Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Excitons in undoped AlGaAs/GaAs wide parabolic quantum wells

In this work the electronic structure of undoped AlGaAs/GaAs wide parabolic quantum wells (PQWs) with different well widths (1000 and 3000 ) were investigated by means of photoluminescence (PL) measurements. Due to the particular potential shape, the sample structure confines photocreated carriers with almost three-dimensional characteristics. Our data show that depending on the well width thickness it is possible to observe very narrow structures in the PL spectra, which were ascribed to emissions associated to the recombination of confined 1s-excitons of the parabolic potential wells. From our measurements, the exciton binding energies (of a few meV) were estimated. Besides the exciton emission, we have also observed PL emissions associated to electrons in the excited subbands of the PQWs. © 2010 IOP Publishing Ltd.

Analysis of confinement potential fluctuation and band-gap renormalization effects on excitonic transition in GaAs/AlGaAs multiquantum wells grown on (100) and (311)A GaAs surfaces

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/AlxGa1-xAs quantum wells grown on [1 0 0] and [3 1 1]A GaAs substrates is evaluated. The results clearly demonstrate the coexistence of the band-tail states filling related to potential fluctuations and the band-gap renormalization caused by an increase in the density of photogenerated carriers during the photoluminescence (PL) experiments. Both phenomena have strong influence on temperature dependence of the PL-peak energy (E-PL(T)). As the photon density increases, the E-PL can shift to either higher or lower energies, depending on the sample temperature. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density. A simple band-tail model with a Gaussian-like distribution of the density of state was used to describe the competition between the band-tail filling and the BGR effects on E-PL(T). (C) 2012 Elsevier B.V. All rights reserved.

Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al0.35Ga0.65As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel. (C) 2011 Elsevier B.V. All rights reserved.

Constraining age distributions of groundwater from public supply wells in diverse hydrogeological settings in Scania, Sweden

Twenty-five public supply wells throughout the hydrogeologically diverse region of Scania, southern Sweden are subjected to environmental tracer analysis (³H���³He,⁴He, CFCs, SF₆ and for one well only also ⁸⁵Kr and ³⁹Ar) to study well and aquifer vulnerability and evaluate possibilities of groundwater age distribution assessment. We find CFC and SF₆ concentrations well above solubility equilibrium with modern atmosphere, indicating local contamination, as well as indications of CFC degradation. The tracer-specific complications considerably constrain possibilities for sound quantitative regional ground- water age distribution assessment and demonstrate the importance of initial qualitative assessment of tracer-specific reliability, as well a need for additional, complementary tracers (e.g. ⁸⁵Kr,³⁹Ar and potentially also ¹⁴C). Lumped parameter modelling yields credible age distribution assessments for representative wells in four type aquifers. Pollution vulnerability of the aquifer types was based on the selected LPM models and qualitative age characterisation. Most vulnerable are unconfined dual porosity and fractured bedrock aquifers, due to a large component of very young groundwater. Unconfined sedimentary aquifers are vulnerable due to young groundwater and a small pre-modern component. Less vulnerable are semi-confined sedimentary or dual-porosity aquifers, due to older age of the modern component and a larger pre-modern component. Confined aquifers appear least vulnerable, due an entirely pre-modern groundwater age distribution (recharged before 1963). Tracer complications aside, environmental tracer analyses and lumped parameter modelling aid in vulnerability assessment and protection of regional groundwater resources.

Spatial distribution of orofacial cleft birth defects and uranium-radium concentrations in tap water in Atascosa, Bee, Brooks, Calhoun, Duval, Goliad, Hidalgo, Jim Hogg, Jim Wells, Karnes, Kleberg, Live Oak, McMullen, Nueces, San Patricio, Refugio, Starr, Victoria, Webb, and Zavala Counties, Texas

Background Past and recent evidence shows that radionuclides in drinking water may be a public health concern. Developmental thresholds for birth defects with respect to chronic low level domestic radiation exposures, such as through drinking water, have not been definitely recognized, and there is a strong need to address this deficiency in information. In this study we examined the geographic distribution of orofacial cleft birth defects in and around uranium mining district Counties in South Texas (Atascosa, Bee, Brooks, Calhoun, Duval, Goliad, Hidalgo, Jim Hogg, Jim Wells, Karnes, Kleberg, Live Oak, McMullen, Nueces, San Patricio, Refugio, Starr, Victoria, Webb, and Zavala), from 1999 to 2007. The probable association of cleft birth defect rates by ZIP codes classified according to uranium and radium concentrations in drinking water supplies was evaluated. Similar associations between orofacial cleft birth defects and radium/radon in drinking water were reported earlier by Cech and co-investigators in another of the Gulf Coast region (Harris County, Texas).50, 55 Since substantial uranium mining activity existed and still exists in South Texas, contamination of drinking water sources with radiation and its relation to birth defects is a ground for concern. ^ Methods Residential addresses of orofacial cleft birth defect cases, as well as live births within the twenty Counties during 1999-2007 were geocoded and mapped. Prevalence rates were calculated by ZIP codes and were mapped accordingly. Locations of drinking water supplies were also geocoded and mapped. ZIP codes were stratified as having high combined uranium (≥30μg/L) vs. low combined uranium (<30μg/L). Likewise, ZIP codes having the uranium isotope, Ra-226 in drinking water, were also stratified as having elevated radium (≥3 pCi/L) vs. low radium (<3 pCi/L). A linear regression was performed using STATA® generalized linear model (GLM) program to evaluate the probable association between cleft birth defect rates by ZIP codes and concentration of uranium and radium via domestic water supply. These rates were further adjusted for potentially confounding variables such as maternal age, education, occupation, and ethnicity. ^ Results This study showed higher rates of cleft births in ZIP codes classified as having high combined uranium versus ZIP codes having low combined uranium. The model was further improved by adding radium stratified as explained above. Adjustment for maternal age and ethnicity did not substantially affect the statistical significance of uranium or radium concentrations in household water supplies. ^ Conclusion Although this study lacks individual exposure levels, the findings suggest a significant association between elevated uranium and radium concentrations in tap water and high orofacial birth defect rates by ZIP codes. Future case-control studies that can measure individual exposure levels and adjust for contending risk factors could result in a better understanding of the exposure-disease association.^

Tab. 1 Mole % CO2 and 13C values of CO2 at baseline and at the final monitoring event for the eight observation wells

During CO2 storage operations in mature oilfields or saline aquifers it is desirable to trace the movement of injected CO2 for verification and safety purposes. We demonstrate the successful use of carbon isotope abundance ratios for tracing the movement of CO2 injected at the Cardium CO2 Storage Monitoring project in Alberta between 2005 and 2007. Injected CO2 had a d13C value of -4.6±1.1 per mil that was more than 10 per mil higher than the carbon isotope ratios of casing gas CO2 prior to CO2 injection with average d13C values ranging from -15.9 to -23.5 per mil. After commencement of CO2 injection, d13C values of casing gas CO2 increased in all observation wells towards those of the injected CO2 consistent with a two-source end-member mixing model. At four wells located in a NE-SW trend with respect to the injection wells, breakthrough of injected CO2 was registered chemically (>50 mol % CO2) and isotopically 1-6 months after commencement of CO2 injection resulting in cumulative CO2 fluxes exceeding 100000 m**3 during the observation period. At four other wells, casing gas CO2 contents remained below 5 mol % resulting in low cumulative CO2 fluxes (<2000 m**3) throughout the entire observation period, but carbon isotope ratios indicated contributions between <30 and 80% of injected CO2. Therefore, we conclude that monitoring the movement of CO2 in the injection reservoir with geochemical and isotopic techniques is an effective approach to determine plume expansion and to identify potential preferential flow paths provided that the isotopic composition of injected CO2 is constant and distinct from that of baseline CO2.

The life and letters of Samuel Wells Williams, L.L.D., missionary, diplomatist, sinologue.

Mode of access: Internet.

Remains of Rev. Joshua Wells Downing...

Mode of access: Internet.

Wells of discontent, a study of the economic, social, and political aspects of the chain store,

Mode of access: Internet.

A Cochrane review (update 2004): Hylan G-F 20 vs placebo for knee OA

Aim of study: As part of a Cochrane review of viscosupplementation in knee OA, randomised controlled trials (RCT) were reviewed to evaluate evidence for the efficacy of viscosupplementation with Hylan G-F 20 compared to placebo. Methods: Electronic searches were conducted of MEDLINE, EMBASE, Premedline, Current Contents, and CENTRAL. Human, RCT involving Hylan G-F 20 compared to placebo, published prior to 1Q2004, were included. Trials were selected and data extracted by two independent reviewers. Methodological quality was assessed with the Jadad criteria by two reviewers. Data on the OARSI and OMERACT core set clinical outcome measures were extracted where possible. Weighted mean difference (WMD), based on post-test scores, and 95% confidence intervals (CI) were calculated for continuous outcome measures and relative risk (RR) for dichotomous outcome measures. Results: Seven RCT met the inclusion criteria. Median methodological quality was 4 (range 1–5). A further two studies were only reported in abstract form (Jadad score Z 1) and contained insufficient extractable data for inclusion in the analysis. Nine RCT, which compared Hylan G-F 20 to other interventions such as intra-articular corticosteroid, physiotherapy, NSAID, appropriate care, intra-articular gaseous oxygen and other hyaluronan, are not reported here. Twenty-three studies failed to meet inclusion criteria and were excluded. Hylan G-F 20 was more efficacious than placebo at 1–4 weeks post-injection for pain on weight-bearing WMD (random effects [RE]) 13 mm on a 0–100 mm VAS (P Z 0.002) based on 6 RCT. This difference was even greater at 5–13 weeks post-injection, 22 m (RE) (P Z 0.001) based on 5 RCT, and at 14–6 weeks postinjection, 21 m (RE) (P Z 0.006) based on 4 RCT. Hylan G-F 20 was more efficacious than placebo at 1–4 weeks post-injection for pain at night, WMD 7 mm on a 0–100 mm VAS (P Z 0.003) based on 5 RCT. This difference was even greater at 5–13 weeks post-injection, 11 mm (P Z 0.008) based on 4 RCT, and at 14–26 weeks post-injection, 17 mm (P ! 0.00001) based on 3 RCT. There was no significant difference (WMD 8 mm) between Hylan G-F 20 C oral placebo and arthrocentesis C oral placebo at 5–13 weeks post-injection for WOMAC Pain, but Hylan G-F 20 C oral placebo was more efficacious than arthrocentesis C oral placebo for WOMAC Function, WMD 9 mm on a 0–100 mm VAS (P Z 0.01) (Dickson, 2001). Hylan G-F 20 was more effective than placebo at 1–4 weeks postinjection for the variable designed treatment efficacy, WMD 22 mm on a 0–100 mm VAS (P ! 0.00001) based on improvement in 4 RCT. This difference was even greater at 5–13 weeks post injection, 35 mm (P ! 0.00001). Conclusions: Evidence from this updated Cochrane review supports the superior efficacy of Hylan G-F 20 compared to placebo on weight-bearing pain, night pain, function and treatment efficacy in the treatment of knee OA.