University of Stuttgart

About: University of Stuttgart is a education organization based out in Stuttgart, Germany. It is known for research contribution in the topics: Laser & Finite element method. The organization has 27715 authors who have published 56370 publications receiving 1363382 citations. The organization is also known as: Universität Stuttgart.

Photon statistics of semiconductor microcavity lasers.

Abstract: We present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.

Structure of nanometer-sized polycrystalline iron investigated by positron lifetime spectroscopy.

Abstract: Nanometer-sized polycrystalline materials are polycrystals prepared by compacting very small crystallites (5--10 nm in diameter) under high pressures. The initial studies of Gleiter and co-workers indicate a wide distribution of interatomic distances within the disordered intercrystalline phase, which can be investigated accurately due to its high relative volume fraction in these materials. In the present paper the investigation of nanometer-sized Fe polycrystals by positron lifetime spectroscopy is reported. The influence of the compacting pressure and thermal annealing was studied. The positron lifetimes ${\ensuremath{\tau}}_{1}$=180\ifmmode\pm\else\textpm\fi{}15 ps, ${\ensuremath{\tau}}_{2}$=360\ifmmode\pm\else\textpm\fi{}30 ps, and long-lived components between 1 and 5 ns, have been observed with saturation trapping of positrons. These values are different from the positron lifetimes in well-annealed bulk iron, in amorphous iron alloys, or in the uncompacted fine nanometer-sized iron crystals (\ensuremath{\tau}=443 ps). Based on the present results the lifetime ${\ensuremath{\tau}}_{1}$ in nanometer-sized Fe polycrystals is attributed to positron trapping in vacancy-size free volumes in the crystallite interfaces. This is in agreement with the hypothesis of an interfacial structure with a wide distribution of interatomic distances. The lifetime ${\ensuremath{\tau}}_{2}$ is ascribed to positron annihilation in microvoids at the intersections of interfaces. Hence, positron lifetime spectroscopy on nanometer-sized polycrystalline materials appears to supply a specific tool for studying the interfacial structure of solids. The long-lived components indicate ortho-positronium (o-Ps) formation in larger voids.

Long-term research challenges in wind energy – a research agenda by the European Academy of Wind Energy

Abstract: . The European Academy of Wind Energy (eawe), representing universities and institutes with a significant wind energy programme in 14 countries, has discussed the long-term research challenges in wind energy. In contrast to research agendas addressing short- to medium-term research activities, this eawe document takes a longer-term perspective, addressing the scientific knowledge base that is required to develop wind energy beyond the applications of today and tomorrow. In other words, this long-term research agenda is driven by problems and curiosity, addressing basic research and fundamental knowledge in 11 research areas, ranging from physics and design to environmental and societal aspects. Because of the very nature of this initiative, this document does not intend to be permanent or complete. It shows the vision of the experts of the eawe, but other views may be possible. We sincerely hope that it will spur an even more intensive discussion worldwide within the wind energy community.

Model of a PT -symmetric Bose-Einstein condensate in a δ -function double-well potential

Abstract: The observation of $\mathcal{PT}$ symmetry in a coupled optical waveguide system that involves a complex refractive index has been demonstrated impressively in the experiment by R\"uter et al. [Nat. Phys. 6, 192 (2010)]. This is, however, only an optical analog of a quantum system, and it would be highly desirable to observe the manifestation of $\mathcal{PT}$ symmetry and the resulting properties also in a real, experimentally accessible, quantum system. Following a suggestion by Klaiman et al. [Phys. Rev. Lett. 101, 080402 (2008)], we investigate a $\mathcal{PT}$-symmetric arrangement of a Bose-Einstein condensate in a double-well potential, where in one well cold atoms are injected while in the other particles are extracted from the condensate. We investigate, in particular, the effects of the nonlinearity in the Gross-Pitaevskii equation on the $\mathcal{PT}$ properties of the condensate. To study these effects we analyze a simple one-dimensional model system in which the condensate is placed into two $\mathcal{PT}$-symmetric $\ensuremath{\delta}$-function traps. The analysis will serve as a useful guide for studies of the behavior of Bose-Einstein condensates in realistic $\mathcal{PT}$-symmetric double wells.