University of Siegen
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Publication Access status: Open Access , A pipeline for megahertz X-ray photon correlation spectroscopy on softmatter samples at the MID instrument of European XFEL(2026-02-22); ; ; ; In this article, we present the experimental protocol and data-processing framework for megahertz X-ray Photon Correlation Spectroscopy (MHz XPCS) experiments on soft matter samples implemented at the Materials Imaging and Dynamics (MID) instrument of the European X-ray Free-Electron Laser (EuXFEL). Due to the introduction of a standard configuration and the implementation of a highly automated data-processing pipeline, MHz-XPCS measurements can now be conducted and analyzed with minimal user intervention. A key challenge lies in managing the extremely large data volumes generated by the Adaptive Gain Integrating Pixel Detector (AGIPD) – often reaching several petabytes within a single experiment. We describe the technical implementation, discuss the hardware requirements related to effective parallel data processing and propose strategies to enhance data quality, in particular related to data reduction strategies and an improvement of the signal-to-noise ratio. Finally, we address strategies for making the processed data FAIR (Findable, Accessible, Interoperable, Reusable), in alignment with the goals of the DAPHNE4NFDI project.Source Type:Article23 9 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Lateral heterostructures, alloys, and the Kirkendall effect in transition metal dichalcogenide monolayers(2026)This dissertation analyzes two-dimensional structures and effects involving Mo, Ta, and S on Au(111), using scanning tunneling microscopy (STM). The structures presented include monolayer dislocation-free concentric lateral heterostructures (CLHs) between MoS2 and TaS2, alloys and a kagome structure. All of these are created utilizing epitaxial growth methods, including a mix of physical vapor and chemical vapor deposition. Physical vapor deposition supplies the transition metals Ta and Mo, and chemical vapor deposition using H2S, supplies the S components. By fine tuning of the growth parameters the differing structures could be realized with precision and consistency. The analyzed CLHs consist of a core of MoS2 and a surrounding envelope of TaS2, which represents a Schottky contact with maximally reduced dimensionality in the z-direction. Structures of this kind are of great interest as model systems for device manufacturing with reduced dimensionalities. While a growth process was already established in previous works, the nature of the interconnection of these two materials, which exhibit a lattice constant mismatch of ∼4.5 %, was left unresolved. This thesis shows that each CLHs presents two types of dislocation-free interfaces, compensating lattice constant mismatch not via dislocations but interface-type dependent deformations. The interface types have been identified from the perviously unknown stacking registries of the CLH with the substrate. These were determined from the distribution of the orientations of CLHs within the hcp- and fcc-stacked areas of the Au(111) substrate, in comparison to free-standing islands of MoS2. Additionally, the edge terminations of the CLHs could be determined from these results. The majority of the deformation was shown to accumulate in the TaS2 envelope. This envelope contracted in both interfaces to match the MoS2 core, which only showed little expansion. In one interface a relaxation of the envelope lattice constant with distance to the interface was observed. Notably, with significant distance, even an extension beyond the equilibrium lattice constant was detected. Empirical potentials and finite elements simulations were performed, showing that most of the effects were explainable by simple mechanics. Modifying the growth process, originally designed to grow CLHs, Mo_(1-x)Ta_(x)S2 alloys were observed. The dependence of alloy formation on the new growth parameters has been analyzed and, utilizing a combination of analysis methods, the ranges of mixtures inside alloys with different growth parameters were estimated. A continuous change from CLHs to alloys with increase in annealing temperature has been found. The investigation of these alloys was further expanded upon by the alteration of the growth process, resulting in the observation of additional structures and variations. Another new structure was discovered as a byproduct of various growth processes. A structure crystallizing in the distinctive kagome lattice, aptly called kagome structure. Topography analysis was used to calculate a first estimation of the lattice constant of the structure. Additionally, a synthesization method to grow large areas of the material was developed. Lastly, Kirkendall holes, the 2D equivalent of the 3D Kirkendall voids, were discovered as a result of the Kirkendall effect. The detection took place after a special growth process which resulted in the formation of Mo_(1-x)Ta_(x)S2 alloys with gradual rates of mixture. Local changes of alloy lattice constants and coverages rates of holes and alloys, in comparison to CLHs, were used to analyze the resulting structures. A model of the growth process was proposed with the help of DFT calculations and a Monte Carlo simulation, showing that the Kirkendall effect can be transferred from 3D to 2D applications.Source Type:Doctoral Thesis6 16 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , In situ studies of conjugated polymers by X-ray diffraction techniques(2026)In this thesis conjugated materials are investigated, with a focus on (semi)conductive polymers and oligomers. These materials are promising candidates for use in organic electronics, such as thin-film organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs). The aim is to elucidate structure-property relationships of active layers of conjugated organic thin films that influence device performance. The structural investigation spans length scales from millimeters, corresponding to macroscopic device architecture, down to nanometers in the range of single crystalline domain sizes, that govern molecular stacking and crystalline order. Such complex structural information is essential for correlating multiscale ordering with device performance. X-ray diffraction is a common technique for structural studies of thin films. However, conventional X-ray laboratory sources lack the brilliance to resolve these features at an advanced level. Therefore, this work is carried out at high-brilliance synchrotron facilities that enable high spatial and temporal resolution X-ray scattering and diffraction studies of active thin film layers. Even at synchrotron light facilities, small- and wide-angle X-ray scattering at conventional beamlines provide average device structures, while advanced nano-focused beamlines enable spatially resolved measurements and localized mappings across device cross-sections with nanometer precision. Besides high-resolution measurements, in situ experimentation is a central part of the thesis. In situ annealing studies show how thermal treatment alters structural properties and its effect on electrical device performance. Understanding annealing dynamics and appropriate thermal budgets is beneficial for processing optimization and for developing robust fabrication protocols. In addition to annealing, other external factors may also have an influence on the structural organization of the film. This work reveals how structural order is affected by an external electric field during film formation, highlighting possibilities for morphology control. The application of a permanent electric field to P3HT induces non-equilibrium aggregation, leading to reduced crystallinity, smaller crystallite size, and increased structural disorder. Such changes provide the advantages to tune morphology, including expanded interfacial area and enhanced nanoscale heterogeneity. In contrast, an alternating electric field allows partial relaxation of polymer chains, resulting in a more equilibrium-like structure with preserved crystallinity. Finally, in operando studies provide direct insights into the structure and electrical response, tracking voltage-induced structural alterations and lattice responses under working conditions. The outcomes of this research include: • In situ studies of conjugated polymer film formation reveal structural changes under an external electrical field. The quality of structural order and the size of crystalline domains can be modified during P3HT drop-casting. This effect is greater with a permanent electrical field than with an oscillating field. While the disordered morphology induced by a permanent field is unfavorable for charge transport in transistors, it can be advantageous for applications such as organic solar cells with increased interfacial area and nanoscale heterogeneity. • The thermal protocol during annealing has a strong effect on structural order. In situ annealing studies show that a low thermal budget preserves and improves the microstructure of polymer-fullerene blends. In contrast, a P3HT:PCBM blend undergoes permanent, irreversible changes during the high thermal budget annealing with PCBM crystallization and phase separation. The structural order of PDOPT decreases until melted but recovers when returning to room temperature at a significantly enhanced level. • X-ray nanobeam diffraction on quasi-freestanding P3HT films demonstrates the feasibility of high spatial resolution diffraction studies on organic thin films revealing local orientation variations. This method is ideal for the investigation of the local crystal structure of organic semiconductors while minimizing substrate induced contributions. Using an extremely focused X-ray beam (spot size approximately 150 nm), local variations in orientation and symmetry within the polymer network are directly detectable. • Nanobeam grazing incidence diffraction (nanoGIXD) experiments revealed a strong gold reorientation and modifications of Au-polymer interfaces during device operation which can be detrimental for applications. • In-operando nanoGIXD measurements revealed a significant anisotropy of the oligomer thin films, under source-drain applied voltages with strong structural variations for both in-plane and out-of-plane directions, including a +1.3 % tensile expansion of the π-stacking distance (d_020). The outcomes underline that high resolution X-ray diffraction techniques are excellent tools for spatially and temporally resolved studies of conjugated organic thin films. The results can support the device processing optimization and development of robust fabrication protocols.Source Type:Doctoral Thesis4 18 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Towards a simultaneous measurement of the top-quark mass and decay width with 13 TeV ATLAS data using bb4l Monte-Carlo predictions(2026)This thesis presents a sensitivity study for a simultaneous measurement of two parameters of the heaviest known elementary particle: the top-quark mass and its decay width. The analysis is based on the full Run 2 dataset, corresponding to an integrated luminosity of 140/fb of proton-proton collisions at a centre-of-mass energy of 13 TeV, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The study focuses on the WWbb production process in the dilepton decay channel and exploits the sensitivity of the invariant mass distribution of a charged lepton and a b-jet to both the top-quark mass and decay width. This sensitivity can be only reliably described using next-to-leading-order predictions for the full WWbb final state in the dilepton channel. A central aspect of this work is the validation of a corresponding Monte-Carlo simulation within the ATLAS Collaboration, including a comprehensive evaluation of the associated modelling uncertainties. This simulation constitutes a valuable input not only for the measurement presented here, but also for future ATLAS analyses targeting the WWbb final state. In the context of this measurement, the use of these improved predictions is shown to significantly enhance the sensitivity to the top-quark decay width, reducing the expected uncertainty by approximately a factor of two compared to the currently most precise measurement.Source Type:Doctoral Thesis6 11 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Energy/Strain Analysis of GaAs and Axial/Radial Core – Shell NWs with GaAs Core(2025-09-08)Nanowires have emerged as a promising platform for the development of advanced hetero-structures, particularly where large lattice mismatches prevent coherent growth in planar configurations. In this thesis, strain and thermal behavior in GaAs and InxGa1-xAs core–shell nanowires are systematically investigated using both theoretical modeling and synchrotron-based experimental techniques. A theoretical approach is first established to model strain distributions in bent nanowires, revealing how mechanical deformation influences band structure and optoelectronic properties. Subsequently, strain relaxation in GaAs / InxGa1-xAs nanowires is experimentally examined through in-plane and axial X-ray diffraction. A critical transition from elastic to plastic strain relaxation is observed for indium concentrations exceeding 50%, with the formation of indium-rich surface mounds coexisting with coherent shells, confirmed by SEM and 3D strain mapping. The transient structural response of nanowires to femtosecond laser pulses is studied via an optical pump–X-ray probe setup. The results show that thermal expansion scales with laser power and is strongly dependent on nanowire geometry. Notably, zinc blende and Wurtzite phases exhibit distinct thermal behaviors under pulsed excitation. Finally, the axial distribution of beryllium dopants near the nanowire surface is probed using spatially resolved X-ray photoelectron spectroscopy (XPS). The Fermi level shifts are used to map local dopant concentrations, revealing a gradient along the nanowire growth axis attributed to flux shadowing effects during molecular beam epitaxy. Together, these findings provide a comprehensive understanding of strain, thermal dynamics, and dopant distribution in core–shell nanowires, offering insights critical for the design and optimization of nanoscale semiconductor devices.Source Type:Doctoral Thesis3 7 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Toward Ultrafast Detection of Quantum Emitters at a High Repetition Rate(2025-12-01)The modern information era is built upon technologies such as telecommunications, cryptography, high-performance computing, and, more recently, artificial intelligence (AI). At the core of all these systems are electronic circuits, whose performance is fundamentally limited by the physical speed of electrons. Photonic technologies, by contrast, rely on light quanta—single photons—that propagate orders of magnitude faster and offer intrinsically higher bandwidth and enhanced security. Replacing electronic components with optical counterparts promises major advances in communication speed, data transfer, secure information processing, and quantum technologies. Central to these developments are ultrabright single-photon sources (SPSs), including quantum dots and color centers in solid-state hosts. However, the brightest SPSs currently require cryogenic temperatures, limiting their practicality in real-world applications. This dissertation proposes an alternative route toward achieving bright, room temperature single-photon emission by exploiting high-repetition-rate excitation. Advances in laser technology now allow solid-state lasers to operate at gigahertz repetition rates. By combining such excitation with hybrid quantum systems—where a quantum emitter is coupled to a nanophotonic structure to enhance its radiative emission through the Purcell effect—ultrabright SPSs can, in principle, be realized at room temperature. At these high radiative rates, however, the fluorescence lifetime becomes extremely short, well below the temporal resolution of conventional single-photon detectors, which are further constrained by cost, cryogenic requirements, bulkiness, and limited synchronization capabilities at gigahertz rates. To address this limitation, this dissertation demonstrates an optical Kerr gate (OKG) operating at a 1-GHz repetition rate with sub-picosecond temporal resolution. The OKG is a third-order (χ(3)) nonlinear optical process that does not require phase matching conditions and is therefore suitable for broadband fluorescence detection. Kerr gating was achieved using two distinct Kerr media: bulk bismuth-borosilicate (BBS) glass and two-dimensional thin graphite films. In both cases, OKG signals were generated using sub-nanojoule gate-pulse energies—remarkably low compared with traditional OKG implementations that rely on millijoule-level amplified pulses, more than six orders of magnitude higher. The temporal resolution achieved was 175 ± 1 fs with BBS glass (∼ 3% gating efficiency) and 141 ± 6 fs with thin graphite films (∼ 10% efficiency). This compact, room-temperature technique requires no cryocooler, vacuum system, or bulky amplification stages. The demonstrated OKG platform can be directly integrated into a standard confocal fluorescence microscope, enabling ultrafast fluorescence measurements from quantum emitters and paving the way for practical room-temperature ultrabright SPSs.Source Type:Doctoral Thesis3 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Applications of Non-Perturbative Methods for Heavy Quark Physics(2026)The Standard Model of Particle Physics (SM) is one of the most successful theories, describing the interactions of the fundamental building blocks of nature. However, a number of questions remain unanswered and require the exploration of physics beyond the SM. Aside from some fundamental problems like dark matter and dark energy, there are also small tensions in the data on flavour physics processes, indicating that physics beyond the SM must exist. Flavour physics, and in particular heavy quark physics, involve a broad range of theoretical methods, including tools to estimate or even compute non-perturbative quantities. The focus of this thesis lies on the application of non-perturbative methods for heavy quark systems. This will be exemplified by describing three projects. First, we discuss the treatment of quark masses within the heavy quark expansion and effective field theory. This framework provides powerful predictions for inclusive heavy hadron decays where the treatment of the quark mass plays a crucial role. Since the quark mass itself is not a physical observable, several short-distance mass schemes have been developed to minimize uncertainties induced by the quark mass. We suggest replacing the quark mass directly with an observable. Then we switch to another method by studying the exclusive decay process B → pΨ, observed as a proton and missing energy in the final state indicating the presence of a dark antibaryon Ψ. We determine the decay width using the QCD light-cone sum rule framework. In order to parameterize the non-perturbative effects in the operator product expansion, we systematically include all contributions up to twist six in the nucleon distribution amplitudes. Finally, we apply lattice QCD to a non-perturbative study of the exclusive Bs → Ds* ℓ νℓ decay parameterized by four form factors. Such b → c quark decays are of great phenomenological interest because they allow extraction of the CKM matrix element |Vcb| or testing of lepton flavour universality. We use ratios of two-point over three-point correlation functions evaluated on RBC/UKQCD’s set of 2+1 flavour gauge field ensembles. Following renormalization and extrapolation to the physical charm mass, we present a preliminary chiral-continuum fit in the simulated q² region.Source Type:Doctoral Thesis3 7 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Controlling light emission and detection using subwavelength interlayers(2026-03-18)Light emission and detection are basic processes at the core of applications in optics and photonics, such as lighting, sensing and telecom. Despite extensive work on the design of light-matter interfaces, the exchange of electromagnetic radiation with matter is still a challenge. Here, we show how subwavelength interlayers can lead to light outcoupling efficiencies approaching 100% in combination with strong directional emission, specifically from materials with a large refractive index. We also highlight potentials for enhanced light detection and control, via structural reciprocity. Our findings are particularly relevant for semiconductor-based nanophotonic devices, which typically suffer from a large mismatch with respect to free-space and guided modes.Source Type:Doctoral Thesis6 9 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Precision studies of soft-collinear QCD dynamics in the presence of heavy quarks(2025)The discovery of the Higgs boson in 2012 marked a milestone in confirming the Standard Model (SM) of particle physics, yet the SM remains incomplete, failing to account for phenomena such as gravity, dark matter, and the matter–antimatter asymmetry in the universe. This motivates precision studies of collider processes, where more accurate theoretical predictions are needed to match the increasing experimental precision. Key challenges arise from Quantum Chromodynamics (QCD), which demands both calculations to high loop-orders in the perturbative regime and a careful separation of perturbative and non-perturbative dynamics. This thesis is split into two parts, each addressing issues in one of these categories. In the first part of this thesis, we present an all-order analysis of double-logarithmic corrections to the soft-overlap contribution in heavy-to-light transition form factors at large hadronic recoil. We focus on $B_c \to \eta_c$ transitions in a perturbative non-relativistic setup, treating both the bottom and charm quarks as heavy, with the hierarchy $m_b \gg m_c \gg \Lambda_{\rm QCD}$. Our diagrammatic analysis identifies two independent sources of double logarithms: soft-gluon effects, described by standard exponential Sudakov factors, and rapidity-ordered soft-quark configurations, which generate a novel set of coupled integral equations. These equations capture the intricate interplay between soft-quark and soft-gluon dynamics at the double-logarithmic level. As an independent consistency check, we employ a bare factorization formula within Soft-Collinear Effective Theory. Although endpoint-divergent convolution integrals prevent its use for resumming logarithmic corrections with renormalization group methods, its structure enables us to derive logarithmic corrections up to the two-loop level. By computing the only unknown contribution, we confirm the correctness of the integral equations to this order. While a closed-form solution of the integral equations remains currently elusive, we provide iterative expressions for the double-logarithmic series and derive the asymptotic behavior of the soft-overlap form factor at infinite recoil, showing that the Sudakov suppression is slightly weakened by the combined effects of soft quarks and soft gluons. In the second part of this thesis, we study the phase-space integral of the double-emission soft limit of generic QCD amplitudes with massless and massive emitters at an arbitrary angle to each other. This is a necessary ingredient to extend the nested soft-collinear subtraction scheme to cases with massive final states at hadron colliders. We employ integration-by-parts identities and the differential equations method to obtain an analytic expression for the expansion around the small-velocity limit of the massive emitter, which is an important cross-check for the exact calculation with full dependence on the velocity.Source Type:Doctoral Thesis3 10 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Quantum thermodynamics and metrology with mechanical degrees of freedom(2025)Quantum metrology and quantum thermodynamics are two relatively young fields that, in the spirit of the second quantum revolution, apply quantum theory to address fundamental questions and to develop technological applications. While the research in these fields is predominantly conducted with discrete, finite-dimensional quantum systems in mind, this thesis makes the case for continuous variable mechanical degrees of freedom. In the first of three projects, we present two realizations of the Otto cycle with a planar rotor as the working medium. As a mechanical system, the planar rotor has a well-defined classical analogue that allows the identification of genuine quantum effects by comparing classical and quantum machine. Here, we mainly focus on the parameter regimes, where the machine admits a certain operation mode, i.e. an engine, a refrigerator, or a heater. In the first realization, we find a systematic disadvantage of the quantum machine. The opposite is true for the second realization: It can be shown that the classical machine can, in general, not be run in a useful operation mode. The quantum machine, on the other hand, admits an engine operation mode for sufficiently cold temperatures of the cold bath. The second project is devoted to the dynamics of a quantum system subjected to a thermal gas. In contrast to repeated interaction models, we consider any gas particle as a motional degree of freedom. We Employ quantum mechanical scattering theory to derive a low-density limit master equation, including gases with internal structure. Then, the thermodynamic consistency of the master equation is shown. A comparison with repeated interaction models makes evident that the inclusion of motional degrees of freedom of the gas plays a curial role for the consistency. Finally, we consider a nonequilibrium scenario, where the internal and motional degrees of freedom of the gas are thermal with respect to different temperatures. We show that, under the influence of this gas, the ergotropy of the system can increase. In the last project, we apply quantum metrology on the measurement of magnetic moments in an electron microscope. We consider two types of sample, one that is static and does not change under the influence of the electron and another one that is described by a quantum system and experiences quantum backaction. For both samples, we derive the scattering operator from first principles. Then, two metrological tasks are considered: First, the sensing of the strength of the magnetic moment. We derive the Fisher information in several bases and find that momentum measurements are already optimal in the case without backaction. With backaction included, a measurement of angular momentum is optimal. The second task, we consider, is the optimal discrimination of the scattered and unscattered motional electron state. We derive the trace distance and find the experimentally achievable basis with the highest classical trace distance, which is still significantly worse than the theoretical optimum.Source Type:Doctoral Thesis10 24
