Eldorado - Repositorium der TU Dortmund

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Bei diesem Service handelt es sich um das Institutionelle Repositorium der Technischen Universität Dortmund. Hier werden Ressourcen aus und für Lehre, Studium und Forschung gespeichert, erschlossen und der Öffentlichkeit zugänglich gemacht.

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Aktuellste Veröffentlichungen

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    Neural networks for control: design and analysis
    (2026) Teichrib, Dieter; Schulze Darup, Moritz; Fabiani, Filippo
    The universal approximation property of neural networks (NN) makes them a popular tool for approximating various functions. Therefore, it is no surprise that they also found their way in control. In control applications, NNs are commonly used to approximate complex control laws or optimal value functions (OVF) and Q-functions, which allows for simplifying optimal control problems. However, the flexibility of NNs comes with a large number of hyperparameters associated with the topology and training of the NN that need to be chosen carefully. Moreover, typical quality measures used during the training of NNs, like mean squared error, are not sufficient to ensure a safe operation of NN-based controllers. Consequently, the use of NNs in control requires the development of tailored methods that address two main challenges. The first is how to choose a suitable NN topology that allows for efficiently approximating control laws or other functions of interest in control, e.g., PWQ OVFs. The second challenge is to ensure that systems controlled by NNs can be operated safely in terms of constraint satisfaction, stability, and privacy. These requirements are unique to control applications and thus typically not considered in classical NN applications, like image or language processing. Therefore, we address these problems by developing tailored methods that enable us to derive topologies for representing continuous PWQ functions, which play a crucial role in MPC. Furthermore, by exploiting the structure of the derived topologies, we present a training method for maxout NNs that allows us to compute parameters for the NN that globally minimize the cost function considered during training. Regarding the safety of NN-based controllers, we present extensions and simplifications for analysis methods that allow us to certify stability and constraint satisfaction for linear systems based on the approximation error of the NN with respect to a stabilizing baseline controller. We also develop methods for analyzing systems controlled by NNs, based on over approximations of reachable sets, which are used to compute robustly positively invariant sets. These methods do not require a stabilizing baseline controller and can be extended for the analysis of nonlinear systems controlled by NNs. Regarding privacy, we develop a method for computing polynomial approximations that enable an efficient encrypted evaluation of NNs. In summary, the methods presented in this thesis cover a wide range of problems of NNs in control and solve or mitigate many of them. At the same time, we highlight still existing challenges, which especially include increasing the computational efficiency of the methods based on mixed-integer programming for enabling a wide use in practice.
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    Electronic structure reorganization in MPS3 via d‐shell‐selective alkali metal doping
    (2026-03-24) Nitschke, Jonah Elias; Bhumla, Preeti; Willershausen, Till; Merisescu, Patrick; Janas, David Maximilian; Sternemann, Lasse; Gutnikov, Michael; Schiller, Karl; Mischke, Valentin; Capra, Michele; Arndt, Mira Sophie; Botti, Silvana; Cinchetti, Mirko
    Semiconducting two-dimensional (2D) antiferromagnetic (AFM) transition-metal thiophosphates (MPS3) offer promising opportunities for spintronic applications due to their highly tunable electronic properties. While alloying and intercalation have been shown to modulate ground states, the role of d-shell filling in governing these transitions remains insufficiently understood. Here, we investigate electron doping effects in MPS3 using angle-resolved photoemission spectroscopy (ARPES), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT+U). Lithium and cesium deposition are employed to induce doping across different MPS3 compounds. We identify two distinct doping mechanisms: in MnPS3, electrons are primarily donated to the P2S6 ligand clusters, with negligible Mn 2p core-level shifts and no major changes in the valence band. In contrast, FePS3, CoPS3, and NiPS3 exhibit clear reductions in transition-metal oxidation states, with a ∼1.0 eV reduction in spin-orbit splitting for Co upon doping. ARPES on CoPS3 reveals a ∼400 meV shift of Co-derived bands toward higher binding energies and new dispersive states up to 1 eV above the valence band maximum, indicating metallic behavior. These results establish a direct correlation between d-shell filling and doping response, highlighting alkali metal doping as a tunable route to tailor the electronic and magnetic properties of 2D AFM semiconductors for spintronic applications.
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    Spin‐selective interface engineering in oxide–ferromagnetic junctions via atomic‐scale oxygen control
    (2026-02-20) Janas, David Maximilian; Arndt, Mira Sophie; Nitschke, Jonah Elias; Sternemann, Lasse; Mischke, Valentin; Feyer, Vitaliy; Cojocariu, Iulia; Baranowski, Daniel; Sala, Alessandro; Windischbacher, Andreas; Puschnig, Peter; Dreiser, Jan; Ponzoni, Stefano; Zamborlini, Giovanni; Cinchetti, Mirko
    Atomic-scale control of oxide–ferromagnet interfaces is crucial for optimizing spintronic heterostructures, yet interfacial oxygen remains difficult to control and verify. Here, we deterministically tune the prototypical MgO/Fe(100) interface from oxygen-free terminations to fully intercalated oxygen layers by reactive growth under controlled O2 exposure, while preserving epitaxy. Momentum-resolved photoemission identifies oxygen-dependent fingerprints in k-space that originate from the buried interface and persist up to a thickness of 8 layers of MgO. Insights from complementary spectroscopic methods link these k-space signatures to interfacial chemistry, structural order, work-function shifts, and an oxygen-induced interface resonance within the MgO gap that alters the tunneling response. The combined results define a calibrated growth protocol that allows reproducibly preparing and identifying three distinct terminations — oxygen-free, partially oxidized, and oxygen-intercalated — and enables post-growth conversion even in thicker films. Complementary spin-resolved experiments reveal that oxygen-free interfaces exhibit pronounced suppression of minority-spin spectral weight at the Fermi level, consistent with coherent spin filtering across crystalline MgO, whereas oxygen intercalation reduces the spin contrast at EF. By turning interfacial oxygen from an uncontrolled variable into a measurable, adjustable parameter, our approach establishes MgO/Fe(100) as a benchmark platform for optimizing spintronic functionality in oxide/metal junctions.
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    Strangeness production in 𝑝-He and 𝑝-Ne collisions at √𝑠NN = 110 GeV using the LHCb fixed-target mode
    (2026) Kolk, Lars; Albrecht, Johannes; Glaser, Christian
    A measurement of multiplicity-dependent strangeness production in fixed-target collisions is conducted using data from the LHCb experiment at CERN. The measurement is motivated by observations of strangeness enhancement, the increase of strangeness production as a function of multiplicity, in various collision systems reported by the ALICE collaboration. Additionally, in the context of astroparticle physics, the presence of strangeness enhancement is of interest as it could explain the muon excess observed in extensive air showers, known as the Muon Puzzle. The analysis is performed on data collected with the fixed-target configuration of the \lhcb experiment called SMOG, employing helium and neon as targets at a nucleon-nucleon centre-of-mass energy of \(110 \, \text{GeV}\). Differential cross-section ratios of the single strange particles \(K_S^0\), \(\Lambda\), and \(\overline{\Lambda}\) to charged pions are measured in intervals of rapidity in the centre-of-mass frame and transverse momentum; for the study of strangeness enhancement, these ratios are additionally provided in intervals of multiplicity. The resulting cross-section ratios are compared with predictions of state-of-the-art hadronic event generators. None of the generators provide a consistent description of the data within experimental uncertainties. No significant hints of multiplicity dependent strangeness production are observed.