An O(n^{2.75}) algorithm for online topological ordering

We present a simple algorithm which maintains the topological order of a directed acyclic graph with n nodes under an online edge insertion sequence in O(n^{2.75}) time, independent of the number of edges m inserted. For dense DAGs, this is an improvement over the previous best result of O(min(m^{3/2} log(n), m^{3/2} + n^2 log(n)) by Katriel and Bodlaender. We also provide an empirical comparison of our algorithm with other algorithms for online topological sorting. Our implementation outperforms them on certain hard instances while it is still competitive on random edge insertion sequences leading to complete DAGs.Comment: 20 pages, long version of SWAT'06 pape

A new method for suppressing excited-state contaminations on the nucleon form factors

One of the most challenging tasks in lattice calculations of baryon form factors is the analysis and control of excited-state contaminations. Taking the isovector axial form factors of the nucleon as an example, both a dispersive representation and a calculation in chiral effective field theory show that the excited-state contributions become dominant at fixed source-sink separation when the axial current is spatially distant from the nucleon source location. We address this effect with a new method in which the axial current is localized by a Gaussian wave-packet and apply it on a CLS ensemble with $N_f=2+1$ flavors of O($a$) improved Wilson fermions with a pion mass of $m_\pi=200\,$MeV.Comment: 7 pages, 6 figures, 1 table, Proceedings for the 36th Annual International Symposium on Lattice Field Theory, 22-28 July 2018, Michigan State University, East Lansing, Michigan, US

Structural and Electronic Investigation of Strongly Correlated Transition Metal Oxide Perovskite Thin Films and Interfaces using In-situ Transmission Electron Microscopy

Discussing the necessity as well as possible details of global strategies to reduce and eventually eliminate the anthropogenic climate change (ACC) is a delicate matter, which easily leads to statements based on ressentiments rather than on scientific facts. Indeed, public polls revealed the volatility of individual beliefs in the existence of ACC correlating with short-term weather phenomena [1] well after a scientific consensus about its impact was found [2–5]. Naturally, the models presented in the cited references do not cover all facets of the cybernetic global system at once and the assessment of resulting forecast uncertainties is part of the careful work of colleagues [6, 7]. However, the included and certainly possible scenario of turning the Earth in an increasingly hostile planet appears to be an unreasonably high stake when betting on the future. Consequently, in order to reduce the emission of greenhouse gases being indisputably linked to global warming [7, 8], progress in sustainable energy sources as well as their actual usage is indispensable. In essence, establishing a prevailing renewable energy supply is a threefold problem as primary conversions need to be followed by storage and transport in order to bridge temporal as well as spatial source heterogeneities [9]. One candidate to master the former challenge is solar energy conversion, which has recently gained a lot in global energy market share as module efficiencies resp. prices are constantly rising resp. falling [10, 11]. In detail, both the optimisation of established solar cells, i.e. most prominently silicon modules, as well as the inclusion of innovative concepts and materials is pursued. The latter approach is also referred to as third-generation photovoltaics and aims for solar cell efficiencies beyond the famous Shockley-Queisser limit [12], which describes the theoretical thermodynamic conversion limit under the assumption of transmission of sub-bandgap photons and complete thermalisation of hot charge carriers before power extraction. A particularly promising example, in which these assumptions are no longer valid, is given by the material class of organic halide perovskites showing an extraordinarily fast increase of related efficiencies over the past years [13–15]. In this work, the inorganic counterpart of transition metal oxide perovskites will be the main subject of study. Certainly, currently achieved solar energy conversion efficiencies in this material class are significantly lower compared with organic halides [16–18]. However, because of their rich phase diagrams emerging due to strong correlations [19–22] they serve as a well- suited model system to study underlying mechanisms (lifting the previously mentioned Shockley-Queisser limit) on a fundamental level. Importantly, this statement is not limited to the context of photovoltaics, but holds also for additional fields such as the study of catalysis [23–25] or resistive random access memory (RRAM) [26]. In more detail, this dissertation focuses on the structural and electronic investigation of transition metal oxide perovskite thin films, being typically the basis of technological devices [26]. It includes significant contributions to the phase diagram of Pr1−xCaxMnO3 epitaxial layers – grown on SrTi1−yNbyO3 substrates – in doping and temperature regimes where ordered phases occur due to correlative exchange interactions of lattice, orbital, and spin degrees of freedom [20]. Importantly, these ordered phases have been demonstrated to correlate with an enhanced photovoltaic acitvity [17, 18, 27, 28] emphasizing the importance of such studies in the context of solar energy conversion. In fact, as nicely described in the cited references, the underlying mechanism of the enhanced photovoltaic activity was found to be a prolonged lifetime of hot carriers due to phonon interactions and, thus, reaches beyond the assumptions of the Shockley-Queisser limit. Consequently, the materials in question are well-suited to explore fundamental processes in third generation photovoltaics. In order to study the mentioned phase transitions in thin films as well as electric properties relevant for solar energy conversion such as the excess charge carrier diffusion length, which happens to be located on the nanoscale [29], high-resolution techniques are needed. Therefore, the transmission electron microscope is employed enabling for versatile and highly- resolved real and reciprocal space signal extraction as well as a large variety of in-situ techniques, e.g. heating, cooling, biasing, and environmental control. In fact, facilitated by the outstanding advances in scientific instrumentation, such in-situ methods are ever-increasingly applied on the micro- and nanoscale and successfully correlated to macroscopic physical, chemical, and biological properties [30–33]. In this study, in-situ heating, cooling, biasing, and environmental control are combined with established techniques such as selective area electron diffraction [34] and electron energy loss spectroscopy [35] as well as with recently emerging methods like four-dimensional scanning transmission electron microscopy [36] and scanning transmission electron beam induced current [29]. Additionally, a substantial part of this thesis focuses on further developments of the latter techniques. Selected highlights are the successful extraction of ordering parameters as well as critical temperatures of phase transitions in Pr1−xCaxMnO3 during heating (in a gaseous environment) and cooling. The observed transitions, i.e. charge ordering for x = 0.34 and an orthorhombic to pseudo- tetragonal (or cubic) transition for x = 0.1, are discussed thoroughly in the context of correlation phenomena and photovoltaic activity and differences to the bulk such as decreased critical temperatures will be pointed out. Furthermore, a structural model is presented linking atomic configurations with the material’s lattice parameters. In addition, experimental and modelling advances in the field of scanning transmission electron beam induced current are demonstrated enabling the observation of diffusion and recombination properties of excess charge carriers in perovskites on the nanoscale as well mapping of a sub-0.1 ppm concentration line of boron in a textured silicon solar cell. Lastly, first interpretations of atomic modulations in electron beam induced current signals are presented.2021-09-1

An evaluation of the shortened high school duration in Germany and its impact on postsecondary education and labor market entry

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Towards multimodal nonlinear microscopy in clinics

Multimodal nonlinear microscopy combining two photon excited fluorescence (TPEF), second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) represents a promising and powerful tool for biomedical diagnostics. The method enables label-free visualization of morphology and chemical composition of complex tissues as well as disease related changes and is as such as detailed as staining histologic methods. In this work a compact microscope utilizing novel fiber laser sources and a new approach for data analysis based on colocalization have been developed and tested for detecting various disease patterns, e.g., atherosclerosis and brain tumors.Mit Hilfe der nichtlinearen Multikontrast-Mikroskopie basierend auf den Prozessen Zweiphotonenfluoreszenz (TPEF), Frequenzverdopplung (SHG) und kohärente anti-Stokes Raman-Streuung (CARS), können Morphologie, chemische Zusammensetzung sowie krankheitsbedingte Veränderungen komplexer Gewebe label-frei analog zu histologischen Färbungen dargestellt werden. Potentiell eignet sich die Methode daher für die in vivo Bildgebung und könnte die medizinische Diagnostik entscheidend verbessern. Im Rahmen dieser Arbeit wurde ein kompaktes TPEF/SHG/CARS-Forschungsmikroskop unter Verwendung neuer Faserlaserquellen speziell für die Verwendung in der Klinik entwickelt. Dabei wurde erforscht, wie sich der Bildkontrast durch nahinfrarote Laser sowie eine hohe spektrale Auflösung verbessern lässt. Zusätzlich wurde an Methoden der Datenanalyse multispektraler CARS-Daten gearbeitet, um mittels der Kolokalisationsanalyse die Verteilung verschiedener molekularer Marker in komplexen Geweben zu visualisieren. Das Potential für klinische Anwendungen wurde an verschiedenen Krankheitsbildern wie Arteriosklerose und Tumoren des Hirns demonstriert

Efficiently Generating Geometric Inhomogeneous and Hyperbolic Random Graphs

Hyperbolic random graphs (HRG) and geometric inhomogeneous random graphs (GIRG) are two similar generative network models that were designed to resemble complex real world networks. In particular, they have a power-law degree distribution with controllable exponent beta, and high clustering that can be controlled via the temperature T. We present the first implementation of an efficient GIRG generator running in expected linear time. Besides varying temperatures, it also supports underlying geometries of higher dimensions. It is capable of generating graphs with ten million edges in under a second on commodity hardware. The algorithm can be adapted to HRGs. Our resulting implementation is the fastest sequential HRG generator, despite the fact that we support non-zero temperatures. Though non-zero temperatures are crucial for many applications, most existing generators are restricted to T = 0. We also support parallelization, although this is not the focus of this paper. Moreover, we note that our generators draw from the correct probability distribution, i.e., they involve no approximation. Besides the generators themselves, we also provide an efficient algorithm to determine the non-trivial dependency between the average degree of the resulting graph and the input parameters of the GIRG model. This makes it possible to specify the desired expected average degree as input. Moreover, we investigate the differences between HRGs and GIRGs, shedding new light on the nature of the relation between the two models. Although HRGs represent, in a certain sense, a special case of the GIRG model, we find that a straight-forward inclusion does not hold in practice. However, the difference is negligible for most use cases

Reversible intracellular translocation of KRas but not HRas in hippocampal neurons regulated by Ca2+/calmodulin

The Ras/MAPK pathway regulates synaptic plasticity and cell survival in neurons of the central nervous system. Here, we show that KRas, but not HRas, acutely translocates from the plasma membrane (PM) to the Golgi complex and early/recycling endosomes in response to neuronal activity. Translocation is reversible and mediated by the polybasic-prenyl membrane targeting motif of KRas. We provide evidence that KRas translocation occurs through sequestration of the polybasic-prenyl motif by Ca2+/calmodulin (Ca2+/CaM) and subsequent release of KRas from the PM, in a process reminiscent of GDP dissociation inhibitor–mediated membrane recycling of Rab and Rho GTPases. KRas translocation was accompanied by partial intracellular redistribution of its activity. We conclude that the polybasic-prenyl motif acts as a Ca2+/CaM-regulated molecular switch that controls PM concentration of KRas and redistributes its activity to internal sites. Our data thus define a novel signaling mechanism that differentially regulates KRas and HRas localization and activity in neurons