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- ItemDFG final report for the Walter Benjamin Programme : Stationary and time-dependent radiative heat transfer with cylindrical waveguides(Hannover : Technische Informationsbibliothek, 2025) Asheichyk, KirylFollowing the progress in development of micro- and nanodevices, the physics of thermal radiation at these small length scales has been studied intensively in the current century. It requires more sophisticated theories and obeys different laws compared to radiative heat exchange at large scales, for example, between the Sun and the Earth. Yet main questions remain similar: How to improve the efficiency of radiative heat transfer, suppress or enhance the cooling rate of an object, thermally isolate a part of a system? Another important and only recently emerged research area concerns systems subject to nonstationary conditions, such as time-dependent temperatures or material properties of the objects, where the heat fluxes can depend on time and show fundamentally different properties compared to the stationary case. In this project, we addressed the aforementioned questions for the stationary heat transfer between two nanoparticles placed close to a nanowire or inside a cylindrical cavity. In the former case, the heat transfer was shown to decay logarithmically with the interparticle distance, thus greatly outperforming the transfer without an additional object or in the presence of objects of other shapes. In the latter case, a cylindrical cavity can largely suppress or resonantly enhance the heat flow, depending on its radius. If we consider that one particle starts to radiate at a certain time, the heat flux to another one is no longer stationary, which requires an extension of the existing theoretical formalisms. Making this extension, we derived a formula for the time-dependent flux, numerically demonstrating it for two isolated nanoparticles, where picosecond-scale oscillatory exponential relaxation to the stationary value was observed. In addition to the aforementioned studies of radiative heat flow through vacuum, we also developed a theory for computing this flow inside media, and derived the corresponding thermal conductivity tensor for an arbitrary object. In agreement with recent experiments, it was found that the radiative part of conductivity of a nanosheet can be larger compared to the phononic part due to the electromagnetic surface waves contribution. The developed theoretical frameworks can be used to further advance the subject area, while the found results may be relevant for practical applications, such as thermal microscopy, thermal logic, or nanomedicine.
- ItemBerechnung, Messung und Kontrolle der Energiebarrieren und der lichtinduzierten Kinetik des ASi-Sii-Defektes(Hannover : Technische Informationsbibliothek, 2025-04-09) Lauer, KevinDie Siliziumtechnologie hat umfangreiche Anwendungsmöglichkeiten, die sich im gegenwärtigen Alltag überall wiederfinden. Anwendungen wie Computer, Smartphones, Sensoren, Detektoren, Solarzellen und viele mehr sind nur möglich geworden durch jahrzehntelange Erforschung des Siliziums und der nötigen technologischen Prozesse. Nichtsdestotrotz gibt es noch immer unverstandene Phänomene und Mechanismen, speziell im Bereich der Defekte und der Degradation. Ein besseres Verständnis ist notwendig, da Defekte zum einen notwendig für die Funktionsweise von Bauelementen sind, sich aber auch negativ auswirken können. Das Projekt untersuchte eine besondere Kategorie von Defekten, die einen Akzeptor und interstitielles Silizium involvieren, so genannte ASi-Sii-Defekte. Sie tragen vermutlich maßgeblich zur licht-induzierten Degradation von Solarzellen und Detektorbauelemente bei. Konkret wurden in diesem Projekt die Energiebarrieren zwischen verschiedenen Defektzuständen der ASi-Sii-Defekte, die teilweise erst identifiziert werden mussten, in enger Zusammenarbeit von Experiment und Theorie erforscht und bestimmt. Die Barrierenhöhen haben einen direkten Einfluss auf die Defektkinetik und -stabilität, weshalb ihre Kenntnis essentiell ist. Gelingt es, die Barrierenhöhen gezielt zu beeinflussen (z.B. durch Wahl des Akzeptors), so können Bauelementeigenschaften ggf. gezielt eingestellt werden, z.B. zur Effizienzsteigerung oder Lebenszeitverlängerung von Solarzellen. Zudem erscheint es reizvoll künftig zu untersuchen, inwieweit Defekte aus dieser Kategorie als Qubit Verwendung finden können und somit für die Silizium-basierte Quantentechnologie interessant sind. Das Projekt hat erfolgreich einen Beitrag zum grundlegenden Verständnis der ASi-Sii-Defekte geleistet und legt den Grundstein für die weitere Erforschung dieser interessanten Defektkategorie.
- ItemCollaborative Research Center 917 : Resistively Switching Chalcogenides for Future Electronics : Structure, Kinetics, and Device Scalability : Final report : 2019/2-2020-2021-2022-2023/1(Hannover : Technische Informationsbibliothek, 2025-03-25) Wuttig, Matthias; Waser, Rainer; Dronskowski, Richard; Dittmann, Regina; Simon, Ulrich; Mayer, JoachimThe goal of SFB 917 has been the development of novel nanoswitches that can be reproducibly and reversibly changed between two states on very short time and length scales. Such nanoswitches can enable new storage and memory devices as well as neuro-inspired architectures for information technology. In the third and final funding period of SFB 917 we have witnessed and contributed to three major trends. The exponential growth in the demand for data storage and processing has continued. Hardware improvements are therefore urgently needed to meet the increased demands for data storage and processing as well as the related increase in energy consumption. SFB 917 strives to realize novel storage devices by exploiting the full potential of chalcogenide-based nanoswitches. Yet, it has become increasingly clear in the last few years, that improvements in device performance alone are insufficient to deal with the exponential growth mentioned above. The advance of Large Language Models (LLMs) like ChatGPT and related software has produced a further increase in data processing related energy consumption. This is a major challenge considering the expected further increase of professional and private usage of such machine learning tools. To minimize the related energy consumption, particularly energy-efficient software and hardware developments are mandatory. Within SFB9917, we have this intensified our efforts to work on effects related to a reduction in energy consumption. While energy-efficient devices can help, an improvement in hardware architecture offers significantly more leverage. We have thus explored the potential of chalcogenide based nano-switches in neuro-inspired computer architectures. To this end, several new large-scale research projects have been initiated (NeuroSys and NeuroTec), which extend our research on these devices in increasingly more complex architectures, offering new opportunities to harvest the findings of SFB 917 in new applications. To tailor chalcogenide-based nano-switches, major advances in instrumentation as well as an in-depth understanding of the origin of underlying phenomena and unconventional properties in these materials have been mandatory. Challenges included the characterization of switching in these materials on nanosecond time and nanometer length scales. Sophisticated tools have been built and utilized. Understanding unconventional properties are required building a bridge between concepts of inorganic chemistry and material properties leading to novel treasure maps which help to identify and tailor chalcogenides for specific applications. These successes are described in detail in the present report. To demonstrate that these findings are also relevant for industry, close cooperation with industrial partners has been established to ensure that the findings made within SFB 917 can also be implemented on the shortest possible time scales.
- ItemSedimentation of binary mixtures: Phase stacking and Nonequilibrium dynamics(Hannover : Technische Informationsbibliothek, 2024-12-30) Schmidt, Matthias; de las Heras, DanielBased on equilibrium sedimentation path theory and the local density functional approximation, we investigated the effects of gravity on several relevant types of binary colloidal mixtures. Settled systems are represented by so-called sedimentation paths, which determine the variation of the species-resolved chemical potentials with altitude. Analysing the resulting line segments in the plane of chemical potentials of the bulk phase diagram allows one to rationalize the full equilibrium stacking phenomenology for a given system under gravity. The approach predicts theoretically the stacking sequences of colloidal rod-plate mixtures that were observed in iconic experiments by van der Kooij and Lekkerkerker. Thereby the occurrence of up to five simultaneous phase layers emerges naturally from the mere interplay of gravity and two-phase bulk coexistence, without invoking particle polydispersity. We studied the effects on equilibrium phase stacking upon varying the buoyant mass ratio of both components and our predictions are testable in experiments by systematic variation of the height of sedimentation columns. We have carried out similar sedimentation studies for: plate-spheres mixtures, mass-polydisperse systems, and hard spherocylinders. We suggest that microscopic particle properties, such as the buoyant mass, can be inferred from macroscopic measurements of layer thicknesses in phase stacking sequences. We addressed gravity-induced nonequilibrium flow and structure formation on the basis of power functional theory, adaptive Brownian dynamics computer simulations, and functional machine learning. Power functional theory allows one to rationalize and to model the nonequilibrium behaviour of many-body systems based on the one-body density and velocity field. We have used the approach to categorize systematically the different types of relevant nonequilibrium force contributions and have developed corresponding analytical gradient approximations. Neural functionals, as trained on the basis of both equilibrium and nonequilibrium computer simulation data, were shown to yield accurate predictions for structure formation and design of nonequilibrium flow. We have formulated force-based density functional theory and have demonstrated that neural density functionals outperform the best available hard sphere fundamental measure functionals. We have developed adaptive Brownian dynamics as a performant and highly stable numerical integration scheme for the temporal integration of overdamped many-body Langevin equations of motion, as demonstrated for a particle gel subject to convective sedimentation flow. We have put forward general frameworks for fluctuations of general hyperobservables, for their associated hyperforce correlation functions, and for the gauge invariance of statistical mechanics, where Noether's theorem yields exact sum rules that constrain correlations, as exemplified for ideal and for active sedimentation.
- ItemEffect of a high-voltage mesh electrode on the volume and surface characteristics of pulsed dielectric barrier discharges(Melville, NY : American Inst. of Physics, 2020) Kettlitz, M.; van Rooij, O.; Höft, H.; Brandenburg, R.; Sobota, A.Electrical breakdown in a pulsed asymmetric dielectric barrier discharge between a glass-covered mesh electrode and a grounded metal electrode in the air at atmospheric pressure is investigated. Volume discharge forms between the metal tip and the dielectric surface and spreads over the dielectric surface. Breakdown and discharge behaviors depend on the polarity of the charged electrode covered with glass compared to the metal rod electrode. In the case of the dielectric cathode (covered mesh), volume discharge features a stronger and longer-lasting emission. Volume discharge is weaker with outstretched surface discharge developing on the opposite glass electrode sustained by the embedded mesh when the metal rod functions as a cathode. The development and spatial distribution of the surface discharge depend on the relative polarity of the dielectrics caused by the charge deposition of the preceding discharge and is independent of the polarity of the applied high voltage. The discharge emission is brighter for the metal cathode and dielectric anode than for the metal anode, with a branching discharge developing and spreading in a star-like structure along the embedded grid, while a ring-like structure was observed for the metal anode and dielectric cathode. The duty cycle influences the discharge development and properties through the effects of the gas phase and surface pre-ionization.