General description of quasi-adiabatic dynamical phenomena near exceptional points

The appearance of so-called exceptional points in the complex spectra of non-Hermitian systems is often associated with phenomena that contradict our physical intuition. One example of particular interest is the state-exchange process predicted for an adiabatic encircling of an exceptional point. In this work we analyse this and related processes for the generic system of two coupled oscillator modes with loss or gain. We identify a characteristic system evolution consisting of periods of quasi-stationarity interrupted by abrupt non-adiabatic transitions, and we present a qualitative and quantitative description of this switching behaviour by connecting the problem to the phenomenon of stability loss delay. This approach makes accurate predictions for the breakdown of the adiabatic theorem as well as the occurrence of chiral behavior observed previously in this context, and provides a general framework to model and understand quasi-adiabatic dynamical effects in non-Hermitian systems.Comment: Main text: 9 pages; 5 figures. Appendix: 4 pages; 1 figur

PLoS ONE / Exposure to indoor allergens in different residential settings and its influence on IgE sensitization in a geographically confined Austrian cohort

Background Exposure to indoor allergens is crucial for IgE sensitization and development of allergic symptoms. Residential settings influence the allergen amount in house dust and hence allergic sensitization. Within this study, we investigated allergen exposure and molecule-based IgE levels in a geographically confined region and evaluated the impact of housing, pets and cleaning. Methods 501 adolescents from Salzburg, Austria participated in this cross-sectional study. House dust samples were examined regarding major mite, cat, dog, and mold allergens using a multiplex assay. Serum samples of participants were analyzed for specific IgE to Der p 1, Der p 2, Fel d 1, Can f 1 and Alt a 1 using the multiplex array ImmunoCAP ISAC. Information on allergies, living areas, dwelling form (house, flat, farm), pets, and household cleanliness were obtained by a questionnaire. Results In investigated house dust samples, the concentration of cat allergen was highest while the prevalence of mold allergens was very low. Participants showed IgE sensitization to Der p 1 (13.2%), Der p 2 (18.2%), Fel d 1 (14.4%), Can f 1 (2.4%) and Alt a 1 (2.0%). In alpine regions, lower mite allergen concentrations were detected which correlated with reduced IgE levels. A trend for increased sensitization prevalence from rural to alpine to urban regions was noted. Living on farms resulted in lower sensitization prevalence to mite and cat allergens, even though exposure to mites was significantly elevated. The presence of cats was associated with a lower sensitization rate and IgE levels to cat and mite allergens, and less frequent allergic diseases. Cleaning did not impact allergen concentrations, while IgE reactivity to mites and allergic diseases were more pronounced when living in cleaner homes. Conclusion Allergen exposure to indoor allergens was influenced by setting of homes. Living in a farm environment and having a cat at home showed a pro tective effect for IgE sensitization and allergies. This cross-sectional study in combination with hereditary and lifestyle factors enables development of risk schemes for a more efficient management and potential prevention of allergic diseases

Encircling of exceptional points : theory and applications

Zusammenfassung in deutscher SpracheExceptional points (EPs) bezeichnen Entartungen, die im Spektrum von nicht- hermiteschen Hamilton Operatoren auftreten und die in letzter Zeit für einiges an Aufsehen gesorgt haben, da sie für viele faszinierende und kontraintuitive Phänomene verantwortlich sind. Einer der wohl verblüffendsten Effekte in diesem Zusammenhang tritt auf, wenn ein EP mittels einer zeitlichen Variation der Systemparameter dynamisch umrundet wird. Dies führt zu einem chiralen Verhalten, bei dem der Endzustand nur durch die Umrundungsrichtung um den EP bestimmt wird. Trotz erheblichem Forschungsaufwand und einer Vielzahl an praktischen Anwendungsmöglichkeiten konnte solch ein Protokoll jedoch noch nicht erfolgreich in einem Experiment umgesetzt werden, da die experimentelle Implementierung sehr anspruchsvoll ist. In der vorliegenden Arbeit wird beschrieben, wie die dynamische Umrundung eines EPs auf die Transmission zweier Moden in einem randmodulierten Mikrowellen Wellenleiter übertragen werden kann, in dem sich ein räumlich variierender Absorber befindet. Dadurch erhält man einen asymmetrischen Modenselektor, der eine spezifische transversale Wellenleiter-Mode rein basierend auf der Seite auswählt, von der aus die Wellen in den Wellenleiter eingespeist werden. Die obige Vorgehensweise ermöglicht uns, in Zusammenarbeit mit Kollegen der Universität Nizza, nicht nur die erste experimentelle Realisierung einer dynamischen EP Umrundung, sondern dient auch als Machbarkeitsnachweis für spezifische Anwendungen, wie zum Beispiel der Verallgemeinerung der weit verbreiteten "Rapid Adiabatic Passage" Technik. Weiters zeigen wir mit numerischen Simulationen und mit analytischen Mitteln, wie neuartige Reflexionsresonanzen den Transport in Wellenleitern mit Oberflächenrauhigkeit entscheidend beeinflussen können. Diese Resonanzen führen zu einer um Größenordnungen verstärkten Rückstreuung und sind deshalb von besonderem Interesse, um Wellenleiter mit spezifischen Transporteigenschaften zu konstruieren.Exceptional points (EPs), degeneracies arising in the spectrum of non-Hermitian Hamiltonians, have attracted considerable attention in the physics community since they are the source of many fascinating and counter-intuitive phenomena. One of the most intriguing effects inherent to EPs is predicted when such a degeneracy is dynamically encircled by way of a smooth temporal variation of the system's parameters, leading to a chiral behavior for which the final state is solely determined by the direction one chooses for the round-trip. However, despite a substantial research effort and the prospect for interesting practical applications, an experiment implementing this protocol was not yet realized due to the challenging experimental requirements that are involved in a successful demonstration of this effect. In this thesis, we describe how to map the temporal dynamics in the encircling of an EP onto the transmission of two modes in a boundary modulated microwave waveguide featuring a spatially varying absorber. We thereby obtain an asymmetric mode-switching device that selects a specific transverse waveguide mode based on the direction in which the waves are injected into this waveguide. In collaboration with our colleagues from Nice University, our research not only lead to the first experimental realization of a dynamical EP encircling, but also provides proof of our concept's potential for applications, and may be seen as a generalization of already existing switching protocols like the rapid adiabatic passage technique. Furthermore, we provide numerical as well as analytical evidence for novel reflection resonances occurring in surface-corrugated waveguides. These resonances lead to an order-of-magnitude enhancement of the waveguide's reflectance and are of special interest for imprinting specific transport properties onto a waveguide.11

DFT studies of graphene intercalation systems : graphene/Ni/Ir and graphene/Ag/Re

Zsfassung in dt. SpracheInterkalation von Ni unter Graphen/Ir(111). Um einen Einblick in die Interkalation von Metallschichten zwischen Graphen (Gr) und einem tragenden Substrat zu erhalten untersuchen wir Diffusionsprozesse eines einzelnen Nickel Atoms durch eine auf Ir(111) adsorbierte Graphen Schicht. Die Interkalation wird numerisch mit Hilfe des ab-initio DFT Codes VASP für verschiedene Realisierungen des Systems untersucht, angefangen mit der Diffusion eines Ni Atoms durch eine freie Graphen Lage. Wir betrachten sowohl ideales Graphen als auch eine Graphen Schicht mit Fehlstellen. Der Prozess selbst wird untersucht indem zunächst der vertikale Abstand des Ni Atoms von der Graphen Oberfläche sukzessive verringert wird. In jedem Schritt wird eine volle Relaxation der benachbarten C Atome zugelassen. Die auf diesem Wege erhaltenen Daten von Energie vs. Reaktionskoordinate ermöglichen in weiterer Folge die Bestimmung des Übergangszustandes mittels der Dimer-Methode, für welche VASP einen systematischen Algorithmus bereitstellt um Sattelpunkte auf der hochdimension- alen Energieoberfläche zu finden. Die Geometrie des Übergangszustandes wird dann auch anschließend auf die Ir(111) Oberfläche transferiert. Unsere Untersuchungen zeigen, dass für die Diffusion durch eine ideale, freie Graphen Schicht eine Energiebarriere von 12 eV überwunden werden muss, ein Wert der sehr hoch ist und daher einen sehr unwahrscheinlichen Mechanismus darstellt. Ist jedoch eine Fehlstelle im Graphen vorhanden, wird die Energiebarriere auf 2 eV reduziert. Eine weitere Reduktion um 0.5 eV wird erreicht sobald diese Graphen Schicht auf das Ir(111) Substrat aufgebracht wird.Graphen/Ag(111)/Re(0001). Photoelektronenspektroskopie (engl. angle-resolved photoemission spectroscopy - ARPES) an Graphen/Re sowie an Graphen/Ag/Re, das eine zusätzliche interkalierte Monolage Ag aufweist, deutet darauf hin dass die Entkopplung von Graphen (Gr) vom Rhenium Substrat keineswegs so ausgeprägt ist wie erwartet. Weiters zeigt das Experiment, dass die Ag Lage epitaktisch aufwächst und eine Hybridisierung der Ag Bänder mit dem Graphen Pi-Zustand zeigt. Wir unter- suchen die elektronische Struktur von Gr/Ag/Re mittels numerischen ab-initio DFT Rechnungen unter der Verwendung von VASP. Wir ermitteln die Eigenschaften dieses Materials in einem vereinfachten Modell, indem wir eine primitive (1x1) Zelle verwenden. Die unterschiedlichen Gitterkonstanten von Graphen bzw.Rhenium machen es nötig, zwei gesonderte Fälle zu betrachten, nämlich entweder die Graphen Lage zu strecken bzw. das Ag/Re Substrat zu komprimieren. In diesen Systemen betrachten wir verschiedene Grenzflächen-Geometrien sowie Graphen-Substrat Abstände um die energetisch günstigste Lage der Kohlenstoff Atome zum Substrat zu bestimmen. Dabei finden wir, dass die top-fcc Struktur, d.h. ein C Atom über (top) einem Ag Atom und das andere in der fcc Lücke, diejenige mit der stabilsten energetischen Konfiguration ist. Aufgrund der Wechselwirkung mit dem Ag/Re Substrat wird die elektronische Struktur von Graphen modifiziert, was in guter Übereinstimmung mit dem oben genannten ARPES Experiment ist. Wir finden eine Absenkung des Dirac Punktes bezüglich der Fermi Energie und weiters eine Hybridisierung der Ag d-Bänder mit Graphens Pi-Band. Beide Phänomene hängen, wie gezeigt wird, stark vom Graphen-Substrat Abstand ab.Intercalation of Ni under graphene/Ir(111). To gain insight into the intercalation of metal layers in-between graphene (Gr) and its supporting substrate, we investigate the diffusion processes of a single nickel atom through a graphene sheet which is adsorbed on Ir(111). Utilizing the ab-initio DFT code VASP, the intercalation is assessed numerically for different instances of the system, starting with diffusion through an unsupported graphene sheet.Both an ideal graphene layer as well as a layer featuring a vacant carbon site are taken into account. The process is modelled by tuning the vertical distance of the Ni atom with respect to the surface in each numerical run, followed by a full relaxation of the nearest carbon atoms. The energy vs. reaction coordinate data is then used to determine the transition state of the particular reaction in more detail by means of the dimer method, for which VASP offers a systematic approach for finding saddle points on high-dimensional potential energy surfaces and thus the reaction mechanism. Having obtained the transition state, the corresponding cell geometry is placed on an Ir(111) support. We find that, if assessing the data for unsupported graphene layers, the diffusion through a pristine sheet is facing a barrier of about 12 eV, which is very large and extremely unlikely to be overcome at typical annealing temperatures. However, at a vacancy, this barrier is lowered significantly to about 2 eV. A further decrease of approximately 0.5 eV is obtained if the vacant graphene is placed on top of the Ir(111) substrate.Graphene/Ag(111)/Re(0001). Data from angle-resolved photoemission spectra (ARPES) on graphene/Re and graphene/Ag/Re, which features an additional intercalated Ag monolayer, demonstrates that the decoupling of graphene (Gr) from the Re substrate is not as pronounced as expected as soon as Ag is introduced.The Ag layer grows epitaxially, and there is evidence of a hybridization of the Ag bands with graphene's Pi-state. We study the electronic structure of Gr/Ag(111)/Re(0001) by means of numerical ab-initio DFT calculations employing the VASP code. The properties of Gr/Ag/Re are determined in a simplified approach by studying a primitive (1x1) cell for both the lattice constant of Re and Gr, i.e. either stretching Gr or compressing the underlying Ag/Re substrate, respectively. Different interface models and graphene-substrate distances were tested to determine the energetically most favorable placement of the carbon atoms relative to the substrate surface, yielding the top-fcc arrangement, i.e. one C on top of Ag and the other on the fcc hollow site, as the structure featuring the lowest energy. Due to interaction with the Ag/Re substrate, the electronic structure of graphene is modified, showing good agreement with the above mentioned ARPES experiments. We observe a downward shift of the Dirac point with respect to the Fermi energy and also a hybridization of the graphene Pi-band with the Ag d-bands. The magnitude of both phenomena is shown to depend on the graphene-substrate distance.7

Beyond the settlement grid: investigating social differences through archaeobiology in waterlogged sites

Waterlogged sites represent an invaluable source of archaeological data. Houses dated to exact calendar years by dendrochronology yield countless artefacts and well preserved organic remains. In 150 years of research, a wealth of economic, environmental and chronological information on the circumalpine Neolithic and Bronze Age has been accumulated. The social historical potential of these sites has however been largely neglected, which is in part due to widely held preconceptions on prevailing social conditions drawn from common knowledge rather than research. Due to uniformly large houses arranged in rows, communities are generally perceived as being egalitarian and economically uniform. In an interdisciplinary case study of the Swiss Arbon Bleiche 3 settlement on Lake Constance, the vast potential of the archaeobiological data from waterlogged sites for investigating social issues is explored. Statistical analyses of animal bones and botanical remains reveal several distinct economic strategies and/or dietary preferences, suggesting the existence of a socially diverse settlement community. Our results not only generate multifaceted social data but also contradict a number of preconceptions on lakeside communities. Methodologically, it becomes clear that archaeozoological and archaeobotanical remains are differentially biased by taphonomic processes and sampling strategies. These systematic differences will have to be addressed in further studies

Coping with crises I : subsistence variety and resilience in the Late Neolithic lakeshore settlement Arbon Bleiche 3 (Switzerland)

The site of Arbon Bleiche 3 is located at Lake Constance, canton Thurgau, Switzerland. About half of the original settlement surface was excavated between 1993 and 1995. Due to the waterlogged conditions, the archaeological sediments and the organic remains are very well preserved. Dendrochronological analyses allowed the precise dating of residential structures and the reconstruction of the settlement history in the excavated area. In−depth interdisciplinary analyses of the layer formation processes showed that hardly any horizontal or vertical mixing took place. The settlement is therefore particularly well suited for studying spatial patterns andsocio−economic strategies, in both individual houses and the settlement as a whole. Besides excellent preservation conditions, a short, single−phased occupation (3384−3370 BC), which ended in a devastating fire makes the site particularly valuable, as we can contrast the snapshot captured in the burnt remains with the contents of the cultural layer accumulated during the 15−year occupation. Based on the rich faunal remains from Arbon Bleiche 3 we suggest the existence of a broad and resilient subsistence diversity, which allowed the inhabitants to successfully cope with economic difficulties that probably occured during the final years of the settlement

Die Rolle der Experimentellen Archäologie in systemdynamischen Modellierungen zu neolithischen Feuchtbodensiedlungen

Neolithikum - Modell - Feuchtbodensiedlung - experimentelle Archäologie - Sozialstruktur - Arbeitsteilung - Subsisten