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Search Results: 1 - 10 of 101123 matches for " U. W. Pohl "
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InGaAs/GaAs Quantum Dots for 1.3 μm Applications
U. W. Pohl, A. Schliwa. I. Kaiander, T. Germann, A. Strittmatter, and D. Dimberg
OAtube Nanotechnology , 2008,
Abstract: Zero-dimensional charge carrier localization in the active region of a semiconductor laser was predicted two decades ago to lead to improved device performance. Self-organized growth of quantum dots (QDs) has since then evolved into the decisive method for defect-free QD fabrication to realize such localization, and the first QD injection laser demonstrated the basic validity of previous predictions [1]. Much effort was subsequently spend to extend the emission wavelength of In(Ga)As QDs in GaAs matrix to the datacom range at 1.3 μm. The basic approach aims at decreasing the energy of the dot’s electronic ground state by lowering the hydrostatic strain exerted on the dot by the matrix. Model calculations proved that strain release induced by a thin InGaAs layer with a lower In content on top of the QDs significantly decreases the ground state energy [2]. Combination of such strain-reducing layer (SRL) with a similar, additional layer underneath the dots leads to the dot-in-a-well (DWELL) approach which was successfully applied using molecular beam epitaxy to fabricate GaAs-based QD devices emitting at 1.3 μm. QD devices reaching this target have been fabricated only very recently using metalorganic vapor phase epitaxy (MOVPE) with its scaling ability for mass production. In the presentation concepts for growing InGaAs dots for 1.3 μm emission are discussed and encouraging latest results are presented. We used tertiarybutylarsine as a favorable arsenic precursor [3] and an individual adjustment of growth parameters within the stack of active QDs in a laser [4]. PL is used as a monitor to identify critical growth parameters. Data indicate a crucial role of the V/III ratio applied during growth [5]. InGaAs/GaAs QDs with a strain-reducing layer grown using a low V/III ratio show a robust thermal stability. The good performance is promising to open a way for 1.3 μm device fabrication using metalorganic vapor phase epitaxy.
Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots
R. Seguin,A. Schliwa,S. Rodt,K. P?tschke,U. W. Pohl,D. Bimberg
Physics , 2006, DOI: 10.1103/PhysRevLett.95.257402
Abstract: A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 $\mu$eV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k.p theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.
Size-dependence of anisotropic exchange interaction in InAs/GaAs quantum dots
R. Seguin,S. Rodt,A. Schliwa,K. P?tschke,U. W. Pohl,D. Bimberg
Statistics , 2006, DOI: 10.1002/pssb.200671519
Abstract: A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to the energetic structure of the charged excited exciton (hot trion). A varying degree of intermixing within the hot trion states leads to varying degrees of polarization of the corresponding emission lines. The emission characteristics change from circularly polarized for small quantum dots to elliptically polarized for large quantum dots. The findings are explained by a change of magnitude of the anisotropic exchange interaction and compared to the related effect of fine-structure splitting in the neutral exciton and biexciton emission.
Ex-situ control of fine-structure splitting and excitonic binding energies in single InAs/GaAs quantum dots
R. Seguin,A. Schliwa,T. D. Germann,S. Rodt,K. P?tschke,U. W. Pohl,D. Bimberg
Physics , 2006, DOI: 10.1063/1.2730192
Abstract: A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. We are able to record single QD cathodoluminescence spectra and trace the evolution of one and the same QD over several steps of annealing. A systematic reduction of the excitonic fine-structure splitting is reported. In addition the binding energies of different excitonic complexes change dramatically. The results are interpreted in terms of a change of electron and hole wavefunction shape and mutual position.
Precipitation observation using microwave backhaul links in the alpine and pre-alpine region of Southern Germany
C. Chwala, A. Gmeiner, W. Qiu, S. Hipp, D. Nienaber, U. Siart, T. Eibert, M. Pohl, J. Seltmann, J. Fritz,H. Kunstmann
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2012,
Abstract: Measuring rain rates over complex terrain is afflicted with large uncertainties, because rain gauges are influenced by orography and weather radars are mostly not able to look into mountain valleys. We apply a new method to estimate near surface rain rates exploiting attenuation data from commercial microwave links in the alpine region of Southern Germany. Received signal level (RSL) data are recorded minutely with small data loggers at the towers and then sent to a database server via GSM (Global System for Mobile Communications). Due to the large RSL fluctuations in periods without rain, the determination of attenuation caused by precipitation is not straightforward. To be able to continuously process the RSL data from July 2010 to October 2010, we introduce a new method to detect wet and dry periods using spectral time series analysis. Its performance and limitations are presented, showing that the mean detection error rates of wet and dry periods can be reduced to 10% for all five links. After, the wet/dry classification rain rates are derived from the RSL and compared to rain gauge and weather radar measurements. The resulting correlations differ for different links and reach values of R2 = 0.81 for the link-gauge comparison and R2 = 0.85 for the link-radar comparison.
Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots
R. Seguin,A. Schliwa,T. D. Germann,S. Rodt,M. Winkelnkemper,K. P?tschke,A. Strittmatter,U. W. Pohl,T. Hammerschmidt,P. Kratzer,D. Bimberg
Physics , 2007, DOI: 10.1063/1.2424446
Abstract: A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within eight-band k.p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.
Automated sliding susceptibility mapping of rock slopes
A. Günther,A. Carstensen,W. Pohl
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2004,
Abstract: We present a suite of extensions for ARCVIEW GIS (ESRI) that allows to map the spatial distribution of first-order mechanical slope-properties in hard rock terrain, e.g. for large slope areas like water reservoir slopes. Besides digital elevation data, this expert-system includes regional continuous grid-based data on geological structures that might act as potential sliding or cutoff planes for rockslides. The system allows rapid automated mapping of geometrical and kinematical slope properties in hard rock, providing the basis for spatially distributed deterministic sliding-susceptibility evaluations on a pixel base. Changing hydrostatic slope conditions and rock mechanical parameters can be implemented and used for simple predictive static stability calculations. Application is demonstrated for a study area in the Harz Mts., Germany.
Quantum magnetism and topological ordering via enhanced Rydberg-dressing near F?rster-resonances
R. M. W. van Bijnen,T. Pohl
Physics , 2014, DOI: 10.1103/PhysRevLett.114.243002
Abstract: We devise a cold-atom approach to realizing a broad range of bi-linear quantum magnets. Our scheme is based on off-resonant single-photon excitation of Rydberg $P$-states (Rydberg-dressing), whose strong interactions are shown to yield controllable XYZ-interactions between effective spins, represented by different atomic ground states. The distinctive features of F\"orster-resonant Rydberg atom interactions are exploited to enhance the effectiveness of Rydberg-dressing and, thereby, yield large spin-interactions that greatly exceed corresponding decoherence rates. We illustrate the concept on a spin-1 chain implemented with cold Rubidium atoms, and demonstrate that this permits the dynamical preparation of topological magnetic phases. Generally, the described approach provides a viable route to exploring quantum magnetism with dynamically tuneable (an)isotropic interactions as well as variable space- and spin-dimensions in cold-atom experiments.
Research Proposal for an Experiment to Search for the Decay μ -> eee
A. Blondel,A. Bravar,M. Pohl,S. Bachmann,N. Berger,M. Kiehn,A. Sch?ning,D. Wiedner,B. Windelband,P. Eckert,H. -C. Schultz-Coulon,W. Shen,P. Fischer,I. Peri?,M. Hildebrandt,P. -R. Kettle,A. Papa,S. Ritt,A. Stoykov,G. Dissertori,C. Grab,R. Wallny,R. Gredig,P. Robmann,U. Straumann
Physics , 2013,
Abstract: We propose an experiment (Mu3e) to search for the lepton flavour violating decay mu+ -> e+e-e+. We aim for an ultimate sensitivity of one in 10^16 mu-decays, four orders of magnitude better than previous searches. This sensitivity is made possible by exploiting modern silicon pixel detectors providing high spatial resolution and hodoscopes using scintillating fibres and tiles providing precise timing information at high particle rates.
Effects of inspiratory muscle training on dynamic hyperinflation in patients with COPD
Petrovic M, Reiter M, Zipko H, Pohl W, Wanke T
International Journal of Chronic Obstructive Pulmonary Disease , 2012, DOI: http://dx.doi.org/10.2147/COPD.S23784
Abstract: ts of inspiratory muscle training on dynamic hyperinflation in patients with COPD Original Research (1696) Total Article Views Authors: Petrovic M, Reiter M, Zipko H, Pohl W, Wanke T Published Date November 2012 Volume 2012:7 Pages 797 - 805 DOI: http://dx.doi.org/10.2147/COPD.S23784 Received: 29 June 2011 Accepted: 20 August 2012 Published: 30 November 2012 Milos Petrovic,1 Michael Reiter,2 Harald Zipko,3 Wolfgang Pohl,1 Theodor Wanke1 1Pulmonary Department and Karl Landsteiner Institute for Clinical and Experimental Pulmology, Hietzing Hospital, Vienna, Austria; 2Department of Respiratory and Critical Care Medicine, Otto Wagner Hospital, Vienna, Austria; 3FH Campus Vienna, University of Applied Sciences – Health Department, Vienna, Austria Abstract: Dynamic hyperinflation has important clinical consequences in patients with chronic obstructive pulmonary disease (COPD). Given that most of these patients have respiratory and peripheral muscle weakness, dyspnea and functional exercise capacity may improve as a result of inspiratory muscle training (IMT). The aim of the study was to analyze the effects of IMT on exercise capacity, dyspnea, and inspiratory fraction (IF) during exercise in patients with COPD. Daily inspiratory muscle strength and endurance training was performed for 8 weeks in 10 patients with COPD GOLD II and III. Ten patients with COPD II and III served as a control group. Maximal inspiratory pressure (Pimax) and endurance time during resistive breathing maneuvers (tlim) served as parameter for inspiratory muscle capacity. Before and after training, the patients performed an incremental symptom limited exercise test to maximum and a constant load test on a cycle ergometer at 75% of the peak work rate obtained in the pretraining incremental test. ET was defined as the duration of loaded pedaling. Following IMT, there was a statistically significant increase in inspiratory muscle performance of the Pimax from 7.75 ± 0.47 to 9.15 ± 0.73 kPa (P < 0.01) and of tlim from 348 ± 54 to 467 ± 58 seconds (P < 0.01). A significant increase in IF, indicating decreased dynamic hyperinflation, was observed during both exercise tests. Further, the ratio of breathing frequency to minute ventilation (bf/V'E) decreased significantly, indicating an improved breathing pattern. A significant decrease in perception of dyspnea was also measured. Peak work rate during the incremental cycle ergometer test remained constant, while ET during the constant load test increased significantly from 597.1 ± 80.8 seconds at baseline to 733.6 ± 74.3 seconds (P < 0.01). No significant changes during either exercise tests were measured in the control group. The present study found that in patients with COPD, IMT results in improvement in performance, exercise capacity, sensation of dyspnea, and improvement in the IF prognostic factor.
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