TY - JOUR A1 - Steigert, Alexander A1 - Kojda, Sandrino Danny A1 - Ibaceta-Jaña, Josefa Fernanda A1 - Abou-Ras, Daniel A1 - Gunder, René A1 - Alktash, Nivin A1 - Habicht, Klaus A1 - Wagner, Markus Raphael A1 - Klenk, Reiner A1 - Raoux, Simone A1 - Szyszka, Bernd A1 - Lauermann, Iver A1 - Muydinov, Ruslan T1 - Water-assisted crystallization of amorphous indium zinc oxide films JF - Materials today. Communications N2 - Transparent conductive materials based on indium oxide remain yet irreplaceable in various optoelectronic applications. Amorphous oxides appear especially attractive for technology as they are isotropic, demonstrate relatively high electron mobility and can be processed at low temperatures. Among them is indium zinc oxide (IZO) with a large zinc content that is crucial for keeping the amorphous state but redundant for the doping. In this work we investigated water-free and water containing IZO films obtained by radio frequency sputtering. The correlation between temperature driven changes of the chemical state, the optical and electrical properties as well as the progression of crystallization was in focus. Such characterization methods as: scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, temperature dependent Hall-effect measurements and others were applied. Temperature dependent electrical properties of amorphous IZO and IZO:H2O films were found to evolve similarly. Based on our experience in In2O3:H2O (In2O3:H or IOH) we proposed an explanation for the changes observed. Water admixture was found to decrease crystallization temperature of IZO significantly from similar to 550 degrees C to similar to 280 degrees C. Herewith, the presence and concentration of water and/or hydroxyls was found to determine Zn distribution in the film. In particular, Zn enrichment was detected at the film's surface respective to the high water and/or hydroxyl amount. Raman spectra revealed a two-dimensional crystallization of w-ZnO which precedes regardless water presence an extensive In2O3 crystallization. An abrupt loss of electron mobility as a result of crystallization was attributed to the formation of ZnO interlayer on grain boundaries. KW - IZO KW - Thin films KW - TCOs KW - Crystallization KW - Water-assisted crystallization Y1 - 2022 U6 - https://doi.org/10.1016/j.mtcomm.2022.103213 SN - 2352-4928 VL - 31 PB - Elsevier CY - Amsterdam ER - TY - THES A1 - Steppa, Constantin Beverly T1 - Modelling the galactic population of very-high-energy gamma-ray sources T1 - Modellierung der Population galaktischer Quellen von sehr hochenergetischer Gammastrahlung N2 - The current generation of ground-based instruments has rapidly extended the limits of the range accessible to us with very-high-energy (VHE) gamma-rays, and more than a hundred sources have now been detected in the Milky Way. These sources represent only the tip of the iceberg, but their number has reached a level that allows population studies. In this work, a model of the global population of VHE gamma-ray sources based on the most comprehensive census of Galactic sources in this energy regime, the H.E.S.S. Galactic plane survey (HGPS), will be presented. A population synthesis approach was followed in the construction of the model. Particular attention was paid to correcting for the strong observational bias inherent in the sample of detected sources. The methods developed for estimating the model parameters have been validated with extensive Monte Carlo simulations and will be shown to provide unbiased estimates of the model parameters. With these methods, five models for different spatial distributions of sources have been constructed. To test the validity of these models, their predictions for the composition of sources within the sensitivity range of the HGPS are compared with the observed sample. With one exception, similar results are obtained for all spatial distributions, showing that the observed longitude profile and the source distribution over photon flux are in fair agreement with observation. Regarding the latitude profile and the source distribution over angular extent, it becomes apparent that the model needs to be further adjusted to bring its predictions in agreement with observation. Based on the model, predictions of the global properties of the Galactic population of VHE gamma-ray sources and the prospects of the Cherenkov Telescope Array (CTA) will be presented. CTA will significantly increase our knowledge of VHE gamma-ray sources by lowering the threshold for source detection, primarily through a larger detection area compared to current-generation instruments. In ground-based gamma-ray astronomy, the sensitivity of an instrument depends strongly, in addition to the detection area, on the ability to distinguish images of air showers produced by gamma-rays from those produced by cosmic rays, which are a strong background. This means that the number of detectable sources depends on the background rejection algorithm used and therefore may also be increased by improving the performance of such algorithms. In this context, in addition to the population model, this work presents a study on the application of deep-learning techniques to the task of gamma-hadron separation in the analysis of data from ground-based gamma-ray instruments. Based on a systematic survey of different neural-network architectures, it is shown that robust classifiers can be constructed with competitive performance compared to the best existing algorithms. Despite the broad coverage of neural-network architectures discussed, only part of the potential offered by the application of deep-learning techniques to the analysis of gamma-ray data is exploited in the context of this study. Nevertheless, it provides an important basis for further research on this topic. N2 - Die aktuelle Generation bodengestützter Instrumente hat die Grenzen des uns mit sehr hoch-energetischer (very-high-energy, VHE) Gammastrahlung zugänglichen Bereichs rasch erweitert, so dass inzwischen bereits mehr als hundert Quellen in der Milchstraße entdeckt wurden. Diese Quellen repräsentieren zwar nur die Spitze des Eisbergs, doch ihre Anzahl hat ein Niveau erreicht, das Populationsstudien ermöglicht. In dieser Arbeit wird ein Modell der globalen Population von VHE Gammastrahlungsquellen vorgestellt, das auf den umfassendsten Zensus galaktischer Quellen in diesem Energiebereich, dem H.E.S.S. Galactic plane survey (HGPS), beruht. Bei der Erstellung des Modells wurde ein Populationssynthese-Ansatz verfolgt. Besonderes Augenmerk wurde auf die Korrektur der starken Beobachtungsverzerrung gelegt, die der Stichprobe detektierter Quellen innewohnt. Die für die Schätzung der Modellparameter entwickelten Methoden wurden mit umfangreichen Monte-Carlo-Simulationen validiert und es wird gezeigt, dass sie akkurate Schätzungen der Modelparameter ermöglichen. Mit diesen Methoden wurden fünf Modelle für verschiedene räumliche Verteilungen von Quellen erstellt. Um die Gültigkeit dieser Modelle zu prüfen, werden ihre Vorhersagen für die Zusammensetzung der Quellen innerhalb des Sensitivitätsbereichs des HGPS mit der beobachteten Stichprobe verglichen. Mit einer Ausnahme werden für alle räumlichen Verteilungen ähnliche Ergebnisse erzielt, die zeigen, dass das beobachtete Longitudenprofil und die Quellenverteilung über den Photonenfluss gut mit der Beobachtung übereinstimmen. Bezüglich des Latitudenprofils und der Quellenverteilung über die Winkelausdehnung zeigt sich, dass das Modell weiter angepasst werden muss, um dessen Vorhersagen mit den Beobachtungen in Einklang zu bringen. Auf der Grundlage des Modells werden Vorhersagen über die globalen Eigenschaften der galaktischen Population von VHE Gammastrahlungsquellen und die Perspektiven des Cherenkov Telescope Array (CTA) vorgestellt. CTA wird unser Wissen über VHE Gammastrahlungsquellen erheblich erweitern, indem es die Detektionsschwelle für die Quellen senkt, vor allem durch einer im Vergleich zu Instrumenten der aktuellen Generation größeren Detektionsfläche. In der bodengebundenen Gammastrahlenastronomie hängt die Empfindlichkeit eines Instruments neben der Detektionsfläche jedoch auch stark von der Fähigkeit ab, Bilder von Luftschauern, die durch Gammastrahlen erzeugt werden, von denen zu unterscheiden, die durch kosmische Strahlung erzeugt werden und einen starken Hintergrund darstellen. Dies bedeutet, dass die Anzahl der detektierbaren Quellen von dem verwendeten Algorithmus zur Hintergrundunterdrückung abhängt und daher möglicherweise auch durch eine Verbesserung der Leistung solcher Algorithmen erhöht werden kann. In diesem Zusammenhang wird in dieser Arbeit zusätzlich zum Populationsmodell eine Studie über die Anwendung von Deep-Learning-Techniken für die Aufgabe der Gamma-Hadron-Trennung bei der Analyse von Daten von bodengestützten Gammastrahleninstrumenten vorgestellt. Auf der Grundlage einer systematischen Untersuchung verschiedener neuronaler Netzwerkarchitekturen wird gezeigt, dass robuste Klassifikatoren konstruiert werden können, die im Vergleich zu den besten bestehenden Algorithmen eine konkurrenzfähige Leistung aufweisen. Trotz des Umfangs der diskutierten neuronalen Netzwerkarchitekturen wird im Rahmen dieser Studie nur ein Teil des Potenzials ausgeschöpft, das die Anwendung von Deep-Learning-Techniken für die Analyse von Daten in der Gammaastronomie bietet. Dennoch bietet sie eine wichtige Grundlage für weitere Forschungen zu diesem Thema. KW - gamma astronomy KW - galactic population KW - very-high energy KW - Gammaastronomie KW - galaktische Population KW - sehr hohe Energien Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-549478 ER - TY - JOUR A1 - Stojkoski, Viktor A1 - Jolakoski, Petar A1 - Pal, Arnab A1 - Sandev, Trifce A1 - Kocarev, Ljupco A1 - Metzler, Ralf T1 - Income inequality and mobility in geometric Brownian motion with stochastic resetting: theoretical results and empirical evidence of non-ergodicity JF - Philosophical transactions of the Royal Society A: Mathematical, physical and engineering sciences N2 - We explore the role of non-ergodicity in the relationship between income inequality, the extent of concentration in the income distribution, and income mobility, the feasibility of an individual to change their position in the income rankings. For this purpose, we use the properties of an established model for income growth that includes 'resetting' as a stabilizing force to ensure stationary dynamics. We find that the dynamics of inequality is regime-dependent: it may range from a strictly non-ergodic state where this phenomenon has an increasing trend, up to a stable regime where inequality is steady and the system efficiently mimics ergodicity. Mobility measures, conversely, are always stable over time, but suggest that economies become less mobile in non-ergodic regimes. By fitting the model to empirical data for the income share of the top earners in the USA, we provide evidence that the income dynamics in this country is consistently in a regime in which non-ergodicity characterizes inequality and immobility. Our results can serve as a simple rationale for the observed real-world income dynamics and as such aid in addressing non-ergodicity in various empirical settings across the globe.This article is part of the theme issue 'Kinetic exchange models of societies and economies'. KW - income inequality KW - income mobility KW - geometric Brownian motion KW - non-ergodicity KW - stochastic resetting Y1 - 2022 U6 - https://doi.org/10.1098/rsta.2021.0157 SN - 1364-503X SN - 1471-2962 VL - 380 IS - 2224 PB - Royal Society CY - London ER - TY - JOUR A1 - Stojkoski, Viktor A1 - Sandev, Trifce A1 - Kocarev, Ljupco A1 - Pal, Arnab T1 - Autocorrelation functions and ergodicity in diffusion with stochastic resetting JF - Journal of physics : A, Mathematical and theoretical N2 - Diffusion with stochastic resetting is a paradigm of resetting processes. Standard renewal or master equation approach are typically used to study steady state and other transport properties such as average, mean squared displacement etc. What remains less explored is the two time point correlation functions whose evaluation is often daunting since it requires the implementation of the exact time dependent probability density functions of the resetting processes which are unknown for most of the problems. We adopt a different approach that allows us to write a stochastic solution for a single trajectory undergoing resetting. Moments and the autocorrelation functions between any two times along the trajectory can then be computed directly using the laws of total expectation. Estimation of autocorrelation functions turns out to be pivotal for investigating the ergodic properties of various observables for this canonical model. In particular, we investigate two observables (i) sample mean which is widely used in economics and (ii) time-averaged-mean-squared-displacement (TAMSD) which is of acute interest in physics. We find that both diffusion and drift-diffusion processes with resetting are ergodic at the mean level unlike their reset-free counterparts. In contrast, resetting renders ergodicity breaking in the TAMSD while both the stochastic processes are ergodic when resetting is absent. We quantify these behaviors with detailed analytical study and corroborate with extensive numerical simulations. Our results can be verified in experimental set-ups that can track single particle trajectories and thus have strong implications in understanding the physics of resetting. KW - autocorrelations KW - ergodicity KW - diffusion KW - stochastic resetting Y1 - 2022 U6 - https://doi.org/10.1088/1751-8121/ac4ce9 SN - 1751-8113 SN - 1751-8121 VL - 55 IS - 10 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Stoll, Andreas T1 - Advanced spectroscopic instruments enabled by integrated optics N2 - The aim of this work is the study of silica Arrayed Waveguide Gratings (AWGs) in the context of applications in astronomy. The specific focus lies on the investigation of the feasibility and technology limits of customized silica AWG devices for high resolution near-infrared spectroscopy. In a series of theoretical and experimental studies, AWG devices of varying geometry, foot-print and spectral resolution are constructed, simulated using a combination of a numerical beam propagation method and Fraunhofer diffraction and fabricated devices are characterized with respect to transmission efficiency, spectral resolution and polarization sensitivity. The impact of effective index non-uniformities on the performance of high-resolution AWG devices is studied numerically. Characterization results of fabricated devices are used to extrapolate the technology limits of the silica platform. The important issues of waveguide birefringence and defocus aberration are discussed theoretically and addressed experimentally by selection of an appropriate aberration-minimizing anastigmatic AWG layout structure. The drawbacks of the anastigmatic AWG geometry are discussed theoretically. From the results of the experimental studies, it is concluded that fabrication-related phase errors and waveguide birefringence are the primary limiting factors for the growth of AWG spectral resolution. It is shown that, without post-processing, the spectral resolving power is phase-error-limited to R < 40, 000 and, in the case of unpolarized light, birefringence-limited to R < 30, 000 in the AWG devices presented in this work. Necessary measures, such as special waveguide geometries and post-fabrication phase error correction are proposed for future designs. The elimination of defocus aberration using an anastigmatic AWG geometry is successfully demonstrated in experiment. Finally, a novel, non-planar dispersive in-fibre waveguide structure is proposed, discussed and studied theoretically. N2 - Das Ziel der vorliegenden Arbeit ist die Untersuchung von Arrayed Waveguide Gratings (AWGs) auf SiO2-Basis im Kontext von Anwendungen in der Astronomie. Der besondere Fokus liegt auf der Untersuchung der Machbarkeit und der technologischen Grenzen von spezialisierten AWGs für die hochauflösende Nahinfrarot-Spektroskopie. In einer Reihe von theoretischen und experimentellen Studien werden AWGs unterschiedlicher Geometrie, Größe und spektraler Auflösung konstruiert, unter Verwendung einer numerischen Beam-Propagation-Methode (BPM) und Fraunhofer-Beugung simuliert und hinsichtlich Effizienz, spektraler Auflösung und Polarisationsempfindlichkeit charakterisiert. Der Einfluss von Variationen des effektiven Brechungsindex auf die Leistung von hochauflösenden AWGs wird numerisch untersucht. Mit hergestellten AWGs gewonnene Messergebnisse werden verwendet, um die technologischen Grenzen der SiO2-Plattform zu extrapolieren. Die relevanten Probleme der Polarisationsempfindlichkeit und der in kanonischen Rowland-Geometrien auftretenden Defokusaberration werden theoretisch diskutiert und experimentell durch die Auswahl einer geeigneten aberrationsminimierenden AWG-Struktur angegangen. Die Nachteile anastigmatischer AWGs werden theoretisch diskutiert. Aus den Ergebnissen der experimentellen Studien wird geschlossen, dass herstellungsbedingte Phasenfehler und Wellenleiter-Doppelbrechung die primären begrenzenden Faktoren für die Erhöhung der AWG-Spektralauflösung sind. Es wird gezeigt, dass das spektrale Auflösungsvermögen der in dieser Arbeit präsentierten AWGs ohne Phasenfehlerkorrektur auf R < 40, 000 und durch Doppelbrechung auf R < 30, 000 im Fall unpolarisierten Lichtes begrenzt ist. Notwendige Maßnahmen wie spezielle Wellenleitergeometrien und Phasenfehlerkorrektur nach der Herstellung werden für zukünftige AWG-Designs empfohlen. Die Eliminierung von Defokussierungsfehlern unter Verwendung einer anastigmatischen AWG-Geometrie wird erfolgreich im Experiment demonstriert. Schließlich wird eine neuartige, in eine optische Faser eingebettete, nicht-planare dispersive Wellenleiterstruktur vorgeschlagen, diskutiert und theoretisch untersucht. KW - Astrophotonics KW - Integrated spectrograph KW - planar lightwave circuit KW - arrayed waveguide grating KW - silica-on-silicon KW - Astrophotonik KW - integrierter Spektrograph KW - planare Lichtwellenleiter KW - Siliziumdioxid-auf-Silizium Y1 - 2022 ER - TY - JOUR A1 - Stolterfoht, Martin A1 - Lang, Felix T1 - All-perovskite tandems get flexible JF - Nature energy N2 - Flexible all-perovskite tandem photovoltaics open up new opportunities for application compared to rigid devices, yet their performance lags behind. Now, researchers show that molecule-bridged interfaces mitigate charge recombination and crack formation, improving the efficiency and mechanical reliability of flexible devices. Y1 - 2022 U6 - https://doi.org/10.1038/s41560-022-01087-6 SN - 2058-7546 VL - 7 IS - 8 SP - 688 EP - 689 PB - Nature Publishing Group CY - London ER - TY - JOUR A1 - Sun, Bowen A1 - Sandberg, Oskar A1 - Neher, Dieter A1 - Armin, Ardalan A1 - Shoaee, Safa T1 - Wave optics of differential absorption spectroscopy in thick-junction organic solar cells BT - optical artifacts and correction strategies JF - Physical review applied / The American Physical Society N2 - Differential absorption spectroscopy techniques serve as powerful techniques to study the excited species in organic solar cells. However, it has always been challenging to employ these techniques for characterizing thick-junction organic solar cells, especially when a reflective top contact is involved. In this work, we present a detailed and systematic study on how a combination of the presence of the interference effect and a nonuniform charge-distribution profile, severely manipulates experimental spectra and the decay dynamics. Furthermore, we provide a practical methodology to correct these optical artifacts in differential absorption spectroscopies. The results and the proposed correction method generally apply to all kinds of differential absorption spectroscopy techniques and various thin-film systems, such as organics, perovskites, kesterites, and two-dimensional materials. Notably, it is found that the shape of differential absorption spectra can be strongly distorted, starting from 150-nm active-layer thickness; this matches the thickness range of thick-junction organic solar cells and most perovskite solar cells and needs to be carefully considered in experiments. In addition, the decay dynamics of differential absorption spectra is found to be disturbed by optical artifacts under certain conditions. With the help of the proposed correction formalism, differential spectra and the decay dynamics can be characterized on the full device of thin-film solar cells in transmission mode and yield accurate and reliable results to provide design rules for further progress. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevApplied.17.054016 SN - 2331-7019 VL - 17 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Sushch, Iurii A1 - Brose, Robert A1 - Pohl, Martin A1 - Plotko, Pavlo A1 - Das, Samata T1 - Leptonic nonthermal emission from supernova remnants evolving in the circumstellar magnetic field JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The very-high-energy (VHE; E > 100 GeV) gamma-ray emission observed from a number of supernova remnants (SNRs) indicates particle acceleration to high energies at the shock of the remnants and a potentially significant contribution to Galactic cosmic rays. It is extremely difficult to determine whether protons (through hadronic interactions and subsequent pion decay) or electrons (through inverse Compton scattering on ambient photon fields) are responsible for this emission. For a successful diagnostic, a good understanding of the spatial and energy distribution of the underlying particle population is crucial. Most SNRs are created in core-collapse explosions and expand into the wind bubble of their progenitor stars. This circumstellar medium features a complex spatial distribution of gas and magnetic field which naturally strongly affects the resulting particle population. In this work, we conduct a detailed study of the spectro-spatial evolution of the electrons accelerated at the forward shock of core-collapse SNRs and their nonthermal radiation, using the RATPaC code that is designed for the time- and spatially dependent treatment of particle acceleration at SNR shocks. We focus on the impact of the spatially inhomogeneous magnetic field through the efficiency of diffusion and synchrotron cooling. It is demonstrated that the structure of the circumstellar magnetic field can leave strong signatures in the spectrum and morphology of the resulting nonthermal emission. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac3cb8 SN - 0004-637X SN - 1538-4357 VL - 926 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Teichmann, Erik A1 - Lewandowski, Heather J. A1 - Alemani, Micol T1 - Investigating students’ views of experimental physics in German laboratory classes JF - Physical Review Physics Education Research N2 - There is a large variety of goals instructors have for laboratory courses, with different courses focusing on different subsets of goals. An often implicit, but crucial, goal is to develop students’ attitudes, views, and expectations about experimental physics to align with practicing experimental physicists. The assessment of laboratory courses upon this one dimension of learning has been intensively studied in U.S. institutions using the Colorado Learning Attitudes about Science Survey for Experimental Physics (E-CLASS). However, there is no such an instrument available to use in Germany, and the influence of laboratory courses on students views about the nature of experimental physics is still unexplored at German-speaking institutions. Motivated by the lack of an assessment tool to investigate this goal in laboratory courses at German-speaking institutions, we present a translated version of the E-CLASS adapted to the context at German-speaking institutions. We call the German version of the E-CLASS, the GE-CLASS. We describe the translation process and the creation of an automated web-based system for instructors to assess their laboratory courses. We also present first results using GE-CLASS obtained at the University of Potsdam. A first comparison between E-CLASS and GE-CLASS results shows clear differences between University of Potsdam and U.S. students’ views and beliefs about experimental physics. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevPhysEducRes.18.010135 SN - 1554-9178 VL - 18 SP - 010135-1 EP - 010135-17 PB - APS CY - College Park, Maryland, United States ET - 1 ER - TY - GEN A1 - Teichmann, Erik A1 - Lewandowski, Heather J. A1 - Alemani, Micol T1 - Investigating students’ views of experimental physics in German laboratory classes T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - There is a large variety of goals instructors have for laboratory courses, with different courses focusing on different subsets of goals. An often implicit, but crucial, goal is to develop students’ attitudes, views, and expectations about experimental physics to align with practicing experimental physicists. The assessment of laboratory courses upon this one dimension of learning has been intensively studied in U.S. institutions using the Colorado Learning Attitudes about Science Survey for Experimental Physics (E-CLASS). However, there is no such an instrument available to use in Germany, and the influence of laboratory courses on students views about the nature of experimental physics is still unexplored at German-speaking institutions. Motivated by the lack of an assessment tool to investigate this goal in laboratory courses at German-speaking institutions, we present a translated version of the E-CLASS adapted to the context at German-speaking institutions. We call the German version of the E-CLASS, the GE-CLASS. We describe the translation process and the creation of an automated web-based system for instructors to assess their laboratory courses. We also present first results using GE-CLASS obtained at the University of Potsdam. A first comparison between E-CLASS and GE-CLASS results shows clear differences between University of Potsdam and U.S. students’ views and beliefs about experimental physics. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1263 Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-560429 SN - 1866-8372 SP - 010135-1 EP - 010135-17 PB - Universitätsverlag Potsdam CY - Potsdam ER -