08 Fakultät Mathematik und Physik

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    High space‐bandwidth‐product (SBP) hologram carriers toward photorealistic 3D holography
    (2024) Li, Jin; Li, Xiaoxun; Huang, Xiangyu; Kaissner, Robin; Neubrech, Frank; Sun, Shuo; Liu, Na
    3D holography capable of reproducing all necessary visual cues is considered the most promising route to present photorealistic 3D images. Three elements involving computer‐generated hologram (CGH) algorithms, hologram carriers, and optical systems are prerequisites to create high‐quality holographic displays for photorealistic 3D holography. Especially, the hologram carrier directly determines the holographic display capability and the design of high space‐bandwidth‐product (SBP) optical systems. Currently, two categories of hologram carriers, i.e., spatial light modulators (SLM) and metasurfaces, are regarded as promising candidates for photorealistic 3D holography. However, most of their SBP capability still cannot match the amount of information generated by the CGH. To address this issue, tremendous efforts are made to improve the capability of hologram carriers. Here, the main hologram carriers (from SLM to metasurfaces) that are widely utilized in holography systems to achieve high SBP capability (high resolution, wide viewing angles, and large sizes) are reviewed. The purpose of this review is to identify the key challenges and future directions of SLM‐based and metasurface‐based holography for photorealistic 3D holographic images.
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    Permeability estimation of regular porous structures : a benchmark for comparison of methods
    (2021) Wagner, Arndt; Eggenweiler, Elissa; Weinhardt, Felix; Trivedi, Zubin; Krach, David; Lohrmann, Christoph; Jain, Kartik; Karadimitriou, Nikolaos; Bringedal, Carina; Voland, Paul; Holm, Christian; Class, Holger; Steeb, Holger; Rybak, Iryna
    The intrinsic permeability is a crucial parameter to characterise and quantify fluid flow through porous media. However, this parameter is typically uncertain, even if the geometry of the pore structure is available. In this paper, we perform a comparative study of experimental, semi-analytical and numerical methods to calculate the permeability of a regular porous structure. In particular, we use the Kozeny-Carman relation, different homogenisation approaches (3D, 2D, very thin porous media and pseudo 2D/3D), pore-scale simulations (lattice Boltzmann method, Smoothed Particle Hydrodynamics and finite-element method) and pore-scale experiments (microfluidics). A conceptual design of a periodic porous structure with regularly positioned solid cylinders is set up as a benchmark problem and treated with all considered methods. The results are discussed with regard to the individual strengths and limitations of the used methods. The applicable homogenisation approaches as well as all considered pore-scale models prove their ability to predict the permeability of the benchmark problem. The underestimation obtained by the microfluidic experiments is analysed in detail using the lattice Boltzmann method, which makes it possible to quantify the influence of experimental setup restrictions.
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    Single-band versus two-band description of magnetism in infinite-layer nickelates
    (2023) Plienbumrung, Tharathep; Daghofer, Maria; Morée, Jean-Baptiste; Oleś, Andrzej M.
    We present a weak-coupling analysis of magnetism in infinite-layer nickelates, where we compare a single-band description with a two-band model. Both models predict that (i) hybridization due to hopping is negligible, and (𝑖𝑖) the magnetic properties are characterized by very similar dynamic structure factors, 𝑆(𝑘⃗ ,𝜔), at the points (𝜋,𝜋,0) and (𝜋,𝜋,𝜋). This gives effectively a two-dimensional description of the magnetic properties.
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    Coupled simulations and parameter inversion for neural system and electrophysiological muscle models
    (2024) Homs‐Pons, Carme; Lautenschlager, Robin; Schmid, Laura; Ernst, Jennifer; Göddeke, Dominik; Röhrle, Oliver; Schulte, Miriam
    The functioning of the neuromuscular system is an important factor for quality of life. With the aim of restoring neuromuscular function after limb amputation, novel clinical techniques such as the agonist‐antagonist myoneural interface (AMI) are being developed. In this technique, the residual muscles of an agonist‐antagonist pair are (re‐)connected via a tendon in order to restore their mechanical and neural interaction. Due to the complexity of the system, the AMI can substantially profit from in silico analysis, in particular to determine the prestretch of the residual muscles that is applied during the procedure and determines the range of motion of the residual muscle pair. We present our computational approach to facilitate this. We extend a detailed multi‐X model for single muscles to the AMI setup, that is, a two‐muscle‐one‐tendon system. The model considers subcellular processes as well as 3D muscle and tendon mechanics and is prepared for neural process simulation. It is solved on high performance computing systems. We present simulation results that show (i) the performance of our numerical coupling between muscles and tendon and (ii) a qualitatively correct dependence of the range of motion of muscles on their prestretch. Simultaneously, we pursue a Bayesian parameter inference approach to invert for parameters of interest. Our approach is independent of the underlying muscle model and represents a first step toward parameter optimization, for instance, finding the prestretch, to be applied during surgery, that maximizes the resulting range of motion. Since our multi‐X fine‐grained model is computationally expensive, we present inversion results for reduced Hill‐type models. Our numerical results for cases with known ground truth show the convergence and robustness of our approach.
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    On the construction of the Stokes flow in a domain with cylindrical ends
    (2024) Wendland, Wolfgang L.
    Based on existence results for the Stokes operator and its solution properties in manifolds with cylindrical ends by Große et al. and Kohr et al., the Stokes flow in a three-dimensional compact domain Ω+ with circular openings Σ𝑗 ( 𝑗 = 1, 2) through which the fluid enters and leaves Ω+ through unbounded cylindrical pipes the Stokes flow is modeled as a mixed boundary value problemΩ+ whereas in the cylindrical ends, the velocities and pressures are constant on every straight line in the cylindrical directions with initial values from the openings Σ𝑗 of Ω+. These values equal the velocities and pressures which are obtained from the mixed boundary values' solution in Ω+ at the openings Σ𝑗.
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    Stabilizing γ‐MgH2 at nanotwins in mechanically constrained nanoparticles
    (2021) Kammerer, Jochen A.; Duan, Xiaoyang; Neubrech, Frank; Schröder, Rasmus R.; Liu, Na; Pfannmöller, Martin
    Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH2 system a prime candidate for solid‐state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable γ‐MgH2. A nanoparticle (NP) design, where γ‐MgH2 forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed. Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of β‐MgH2 via (301)β twinning. It is proposed that these twins nucleate γ‐MgH2 nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle‐stable, Mg‐based dynamic plasmonic and hydrogen‐storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform γ‐MgH2 during each rehydrogenation.
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    Atomistic simulation of ultra-short pulsed laser ablation of Al : an extension for non-thermalized electrons and ballistic transport
    (2022) Eisfeld, Eugen; Förster, Daniel; Klein, Dominic; Roth, Johannes
    For our model material aluminum, the influence of laser pulse duration in the range between 0.5 and 16 ps on the ablation depth is investigated in a computational study with a hybrid approach, combining molecular dynamics with the well known two-temperature model. A simple, yet expedient extension is proposed to account for the delayed thermalization as well as ballistic transport of the excited electrons. Comparing the simulated ablation depths to a series of our own experiments, the extension is found to considerably increase the predictive power of the model.
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    Is minimally invasive multi-vessel off-pump coronary surgery as safe and effective as MIDCAB?
    (2024) Rufa, Magdalena I.; Ursulescu, Adrian; Dippon, Juergen; Aktuerk, Dincer; Nagib, Ragi; Albert, Marc; Franke, Ulrich F. W.
    The safety and efficacy of minimally invasive direct coronary artery bypass (MIDCAB) surgery has been confirmed in numerous reports. However, minimally invasive multi-vessel off-pump coronary artery bypass grafting (MICS CABG) has lower uptake and has not yet gained widespread adoption. The study aimed to investigate the non-inferiority of MICS CABG to MIDCAB in long-term follow-up for several clinical outcomes, including angina pectoris, major adverse cardiac and cerebrovascular events (MACCE) and overall survival. Methods This is an observational, retrospective, single center study of 1,149 patients who underwent either MIDCAB (n= 626) or MICS CABG (n= 523) at our institution between 2007 and 2018. The left internal thoracic artery and portions of the radial artery and saphenous vein were used for the patients’ single-, double-, or triple-vessel revascularization procedures. We used gradient boosted propensity-score estimation to account for possible interactions between variables. After propensity-score adjustment, the two groups were similar in terms of preoperative demographics and risk profile. Long-term follow-up (mean 5.87, median 5.6 years) was available for 1,089 patients (94.8%). Results A total of 626, 454 and 69 patients underwent single, double and triple coronary revascularization, respectively. The long-term outcomes of freedom from angina pectoris, acute myocardial infarction, and revascularization rate were similar between the two groups. During follow-up, there were 123 deaths in the MIDCAB group and 96 in the MICS CABG group. The 1-, 3-, 5-, and 10-year survival rates were 97%, 92%, 85%, and 69% for the MIDCAB group and 97%, 93%, 89%, and 74% for the MICS CABG group, respectively. The hazard ratio of overall survival for patients with two or more bypass grafts compared to those with one bypass graft was 1.190 (p -value = 0.234, 95% CI: 0.893-1.586). This indicates that there was no significant difference in survival between the two groups. Furthermore, if we consider a hazard ratio of 1.2 to be clinically non-relevant, surgery with two or more grafts was significantly non-inferior to surgery with just one graft (p-value = 0.0057). Conclusion In experienced hands, MICS CABG is a safe and effective procedure. Survival and durability are comparable with MIDCAB.
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    Integrated optoelectronic devices using lab‐on‐fiber technology
    (2022) Ricciardi, Armando; Zimmer, Michael; Witz, Norbert; Micco, Alberto; Piccirillo, Federica; Giaquinto, Martino; Kaschel, Mathias; Burghartz, Joachim; Jetter, Michael; Michler, Peter; Cusano, Andrea; Portalupi, Simone Luca
    Silica fibers are nowadays cornerstones in several technological implementations from long‐distance communication, to sensing applications in many scenarios. To further enlarge the functionalities, the compactness, and the performances of fiber‐based devices, one needs to reliably integrate small‐footprint components such as sensors, light sources, and detectors onto single optical fiber substrates. Here, a novel proof of concept is presented to deterministically integrate optoelectronic chips onto the facet of an optical fiber, further implementing the electrical contacting between the chip and fiber itself. The CMOS‐compatible procedure is based on a suitable combination of metal deposition, laser machining, and micromanipulation, directly applied onto the fiber tip. The proposed method is validated by transferring, aligning, and bonding a quantum‐well based laser on the core of a multimode optical fiber. The successful monolithic device integration on fiber shows simultaneously electrical contacting between the laser and the ferrule, and 20% light in‐coupling in the fiber. These results pave new ways to develop the next generation of optoelectronic systems on fiber. The technological approach will set a new relevant milestone along the lab‐on‐fiber roadmap, opening new avenues for a novel class of integrated optoelectronic fiber platforms, featuring unrivaled miniaturization, compactness, and performances levels, designed for specific applications.
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    Tuning electrode and separator sizes for enhanced performance of electrical double‐layer capacitors
    (2024) Paolini, Daniele; Antony, Lintymol; Seeta Rama Raju, Ganji; Kuzmak, Andrij; Verkholyak, Taras; Kondrat, Svyatoslav
    An electrical double‐layer capacitor (EDLC) comprises two porous electrodes sandwiching an electrolyte‐permeable separator, which prevents the electrodes from short‐circuiting. While previous studies have mainly focused on electrolyte and electrode properties of EDLCs, the device configuration in terms of electrode and separator sizes received less attention, with separators often simplistically modelled as infinitely large reservoirs of ions. Herein, we investigate how the relationship between electrode and separator thicknesses impacts EDLC charging. We find that the assumption of bulk reservoir holds only under specific conditions. Moreover, we identify a tradeoff between stored energy density and pressure variations within the separator, potentially jeopardizing the EDLC durability. We also explore the influence of ionic liquid additives on EDLC charging. While prior research has shown that trace amounts of uncharged additives with strong electrode affinity can significantly enhance energy storage, we observe this effect as negligible for electrodes and separators of comparable sizes. Instead, we show how to optimize EDLC performance by fine‐tuning the concentration of additives and separator‐to‐electrode size ratio to maximize stored energy density.