12 research outputs found
Forming and confining of dipolar excitons by quantizing magnetic fields
We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum
well efficiently traps dipolar excitons and leads to the stabilization of the
excitonic formation and confinement in the illumination area. Hereby, the
density of dipolar excitons is remarkably enhanced up to . By means of Landau level spectroscopy we study the density of excess
holes in the illuminated region. Depending on the excitation power and the
applied electric field, the hole density can be tuned over one order of
magnitude up to - a value comparable with typical
carrier densities in modulation-doped structures.Comment: 4.3 Pages, 4 Figure
Long exciton spin memory in coupled quantum wells
Spatially indirect excitons in a coupled quantum well structure were studied
by means of polarization and time resolved photoluminescence. A strong degree
of circular polarization (> 50%) in emission was achieved when the excitation
energy was tuned into resonance with the direct exciton state. The indirect
transition remained polarized several tens of nanoseconds after the pumping
laser pulse, demonstrating directly a very long relaxation time of exciton
spin. The observed spin memory effect exceeds the radiative lifetime of the
indirect excitons.Comment: 4 pages, 2 figure
Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves
Individual self-assembled Quantum Dots and Quantum Posts are studied under
the influence of a surface acoustic wave. In optical experiments we observe an
acoustically induced switching of the occupancy of the nanostructures along
with an overall increase of the emission intensity. For Quantum Posts,
switching occurs continuously from predominantely charged excitons (dissimilar
number of electrons and holes) to neutral excitons (same number of electrons
and holes) and is independent on whether the surface acoustic wave amplitude is
increased or decreased. For quantum dots, switching is non-monotonic and shows
a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of
positively charged and neutral excitons is observed at high surface acoustic
wave amplitudes. These findings are explained by carrier trapping and
localization in the thin and disordered two-dimensional wetting layer on top of
which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts
where acoustically induced charge transport is highly efficient in a wide
lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure
Blunted endocrine response to a combined physical-cognitive stressor in adults with early life adversity
The negative health effects of early life adversity (ELA) continue long into adulthood. Changes in the physiological response to psychosocial stressors have been proposed to mediate this effect. However, many previous studies have come to contradicting conclusions as to whether ELA induces a long-term increase or decrease in stress reactivity. Therefore, we tested the association of ELA exposure and adult stress reactivity in a sample of early life adoptees and controls.Two previously validated stressful elements (bilateral feet CPT and the Paced Auditory Serial Addition Task (PASAT)) were combined in an extended Cold Pressor Test (CPT). This test was performed on 22 participants who had experienced severe ELA (separation from biological parents, institutionalization, and adoption in early childhood), and in 22 age-matched control participants.A prior history of ELA was associated with blunted reactivity of the hypothalamic-pituitary-adrenal (HPA) axis (CohenÂŽs dâŻ=âŻ0.680). Cardiovascular reactivity remained unchanged, and affective reactivity (self-report ratings) were increased in participants exposed to ELA compared to the control group (range CohenÂŽs d: 0.642â0.879).Our results suggest that the activity of the HPA axis reactivity was inhibited in ELA participants. Importantly, cardiovascular stress responsiveness was not affected by ELA. This separation of the HPA axis and cardiovascular stress responses may best be explained by ELA selectively enhancing central feedback-sensitivity to glucocorticoids, but preserving cardiovascular/ autonomic stress reactivity
Evaluating an Accelerometer-Based System for Spine Shape Monitoring
In western societies a huge percentage of the population suffers from some kind of back pain at least once in their life. There are several approaches addressing back pain by postural modifications. Postural training and activity can be tracked by various wearable devices most of which are based on accelerometers. We present research on the accuracy of accelerometer-based posture measurements. To this end, we took simultaneous recordings using an optical motion capture system and a system consisting of five accelerometers in three different settings: On a test robot, in a template, and on actual human backs. We compare the accelerometer-based spine curve reconstruction against the motion capture data. Results show that tilt values from the accelerometers are captured highly accurate, and the spine curve reconstruction works well
The indirect CO2 emission implications of energy system pathways: Linking IO and TIMES models for the UK
Radical changes to the current national energy systems â including energy efficiency and the decarbonisation of electricity â will be required in order to meet challenging carbon emission reduction commitments. Technology explicit energy system optimisation models (ESOMs) are widely used to define and assess such low-carbon pathways, but these models only account for the emissions associated with energy combustion and either do not account for or do not correctly allocate emissions arising from infrastructure, manufacturing, construction and transport associated with energy technologies and fuels. This paper addresses this shortcoming, through a hybrid approach that estimates the upstream CO2 emissions across current and future energy technologies for the UK using a multi-regional environmentally extended input output model, and explicitly models the direct and indirect CO2 emissions of energy supply and infrastructure technologies within a national ESOM (the UK TIMES model). Results indicate the large significance of non-domestic indirect emissions, particularly coming from fossil fuel imports, and finds that the marginal abatement cost of mitigating all emissions associated with UK energy supply is roughly double that of mitigating only direct emissions