Effect of the Pauli principle on photoelectron spin transport in p+p^+ GaAs

In p+ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly-focussed light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about $2 \; \mu m$ from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photo-electron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and bandgap renormalization are negligible due to electrostatic screening by the hole gas

Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density

Carrier and spin recombination are investigated in p-type GaAs of acceptor concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly neutral and photoelectrons can either recombine with holes bound to acceptors (e-A0 line) or form excitons which are mostly trapped on neutral acceptors forming the (A0X) complex. It is found that the spin lifetime is shorter for electrons that recombine through the e-A0 transition due to spin relaxation generated by the exchange scattering of free electrons with either trapped or free holes, whereas spin flip processes are less likely to occur once the electron forms with a free hole an exciton bound to a neutral acceptor. An increase of exci- tation power induces a cross-over to a regime where the bimolecular band-to-band (b-b) emission becomes more favorable due to screening of the electron-hole Coulomb interaction and ionization of excitonic complexes and free excitons. Then, the formation of excitons is no longer possible, the carrier recombination lifetime increases and the spin lifetime is found to decrease dramatically with concentration due to fast spin relaxation with free photoholes. In this high density regime, both the electrons that recombine through the e-A0 transition and through the b-b transition have the same spin relaxation time.Comment: 4 pages, 5 figure

Absence of an intrinsic value for the surface recombination velocity in doped semiconductors

A self-consistent expression for the surface recombination velocity $S$ and the surface Fermi level unpinning energy as a function of light excitation power ($P$) is presented for n- and p-type semiconductors doped above the 10$^{16}$ cm$^{-3}$ range. Measurements of $S$ on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface $S$ is described by a power law in $P$ whereas for a passivated surface $S^{-1}$ varies logarithmically with $P$. Furthermore, the variation in $S$ with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that $S$ depends on $P$ throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of $S$ on a range of semiconducting materials are thus only valid for a specific excitation power.Comment: 10 pages, 7 figure

Exploring the best practices of the Youth for Environment in Schools – Organization (YES-O)

Increasing community awareness and knowledge about environmental problems is the primary goal of environmental education. This study explores the best practices of the Youth for Environment in Schools-Organization (YES-O) in selected schools in the National Capital Region, Philippines. A qualitative study involving interviews and focus group discussions directed the collection and analysis of data. The study discovered that the implementation of the YES-O program was successful due to the following best practices: (a) personal advocacy, (b) resource availability, (c) flexibility, (d) integrated school efforts, (e) participatory approach, and (f) monitoring and evaluation strategies, which were all supported by the emerging themes. Meanwhile, capacity building and strong networks emerged as important themes supporting the YES-O program implementation in secondary schools. The schools can continue their practices to sustain the implementation of the program. However, schools should take into account certain indicators that require improvement to enhance their performance. These may include strengthening their networking, linkages, and partnership strategies to better engage and synergize environmental education given the mandated programs, projects, and activities of the YES-O program implementation

Optical test of the DS1 prototype concentrating surface

This paper describes the optical test of the DS1 prototype concentrating surface carried out by CTAER. The DS1 is a parabolic Stirling dish developed under the framework of “SOLARDIS” project. The aim of this investigation was to characterize the DS1 prototype optical parameters. For this purpose the real and the theoretical flux distribution was calculated on a target placed at the focal length and a comparison between them reported about the value of some relevant parameters. The theoretical flux distribution was obtained by photogrammetry technique and ray tracing tools; the real flux distribution was measured by photographic flux mapping technique of lunar images. The results comparison showed that the dish surface had an average optical error of 2.5mrad (it includes errors due to deviation of surface normal vector and the effect of the specularity, but it does not include the sun shape error) and an estimated spillage value of 7%, for this geometry

Optical test of the DS1 prototype concentrating surface

This paper describes the optical test of the DS1 prototype concentrating surface carried out by CTAER. The DS1 is a parabolic Stirling dish developed under the framework of “SOLARDIS” project. The aim of this investigation was to characterize the DS1 prototype optical parameters. For this purpose the real and the theoretical flux distribution was calculated on a target placed at the focal length and a comparison between them reported about the value of some relevant parameters. The theoretical flux distribution was obtained by photogrammetry technique and ray tracing tools; the real flux distribution was measured by photographic flux mapping technique of lunar images. The results comparison showed that the dish surface had an average optical error of 2.5mrad (it includes errors due to deviation of surface normal vector and the effect of the specularity, but it does not include the sun shape error) and an estimated spillage value of 7%, for this geometry

Decolorization of Synthetic Textile Dyes by Fungal Endophytes Isolated from the Leaves of Philippine Mangrove (Avicennia marina)

Textile dyes in wastewater can be harmful pollutants when released into the environment without treatment. Biodegradation of textile dye effluents by different microbes, including fungi, has become popular as an alternative to physicochemical methods. The mangrove Avicennia marina is known to harbor endophytic fungi which have the potential to carry out dye degradation. Therefore, this study assessed the ability to decolorize synthetic dyes of endophytic fungi isolated from the leaves of A. marina. Of the nine fungal endophytes, Aspergillus niger, Syncephalastrum racemosum and Penicillium citrinum exhibited the highest mycelial growths in solid media, while all endophytes adsorbed Congo red. Through liquid decolorization assay, four isolates decolorized Congo red at greater than 89% decolorization rates. P. citrinum (55.45%), Mycelia sterilia (85.19%), A. flavus (44.91%) showed the highest decolorization rates of Methylene blue, Malachite green and Rhodamine B, respectively. The ligninolytic enzymes produced by the endophytic fungi, laccase exhibited the highest activity with values higher than the positive control

Exploratory Investigation on the Occurrence, Spatial Distribution, and Risk Factors of Selected Zoonotic Enteropathogens in Davao City Backyard Farms

The swine industry is the second largest economic contributor to Philippine agriculture and is dominated by backyard farms, which are plagued by outdated management practices and poor animal health support that promote the spread of pathogens. Zoonotic enteropathogens pose a public health threat, especially to backyard farmers who have daily close contact with the infected animals and their waste. Hence, there is a need to survey such pathogens. This exploratory study generated baseline information on enteropathogen occurrence in backyard farms of Davao City, Philippines; the spatial distribution of affected farms; and the risk factors for enteropathogen occurrence. Protozoans such as Blastocystis sp., Balantidium coli, Entamoeba sp., Iodamoeba sp., Giardia sp., and coccidia, while helminths such as hookworm and strongylids were identified by direct wet smear. Rotavirus A was detected by reverse transcription-nested polymerase chain reaction. Almost 73% of the farms harbored enteropathogens with mostly asymptomatic infections, and weaners and growers are major carriers. Geospatial analysis identified Barangay Bato in Toril District as a hotspot for the pathogens. Probit regression analysis revealed that use of treatments increased the likelihood of pathogen occurrence by 24%, possibly due to misapplication of medications such as anthelmintics. On the other hand, there was 40% reduced likelihood for farms that use traditional feeds, which can promote gut immunity. Therefore, high-fiber diet can be explored for broad-spectrum protection against enteropathogens. Promoting awareness on the benefits of traditional feeds and education on the proper use of medication are also recommended, especially for vulnerable farms in hotspot areas