4,973 research outputs found
Tuning of structure inversion asymmetry by the -doping position in (001)-grown GaAs quantum wells
Structure and bulk inversion asymmetry in doped (001)-grown GaAs quantum
wells is investigated by applying the magnetic field induced photogalvanic
effect. We demonstrate that the structure inversion asymmetry (SIA) can be
tailored by variation of the delta-doping layer position. Symmetrically-doped
structures exhibit a substantial SIA due to impurity segregation during the
growth process. Tuning the SIA by the delta-doping position we grow samples
with almost equal degrees of structure and bulk inversion asymmetry.Comment: 4 pages 2 figure
Drift mobility of long-living excitons in coupled GaAs quantum wells
We observe high-mobility transport of indirect excitons in coupled GaAs
quantum wells. A voltage-tunable in-plane potential gradient is defined for
excitons by exploiting the quantum confined Stark effect in combination with a
lithographically designed resistive top gate. Excitonic photoluminescence
resolved in space, energy, and time provides insight into the in-plane drift
dynamics. Across several hundreds of microns an excitonic mobility of >10^5
cm2/eVs is observed for temperatures below 10 K. With increasing temperature
the excitonic mobility decreases due to exciton-phonon scattering.Comment: 3 pages, 3 figure
Magnetoresistance Induced by Rare Strong Scatterers in a High Mobility 2DEG
We observe a strong negative magnetoresistance at non-quantizing magnetic
fields in a high-mobility two-dimensional electron gas (2DEG). This strong
negative magnetoresistance consists of a narrow peak around zero magnetic field
and a huge magnetoresistance at larger fields. The peak shows parabolic
magnetic field dependence and is attributed to the interplay of smooth disorder
and rare strong scatterers. We identify the rare strong scatterers as
macroscopic defects in the material and determine their density from the peak
curvature.Comment: 5 pages, 4 figure
Spin-sensitive Bleaching and Spin-Relaxation in QW's
Spin-sensitive saturation of absorption of infrared radiation has been
investigated in p-type GaAs QWs. It is shown that the absorption saturation of
circularly polarized radiation is mostly controlled by the spin relaxation time
of the holes. The saturation behavior has been investigated for different QW
widths and in dependence on the temperature with the result that the saturation
intensity substantially decreases with narrowing of QWs. Spin relaxation times
were experimentally obtained by making use of calculated (linear) absorption
coefficients for inter-subband transitions
Aluminum arsenide cleaved-edge overgrown quantum wires
We report conductance measurements in quantum wires made of aluminum
arsenide, a heavy-mass, multi-valley one-dimensional (1D) system. Zero-bias
conductance steps are observed as the electron density in the wire is lowered,
with additional steps observable upon applying a finite dc bias. We attribute
these steps to depopulation of successive 1D subbands. The quantum conductance
is substantially reduced with respect to the anticipated value for a spin- and
valley-degenerate 1D system. This reduction is consistent with
disorder-induced, intra-wire backscattering which suppresses the transmission
of 1D modes. Calculations are presented to demonstrate the role of strain in
the 1D states of this cleaved-edge structure.Comment: Submitted to Applied Physics Letter
Lunar and Mars Exploration: The Autonomy Factor
Long duration space flight crews have relied heavily on almost constant communication with ground control mission support. Ground control teams provide vehicle status and system monitoring, while offering near real time support for specific tasks, emergencies, and ensuring crew health and well being. With extended exploration goals to lunar and Mars outposts, real time communication with ground control teams and the ground s ability to conduct mission monitoring will be very limited compared to the resources provided to current International Space Station (ISS) crews. An operational shift toward more autonomy and a heavier reliance on the crew to monitor their vehicle and operations will be required for these future missions. NASA s future exploration endeavors and the subsequent increased autonomy will require a shift in crew skill composition, i.e. engineer, doctor, mission specialist etc. and lead to new training challenges and mission scenarios. Specifically, operational and design changes will be necessary in many areas including: Habitat Infrastructure and Support Systems, Crew Composition, Training, Procedures and Mission Planning. This paper will specifically address how to apply ISS lessons learned to further use ISS as a test bed to address decreased amounts of ground support to achieve full autonomous operations for lunar and Mars missions. Understanding these lessons learned and applying them to current operations will help to address the future impacts of increased crew autonomy for the lunar and Mars outposts and pave the way for success in increasingly longer mission durations
Unterschiede im Leistungsverhalten von Gesunden zwischen Fahrrad- und Laufbandergometrie
Problemstellung: Zur Überprüfung des Ausdauertrainingseffektes während einer Terrainkur ist die Laufbandergometrie der Belastung auf dem Fahrrad vorzuziehen, da hierbei dieselben Muskeln beansprucht werden wie während des Trainings. Allerdings gelingt es wegen der frühzeitigen subjektiven Erschöpfung der Kurpatienten nur selten, Milchsäurewerte oberhalb der aeroben/anaeroben Schwelle zu erzielen.
Gegenstand: Die vorliegende Studie mit Gesunden befaĂźt sich mit den Fragen, warum sich Kurpatienten auf dem Laufband metabolisch betrachtet nicht genĂĽgend ausbelasten lassen, und welches die entscheidenden Unterschiede in der Art der Belastung zwischen Laufband- und Fahrradergometer sind.
Versuchsplan: 27 gesunde Testpersonen wurden auf dem Fahrrad- und Laufbandergometer submaximal belastet. Die Belastung wurde nach einem standardisierten Verfahren in 0,33 W/kg-Schritten gesteigert. Wichtigste Meßparameter waren Herzfrequenz und Milchsäurespiegel.
Ergebnisse: Während der Laufbandergometrie lagen Herzfrequenz und Sauerstoffbedarf des Myokards zum Erreichen der gleichen Wattstufe signifikant höher (1,0 W/kg:p≤0,001; 1,33 W/kg: p≤0,01; 2,0 W/kg: n.s.; Erholung: p≤0,001) als auf dem Fahrrad. Dagegen zeigte sich bei gegebener Herzfrequenz von 130/min und bei derselben höchsten erreichten Wattstufe auf dem Laufband ein um 0,7 mmol/1 bzw. l,0mmol/l signifikant (p≤0,01) niedrigerer Laktatanstieg als auf dem Fahrradergometer.
Schlußfolgerung: Die Ergebnisse zeigten somit eine stärkere kardiale Ausschöpfung auf dem Laufbandergometer. Die aeroben/anaerobe Schwelle wird dagegen auf dem Laufband später als auf dem Fahrrad erreicht
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