ALBERT

Notes: A model is proposed for the description of the current instabilities in GaAs-AlGaAs heterojunctions. It consists of three parts: the injection of electrons via the contact into the AlGaAs layer, the partial capture of these electrons in deep centers, and the change with time of the band structure. This last ingredient is crucial, since due to the increase of the total number of electrons in the AlGaAs layer the band bending decreases making real-space transfer from the AlGaAs layer to the two-dimensional electron gas possible. We have performed quasistationary simulations of the time dependence of the current. The velocities, average energies, capture rates, etc. were taken from Monte Carlo simulations. It turned out, that the parameters for the modeling of the contact, which are to a high degree unknown, play an essential role. © 1996 American Institute of Physics.

Notes: The ultrafast carrier dynamics in the high electric field at an Au-GaAs interface has been studied experimentally as well as theoretically. The photoluminescence decay time is related directly to the carrier sweepout from the GaAs depletion region, i.e., to the time needed for photoexcited electrons and holes to leave this region. This decay time has been found to increase drastically with laser input power, ranging from a few picoseconds at low excitation to values of 10–20 ps at high excitation. These results indicate a significant retardation of the sweepout, which cannot be explained by intervalley scattering and space-charge effects. From our Monte Carlo calculations it has been found that the applied electric field collapses totally almost instantaneously after laser excitation due to the enormous excess of photoexcited charges. The sweepout only recovers after some time needed to recharge the device. © 1996 American Institute of Physics.

Notes: Abstract Pore water and solid phase distributions of C, N, P and Si in sediments of the Arctic Ocean (Svalbard area) have been investigated. Concentrations of organic carbon (Corg) in the solid phase of the sediment varied from 1.3 to 2.8% (mean 1.9%), with highest concentrations found at shallow stations south/southwest of Svalbard. Relatively low concentrations were obtained at the deeper stations north/northeast of Svalbard. Atomic carbon to nitrogen ratios in the surface sediment ranged from below 8 to above 10. For some stations, high C/N ratios together with high concentrations of Corg suggest that sedimentary organic matter is mainly of terrigenous origin and not from overall biological activity in the water column. Organic matter reactivity (defined as the total sediment oxygen consumption rate normalized to the organic carbon content of the surface sediment) correlated with water depth at all investigated stations. However, the stations could be divided into two separate groups with different reactivity characteristics, representing the two most dominant hydrographic regimes: the region west of Svalbard mainly influenced by the West Spitsbergen Current, and the area east of Svalbard where Arctic polar water set the environmental conditions. Decreasing sediment reactivity with water depth was confirmed by the partitioning between organic and inorganic carbon of the surface sediment. The ratio between organic and inorganic carbon at the sediment-water interface decreased exponentially with water depth: from indefinite values at shallow stations in the central Barents Sea, to approximately 1 at deep stations north of Svalbard. At stations east of Svalbard there was an inverse linear correlation between the organic matter reactivity (as defined above) and concentration of dissolved organic carbon (DOC) in the pore water. The more reactive the sediment, the less DOC existed in the pore water and the more total carbonate (Ct or ΣCO2) was present. This observation suggests that DOC produced in reactive sediments is easily metabolizable to CO2. Sediment accumulation rates of opaline silica ranged from 0.35 to 5.7 µmol SiO2 m−2d−1 (mean 1.3 µmol SiO2 m−2d−1), i.e. almost 300 times lower than rates previously reported for the Ross Sea, Antarctica. Concentrations of ammonium and nitrate in the pore water at the sediment-water interface were related to organic matter input and water depth. In shallow regions with highly reactive organic matter, a pool of ammonium was present in the pore water, while nitrate conoentrations were low. In areas where less reactive organic matter was deposited at the sediment surface, the deeper zone of nitrification caused a build-up of nitrate in the pore water while ammonium was almost depleted. Nitrate penetrated from 1.8 to ≥ 5.8 cm into the investigated sediments. Significantly higher concentrations of “total” dissolved nitrogen (defined as the sum of NO3, NO2, NH4 and urea) in sediment pore water were found west compared to east of Svalbard. The differences in organic matter reactivity, as well as in pore water distribution patterns of “total” dissolved nitrogen between the two areas, probably reflect hydrographic factors (such as ice coverage and production/import of particulate organic material) related to the dominant water mass (Atlantic or Arctic Polar) in each of the two areas.

Notes: Abstract: The optimum suction distribution which gives the longest laminar region for a given total suction is computed. The goal here is to provide the designer with a method of finding the best suction distribution subject to some overall constraints applied to the suction. We formulate the problem using the Lagrangian multiplier method with constraints. The resulting nonlinear system of equations is solved using the Newton–Raphson technique. The computations are performed for a Blasius boundary layer on flat-plate and crossflow cases. For the Blasius boundary layer, the optimum suction distribution peaks upstream of the maximum growth rate region and remains flat in the middle before it decreases to zero at the end of the transition point. For the stationary and travelling crossflow instability, the optimum suction peaks upstream of the maximum growth rate region and decreases gradually to zero.

Notes: Abstract: The response of the fastest growing linear Görtler vortex to wall suction or blowing is investigated. The fastest growing mode occurs in flows of high local curvature and we show how its growth may be inhibited using very small amounts of suction or blowing. We show how the suction or blowing should be distributed in order to minimize the growth of a disturbance over a specified interval.