First-principles many-body study of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources

We present a comprehensive first-principles investigation of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources for particle accelerators. Our study, based on density-functional theory and many-body perturbation theory, provides all the ingredients to model the emission of this material as a photocathode, including band gap, band dispersion, and optical absorption. An accurate description of these properties beyond the mean-field picture is relevant to take into account many-body effects. We discuss our results in the context of state-of-the-art electron sources for particle accelerators to set the stage towards improved modeling of quantum efficiency, intrinsic emittance, and other relevant quantities determining the macroscopic characteristics of photocathodes for ultra-bright beams.Peer Reviewe

Changes in neural resting state activity in primary and higher-order motor areas induced by a short sensorimotor intervention based on the Feldenkrais method

We use functional magnetic resonance imaging to investigate short-term neural effects of a brief sensorimotor intervention adapted from the Feldenkrais method, a movement-based learning method. Twenty-one participants (10 men, 19-30 years) took part in the study. Participants were in a supine position in the scanner with extended legs while an experienced Feldenkrais practitioner used a planar board to touch and apply minimal force to different parts of the sole and toes of their left foot under two experimental conditions. In the local condition, the practitioner explored movement within foot and ankle. In the global condition, the practitioner focused on the connection and support from the foot to the rest of the body. Before (baseline) and after each intervention (post-local, post-global), we measured brain activity during intermittent pushing/releasing with the left leg and during resting state. Independent localizer tasks were used to identify regions of interest (ROI).Brain activity during left-foot pushing did not significantly differ between conditions in sensorimotor areas. Resting state activity (regional homogeneity, ReHo) increased from baseline to post-local in medial right motor cortex, and from baseline to post-global in the left supplementary/cingulate motor area. Contrasting post-global to post-local showed higher ReHo in right lateral motor cortex. ROI analyses showed significant increases in ReHo in pushing-related areas from baseline to both post-local and post-global, and this increase tended to be more pronounced post-local. The results of this exploratory study show that a short, non-intrusive sensorimotor intervention can have short-term effects on spontaneous cortical activity in functionally related brain regions. Increased resting state activity in higher-order motor areas supports the hypothesis that the global intervention engages action-related neural processes

Dopamine-dependent scaling of subthalamic gamma bursts with movement velocity in patients with Parkinson’s disease

Gamma synchronization increases during movement and scales with kinematic parameters. Here, disease-specific characteristics of this synchronization and the dopamine-dependence of its scaling in Parkinson’s disease are investigated. In 16 patients undergoing deep brain stimulation surgery, movements of different velocities revealed that subthalamic gamma power peaked in the sensorimotor part of the subthalamic nucleus, correlated positively with maximal velocity and negatively with symptom severity. These effects relied on movement-related bursts of transient synchrony in the gamma band. The gamma burst rate highly correlated with averaged power, increased gradually with larger movements and correlated with symptom severity. In the dopamine-depleted state, gamma power and burst rate significantly decreased, particularly when peak velocity was slower than ON medication. Burst amplitude and duration were unaffected by the medication state. We propose that insufficient recruitment of fast gamma bursts during movement may underlie bradykinesia as one of the cardinal symptoms in Parkinson’s disease

Zusammensetzung und Chemie von Pd-Ni-Co Dünnschichtlegierungen für wasserstoffbezogene Anwendungen

The discovery and design of high performance Pd-alloys is of great interest for the use of hydrogen as a future energy carrier. Therefore hydrogen has to be detected, separated from other gases and stored. In this respect this thesis presents the combinatorial synthesis and characterization of the ternary Pd-Ni-Co alloy system over a wide composition range based on so-called thin film alloy libraries. Those libraries are model systems to characterize a large number of alloy compositions at the same time. The sputter-deposition process is optimized for the gradient of the Pd concentration on the surface of the alloy library by the use of electron-excited Auger electron spectroscopy. The scientific goal of this work is the experimental investigation of adsorbate-induced surface segregation phenomena on alloy libraries. The surface and bulk compositions of an alloy library are studied after deposition, hydrogen exposure and hydrogen sulfide poisoning. The co-segregation of Ni and Co to the surface is observed. The segregation process is influenced by the oxidation of Ni and Co due to the contact with ambient air, by hydrogen and by hydrogen sulfide poisoning. Also at very high Pd concentrations in the range of 87 at.% to 97 at.%, which is interesting for the detection of very low hydrogen concentrations in air, the co-segregation of Ni and Co takes place. The poisoning effects were investigated in detail on a pre-selected Pd-Ni-Co alloy by photoelectron spectroscopy (XPS, HAXPES) in addition to AES and EDX. The composition profile of the alloy on the nm scale is acquired and the surface and bulk chemistry is discussed before and after poisoning. The composition of the alloy only changed within the first 3 nm due to hydrogen sulfide exposure. In the ternary Pd-Ni-Co alloy system Pd is present in its metallic state, while Ni and Co show several oxidation states. The presented concepts of ternary alloy development pave the way for the systematic synthesis and characterization of new ternary transition metal alloy systems.Die Entwicklung leistungsfähiger Pd-Legierungen ist von großem Interesse für den Einsatz von Wasserstoff als Energieträger der Zukunft. Dafür muss Wasserstoff detektiert, von anderen Gasen getrennt und gespeichert werden. In diesem Zusammenhang verfolgt diese Arbeit die kombinatorische Synthese und Charakterisierung des ternären Pd-Ni-Co Legierungssystems über einen weiten Konzentrationsbereich basierend auf sogenannten Legierungsbibliotheken. Solche Bibliotheken dienen als Modellsysteme um möglichst viele Legierungskompositionen gleichzeitig zu charakterisieren. Der Herstellungsprozess ist mit Hilfe der elektronenangeregten Auger-Elektronspektroskopie hinsichtlich des Gradienten der Palladium-Konzentration an der Oberfläche der Legierungsbibliothek optimiert worden. Ein weiteres Ziel dieser Arbeit bestand darin, adsorbat-induzierte Oberflächensegregationsphänomene an diesen Legierungsbibliotheken experimentell zu untersuchen. Die Zusammensetzung der Legierungsbibliotheken wurde an der Oberfläche und im Volumen nach der Herstellung, nach der Behandlung mit Wasserstoff und nach der Vergiftung mit Schwefelwasserstoff untersucht. Die Segregation von Ni und Co zur Oberfläche wurde über den gesamten untersuchten Konzentrationsbereich beobachtet. Der Oberflächensegregationsprozess wird durch Wasserstoff und Schwefelwasserstoff beeinflusst und hängt mit der Oxidation von Ni und Co durch Luftsauerstoff zusammen. Auch bei sehr hohen Pd Konzentrationen zwischen 87 at.% und 97 at.%, interessant für die Wasserstoffdetektion an Luft, ließ sich die Segregation von Ni und Co beobachten. Ein besonderes Augenmerk lag auf der Vergiftung einer vorausgewählten Pd-Ni-Co Legierung. Diese wurde zusätzlich zu AES und EDX mit Methoden der Photoelektronenspektroskopie (XPS, HAXPES) untersucht. Das tiefenabhängige Zusammensetzungsprofil der Legierung vor und nach der Vergiftung konnte so aufgenommen und ein Einblick in die Oberflächen- und Volumenchemie des Systems gewonnen werden. Die Vergiftung veränderte die Legierungskomposition nur innerhalb der ersten 3 nm. Die gezielte Methodenkombination in dieser Arbeit ermöglicht eine umfassende und zerstörungsfreie Charakterisierung von ternären Legierungsbibliotheken und die Untersuchung von Oberflächensegregationsphänomenen.DFG, WU80/10-2, Untersuchungen zur katalytischen Aktivität ternärer Legierungen und zum Mechanismus chemischer Sensorreaktione