Theoretical and experimental study of the excitonic binding energy in GaAs/AlGaAs single and coupled double quantum wells

This paper discusses the theoretical and experimental results obtained for the excitonic binding energy (Eb) in a set of single and coupled double quantum wells (SQWs and CDQWs) of GaAs/AlGaAs with different Al concentrations (Al%) and inter-well barrier thicknesses. To obtain the theoretical Eb the method proposed by Mathieu, Lefebvre and Christol (MLC) was used, which is based on the idea of fractional-dimension space, together with the approach proposed by Zhao et al., which extends the MLC method for application in CDQWs. Through magnetophotoluminescence (MPL) measurements performed at 4 K with magnetic fields ranging from 0 T to 12 T, the diamagnetic shift curves were plotted and adjusted using two expressions: one appropriate to fit the curve in the range of low intensity fields and another for the range of high intensity fields, providing the experimental Eb values. The effects of increasing the Al% and the inter-well barrier thickness on Eb are discussed. The Eb reduction when going from the SQW to the CDQW with 5 Å inter-well barrier is clearly observed experimentally for 35% Al concentration and this trend can be noticed even for concentrations as low as 25% and 15%, although the Eb variations in these latter cases are within the error bars. As the Zhao's approach is unable to describe this effect, the wave functions and the probability densities for electrons and holes were calculated, allowing us to explain this effect as being due to a decrease in the spatial superposition of the wave functions caused by the thin inter-well barrier.

Morphology and surface sedimentation on the upper and middle continental slope off Uruguay

This paper presents for the first time a morphological and surface sediment characterization of the Uruguayan outer continental shelf and slope. The study is based on a high-resolution coverage using hydrographical, geomorphological and sedimentological sampling and several textural and productivity proxies. Along slope terraces and an important canyon system characterizes continental slope morphology, indicating that across- and down-slope sedimentary processes control large-scale sedimentation. Terraces represent the prolongation of the Argentinean Contouritic Depositional System that vanishes in the study area, presumably as a result of the dynamic of the Brazil-Malvinas confluence. Canyons incised in the upper slope are likely related to low-stand sea level conditions. At the outer shelf and shallow upper slope (170-250 m depth), off-shelf sand transport is inferred from the distribution of relict sand and reworked biogenic gravel. In the upper continental slope, the northern region is characterized by an erosive environment controlled by a steep slope and the southward flowing Brazil current. In the south, a depositional environment is enhanced by the presence of a gentler slope and seaward incised canyons and is mainly controlled by hemipelagic processes associated with nutrient-rich Sub-Antarctic Waters (SAW), by its confluence with South Atlantic Central Waters (SACW) and by the Rio de la Plata’s (RdlP) influence. Additionally, within the upper slope, the occurrence of igneous-metamorphic cobbles and pebbles in canyon and mound lag deposits suggests the influence of glacial fluvial discharge and/or iceberg transport processes. In the middle slope, sedimentation is controlled by thermohaline-induced deep-water bottom currents. The decreasing influence of the erosive Antarctic Intermediate Water (AAIW) is evident in a northward diminution in grain size. The variety of transport and sedimentary processes identified reflect the control of the Brazil-Malvinas confluence zone and the Rio de la Plata’s discharge.