > Ni(II) > Pb(II) > Hg(II) in mmol/g dry beads basis. Elution from the loaded resins at maximum capacity was studied by using HCl and HNO₃ as eluents at different concentrations.     

Cadmium tellutide etched surfaces

Studies of etched surfaces of single-crystal cadmium telluride (CdTe) are presented. A CdTe-electrolyte system is used for electrical measurements. The possibility of carrying out electrochemical processes to alter the etched surface is demonstrated. Auger electron spectroscopy and electron loss spectroscopy depth profiles are obtained for the etched and electrochemically treated CdTe surfaces. Disturbed layers whose stoichiometry varies with depth differently for p- and n-type CdTe were detected on the surfaces.

Precise interpretation of the data is quite complex, but the model for a “nonequilibrium metal/insulator/semiconductor” structure is adopted to explain the elimination of the “pinning” effect with a reduction in the surface layer thickness.     

Evaluation of two control strategies for a sequencing batch reactor degrading high concentration peaks of 4-chlorophenol

The operation of a sequencing batch reactor (SBR) exposed to high concentration peaks (shock loads) of a toxic compound (4-chlorophenol, 4CP) was evaluated. Two control strategies based on on-line measurements of the dissolved oxygen concentration were tested. The first strategy, called variable timing control (VTC), detects the end of the reaction period to stop it. In the second control strategy, called observer-based time optimal control (OB-TOC), the automated system tries to maintain the critical specific growth rate by controlling the feed rate, i.e. the maximum growth rate when the substrate is toxic. The system operating under the VTC strategy presented a stable and efficient operation when the acclimated microorganisms (to an initial concentration of 350 mg 4CP/L) were exposed to punctual concentration peaks of 700 mg 4CP/L. A 4CP concentration peak higher than or equal to 1050 mg/L disturbed the system (1 month to recover). A 1400 mg/L peak caused strong inhibition that shut down the metabolic activity of the microorganisms, leading to reactor failure. With the OB-TOC strategy, the system was stable and worked efficiently when punctual concentration peaks of 700, 1050 and 1400 mg 4CP/L were fed. The system controlled by the OB-TOC strategy treated 1400 mg 4CP/L in less than 8h without affecting the operation of the reactor. The conclusion is that the OB-TOC strategy is more efficient than the VTC strategy to control a bioreactor when there are variations of concentrations of toxic organic compounds.     

Effect of coulomb correlations on luminescence and absorption in compensated semiconductors

The spectra of donor–acceptor light absorption and luminescence in lightly doped and lightly compensated semiconductors are calculated. In the photoluminescence calculation, two limiting cases of long and short carrier lifetimes relative to the carrier-energy relaxation time are considered. It is shown that, at long lifetimes, the photoluminescence spectrum is significantly shifted toward longer wavelengths due to the relaxation of minority charge carriers. At intermediate lifetimes, the photoluminescence spectrum consists of two peaks, which is in good agreement with the experimental data.     

Gut Microbiota Influences the Photoperiod Effects on Proanthocyanidins Bioavailability in Diet-Induced Obese Rats

Scope

Polyphenols health effects on obesity are mainly attributed to their metabolites generated after their gastrointestinal digestion, in which gut microbiota plays an important role. Moreover, gut microbiota composition and polyphenols bioavailability are influenced by differences in day light length (photoperiod). Thus, this study evaluates if a grape seed proanthocyanidins (GSPEs) extract bioavailability is influenced by different photoperiod exposure via gut microbiota modulation in an obesogenic context.

Methods and results

Cafeteria diet-induced obese Fischer 344 rats are housed under different photoperiod conditions (6, 12, or 18 h of light per day) during 9 weeks and administered with GSPE (25 mg kg⁻¹) or GSPE and an antibiotic cocktail (ABX) for the last 4 weeks. Serum GSPE-derived metabolites are quantified by HPLC-MS/MS.

Conclusion

A higher bioavailability is observed under 6 h light/18 h darkness (L6) compared to 18 h light/6 h darkness (L18). Individual metabolites, especially those from the gut microbiota, are affected by photoperiods. ABX treatment alters these photoperiod-mediated changes. Therefore, these results suggest that gut microbiota plays a key role in the photoperiod effects on GSPE bioavailability in obese rats.     

Next generation of broadcast multimedia services to mobile receivers in urban environments

This paper analyses the possibility of receiving high data rate multimedia services in the non-stationary urban mobile scenario using the digital video broadcast standard DVB-T2. The work focuses on the complex urban mobile environment and presents a comparison between the different configuration parameters and the data rates associated to different video services. The study is based on the experience operating an experimental DVB-T2 network in urban environments, where portable and vehicular reception scenarios have been tested. The results show the system performance and the feasible video quality. The paper explores the coverage for various video services, including HDTV and 3DTV options, and proposes some scenarios for the deployment of broadcasting networks transmitting multimedia services to mobile receivers.