Universität Stuttgart
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Item Open Access Exploring the growth of refractory metal and sapphire films by thermal laser epitaxy(2024) Majer, Lena N.; Mannhart, Jochen (Prof. Dr.)Item Open Access Self-organized structures and excitations in dipolar quantum fluids(2024) Hertkorn, Jens; Pfau, Tilman (Prof. Dr.)Quantum many-body phenomena at a macroscopic scale, such as superfluidity and superconductivity, are rooted in the interplay between microscopic particles, governed by the laws of quantum mechanics. Exploring how this interplay leads to quantum behavior at a large scale allows us to gain a deeper understanding of nature and to discover new quantum phases. An elusive quantum phase in which the frictionless flow of superfluids and the crystal structure of solids coexists - the supersolid - was recently realized with quantum droplets in dipolar Bose-Einstein condensates. In this thesis we investigate self-organized structures, their formation mechanism, and excitations in dipolar quantum fluids created from such Bose-Einstein condensates. We show that the supersolid formation mechanism is driven by density fluctuations due to low-energy roton excitations, leading to a crystal structure of quantum droplets that are immersed in a superfluid background. These roton excitations split into a Goldstone mode and a Higgs amplitude mode, associated to the broken translational symmetry in the supersolid. We investigate the symmetry breaking of dipolar quantum fluids in a range of confinement geometries and establish a comprehensive description of elementary excitations across the superfluid to supersolid droplet phase transition. The droplets are stabilized by an interplay between interactions and the presence of quantum fluctuations. We show how this interplay can be used to find regimes where droplets are immersed in a high superfluid background, allowing for frictionless flow throughout the crystal. Moreover we show that towards higher densities beyond the quantum droplet phase, this interplay leads to several new self-organized structures in the phase diagram of dipolar quantum fluids. We theoretically predict new supersolid honeycomb, amorphous labyrinth, and other phases in oblate dipolar quantum fluids. Finally, we present a new experimental setup for the exploration of self-organized phases in dipolar quantum fluids and which also lays the foundation for the implementation of a quantum gas microscope. The results of this thesis present a complete framework for understanding and creating exotic phases in dipolar quantum fluids. The versatile structure formation, governed by a competition of controllable interactions and the presence of quantum fluctuations, positions dipolar quantum fluids as a model system for exploring self-organized equilibrium in weakly-interacting quantum many-body systems.Item Open Access Rigorous compilation for near-term quantum computers(2024) Brandhofer, Sebastian; Polian, Ilia (Prof.)Quantum computing promises an exponential speedup for computational problems in material sciences, cryptography and drug design that are infeasible to resolve by traditional classical systems. As quantum computing technology matures, larger and more complex quantum states can be prepared on a quantum computer, enabling the resolution of larger problem instances, e.g. breaking larger cryptographic keys or modelling larger molecules accurately for the exploration of novel drugs. Near-term quantum computers, however, are characterized by large error rates, a relatively low number of qubits and a low connectivity between qubits. These characteristics impose strict requirements on the structure of quantum computations that must be incorporated by compilation methods targeting near-term quantum computers in order to ensure compatibility and yield highly accurate results. Rigorous compilation methods have been explored for addressing these requirements as they exactly explore the solution space and thus yield a quantum computation that is optimal with respect to the incorporated requirements. However, previous rigorous compilation methods demonstrate limited applicability and typically focus on one aspect of the imposed requirements, i.e. reducing the duration or the number of swap gates in a quantum computation. In this work, opportunities for improving near-term quantum computations through compilation are explored first. These compilation opportunities are included in rigorous compilation methods to investigate each aspect of the imposed requirements, i.e. the number of qubits, connectivity of qubits, duration and incurred errors. The developed rigorous compilation methods are then evaluated with respect to their ability to enable quantum computations that are otherwise not accessible with near-term quantum technology. Experimental results demonstrate the ability of the developed rigorous compilation methods to extend the computational reach of near-term quantum computers by generating quantum computations with a reduced requirement on the number and connectivity of qubits as well as reducing the duration and incurred errors of performed quantum computations. Furthermore, the developed rigorous compilation methods extend their applicability to quantum circuit partitioning, qubit reuse and the translation between quantum computations generated for distinct quantum technologies. Specifically, a developed rigorous compilation method exploiting the structure of a quantum computation to reuse qubits at runtime yielded a reduction in the required number of qubits of up to 5x and result error by up to 33%. The developed quantum circuit partitioning method optimally distributes a quantum computation to distinct separate partitions, reducing the required number of qubits by 40% and the cost of partitioning by 41% on average. Furthermore, a rigorous compilation method was developed for quantum computers based on neutral atoms that combines swap gate insertions and topology changes to reduce the impact of limited qubit connectivity on the quantum computation duration by up to 58% and on the result fidelity by up to 29%. Finally, the developed quantum circuit adaptation method enables to translate between distinct quantum technologies while considering heterogeneous computational primitives with distinct characteristics to reduce the idle time of qubits by up to 87% and the result fidelity by up to 40%.Item Open Access Classical and semiclassical approaches to excitons in cuprous oxide(2024) Ertl, Jan; Main, Jörg (Prof. Dr.)When an electron is excited from the valence into the conduction band it leaves behind a positively charged hole in the valence band to which it can couple through the Coulomb interaction. Bound states of electrons and holes, the excitons, are the solid state analogue of the hydrogen atom. As such they follow a Rydberg series. T. Kazimierczuk et al. [Nature 514, 343 (2014)] were able to show the existence of Rydberg excitons in cuprous oxide up to principle quantum number n=25. These states then have extensions in the µm range and thus lie in a region where the correspondence principle is applicable and quantum mechanics turns into classical mechanics. A more precise study of experimental spectra reveals significant deviations from a purely hydrogen-like behavior. These deviations can be traced to the complex valence band structure of cuprous oxide which inherits the cubic symmetry of the system. A theoretical description of the band structure introduces new degrees of freedom, i.e., a quasispin I=1 describing the three-fold degenerate valence band. Due to the coupling of quasispin and hole spin the valence band splits resulting in a yellow exciton series and two green exciton series with light and heavy holes. In this thesis we provide a semiclassical interpretation for excitons in cuprous oxide beyond the hydrogen-like model. To this end we introduce an adiabatic approach diagonalizing the band structure part of the Hamiltonian in a basis for quasi- and hole spin. This leads to a description via energy surfaces in momentum space, which correspond to the different exciton series. Classical dynamics can be calculated by choosing the energy surface of the series under interest and integrating Hamilton's equations of motion. Due to the energy surfaces the symmetry is drastically reduced compared to the hydrogen-like problem now allowing for the existence of fully three-dimensional orbits as well as the possibility of chaotic dynamics. For the yellow exciton series we find mostly regular phase space regions with quasi-periodic motion on near-integrable tori and small chaotic phase space regions. To demonstrate the existence of classical exciton orbits in the quantum spectra we show that the quantum mechanical recurrence spectra exhibit peaks, which, by application of semiclassical theories and a scaling transformation, can be directly related to classical periodic exciton orbits. An analysis of the energy dependence reveals that the dynamics deviations' from a purely hydrogen-like behavior increase with decreasing energy. Starting from the full Hamiltonian we develop a spherical model from which we are able to derive the quantum defects of the yellow exciton series using a semiclassical torus quantization. A comparison with quantum mechanical calculations show good agreement with our semiclassical results, thus allowing to identify individual quantum states by a corresponding classical exciton orbit in analogy to Bohr's atomic model. Finally, we provide a comparison of yellow exciton series with the two distinct green exciton series. The phase space is analyzed by application of Poincaré surfaces of section and Lagrangian descriptors. In addition, we investigate the Lyapunov stability of individual orbits. The analysis reveals the existence of a classically chaotic exciton dynamics for both yellow and green excitons, however, the chaotic regions are more pronounced for the green than for the yellow excitons. Excitons in cuprous oxide thus provide an example of a two-particle system with chaos even without the application of external fields.Item Open Access Über die Lösung der Navier-Stokes-Gleichungen mit Hilfe der Moore-Penrose-Inversen des Laplace-Operators im Vektorraum der Polynomkoeffizienten(2024) Große-Wöhrmann, Bärbel; Resch, Michael (Prof. Dr.-Ing.)Die bekannten numerischen Standard-Verfahren zur Lösung partieller Differentialgleichungen basieren auf einer räumlichen Diskretisierung des Berechnungsgebiets. Ihre Performance und Skalierbarkeit auf modernen massiv-parallelen Höchstleistungsrechnern ist von der Verfügbarkeit effizienter numerischer Verfahren zur Lösung linearer Gleichungssysteme abhängig. Angesichts grundlegender Herausforderungen erscheint die Entwicklung neuer Lösungsansätze sinnvoll. Ich stelle in dieser Arbeit einen Polynomansatz zur Lösung partieller Differentialgleichungen vor, der nicht auf einer räumlichen Diskretisierung beruht und mit Hilfe der Moore-Penrose-Inversen des Laplace-Operators die Entkopplung der Navier-Stokes-Gleichungen ermöglicht. Dabei ist der Grad der Polynome nicht grundsätzlich beschränkt, so dass eine hohe räumliche Auflösung erreicht werden kann.Item Open Access Physics-driven machine learning : from biomolecules to crystals(2024) Díaz Carral, Ángel; Schmauder, Siegfried (Prof. Dr. rer. nat. Dr. h. c.)Physical systems and their interactions exhibit inherent equivariance. In machine learning (ML), predicting quantities derived from these interactions follows two main approaches: constructing invariant scalar features as inputs to invariant models or employing equivariant models directly. This thesis focuses on the former, investigating feature extraction and data representation in the context of physics-driven machine learning (PDML). PDML leverages prior physical knowledge to construct descriptors that encode symmetries inherent in the data, thereby reducing dimensionality, enhancing interpretability, and improving generalization performance. The research addresses critical questions such as the limitations of physics-informed descriptors, the feasibility of dimensionality reduction without compromising prediction accuracy, the comparative performance of PDML against traditional ML methods, and the scalability of PDML in atomistic systems. Key investigations include: 1. Copper-based alloys: Combining molecular simulations and active learning (AL) to discover stable precipitate phases and assess mechanical properties. This involves density functional theory (DFT) simulations and the development of machine learning interatomic potentials (MLIPs) using moment tensor potentials (MTPs), leveraging invariant polynomials to model multi-component alloys. 2. Nanopore translocations: Improving DNA sequencing accuracy by training ML models on experimental ionic blockade data from DNA translocation through nanopores. The approach employs dimensionality reduction through a set of physical descriptors to efficiently classify nucleotide identities, with an emphasis on increasing readout accuracy and reducing model complexity. 3. High-Tc superconductivity: Proposing an effective PDML model to predict critical temperatures of superconductors by extracting key electronic and atomic features. Despite the reduced feature space, the model achieves high accuracy, offering a streamlined approach to predicting superconductor properties with minimal computational overhead. This work bridges the gap between machine learning and physics by embedding physical principles into ML feature representations, enhancing the ability to model, predict, and control complex physical systems with greater precision and efficiency. These advancements aim to unlock transformative applications and discoveries across a range of scientific and technological domains.Item Open Access Gold nanoparticle-mediated DNA origami nanoarchitectures(2024) Peil, Andreas; Na Liu, Laura (Prof. Dr.)Since its origin in the 1980s, DNA (deoxyribonucleic acid) nanotechnology has established itself as a captivating nanofabrication technique with ever increasing impact that combines aspects from physics, chemistry, and biology to construct artificial nanosystems by means of molecular self assembly. Within the field of DNA nanotechnology, the DNA origami technique represents one of the most versatile fabrication tools to craft functional two-dimensional (2D) and three-dimensional (3D) nanostructures from the bottom up. These structures offer precisely tailored geometries along with programmable functions, featuring positional addressability with sub-5 nm resolution and exceptional spatiotemporal accuracy. This thesis discusses strategies to employ the DNA origami technique to assemble intricate hybrid nanosystems with synergistically integrated gold nanoparticles (AuNPs). The AuNPs take over different roles; they grant (i) structural and (ii) functional features and enable the (iii) optical monitoring of the systems. This approach allows the fabrication of nanostructures piece by piece to explore their structural and functional properties at the nanoscale in detail. The first publication covers different strategies for the hierarchical assembly of topological DNA origami structures using a AuNP-templated self-assembly approach. The assembly of [2], [3], and [4]catenanes with interconnecting AuNPs is elucidated. The AuNPs can be controllably released to disconnect the individual rings, leaving only the mechanical bond of the catenane chain. In the second publication, a dynamic AuNP-DNA origami gear system is presented that is designed to emulate a planetary gearset with precise spatiotemporal control over its rotation dynamics. The AuNPs serve three crucial tasks. They (i) structurally link the origami ring modules, (ii) mediate the rotation and (iii) enable the real time optical tracking of the rotation via fluorescence spectroscopy. The system enables tightly orchestrated and programmable bidirectional rotations. In the third publication, reconfigurable chiral metastructures comprising multiple plasmonic particles that are accurately positioned in a helical manner around a DNA origami template are discussed. The implementation of a DNA ‘swingarm strategy’ enables the simultaneous and efficient relocation of multiple closely spaced AuNPs over large distances to precisely tune the chiroptical response of the system. The presented publications illustrate the beneficial synergies between DNA origami systems and rationally integrated AuNPs with the aim to advance and expand the application spectrum of these hybrid nanosystems within their scientific disciplines.Item Open Access Nonlinear dynamics of and coherent Raman imaging with fiber-feedback optical parametric oscillators(2024) Floess, Moritz; Giessen, Harald (Prof. Dr.)Item Open Access Optimizing aerosol-based sample delivery for single particle imaging at X-ray Free-Electron Lasers : pushing the limits: boosting data collection, minimizing background noise, and expanding sample compatibility(2024) Rafie-Zinedine, Safi; Heymann, Michael (Jun.Prof. Dr.)One of the most promising applications of X-ray Free Electron Lasers (XFELs) is the imaging of isolated particles, such as proteins, using single-particle X-ray diffractive imaging (SPI). This technique can provide high-resolution structural information on individual particles and facilitate the study of dynamic processes at the nanoscale. SPI, employing gas phase injection through an aerodynamic lens stack (ALS), has attracted significant attention due to its low background scattering and suitability for high-rate data collection. Despite these advantages, these SPI experiments encounter several challenges, especially with smaller and lighter biomolecule particles. These include low signal strength, limited collected datasets, high background scattering, and issues with sample compatibility in delivery system. In this doctoral thesis, I address the latter three challenges by developing and optimizing traditional electrospray-based gas phase sample delivery systems for SPI at XFELs. My research aims to enhance particle transmission efficiency, reduce background scattering, and expand the conductivity range of these systems to enable high-resolution imaging of smaller biological particles. I have developed three modified electrospray systems based on the traditional system to improve SPI at XFELs: enhanced electrospray, helium electrospray (He-ES), and coaxial helium electrospray (CHeES). The enhanced electrospray, upgraded from the traditional system by exploring different neutralizers and geometries, achieves an eightfold increase in particle transmission efficiency by employing a VUV neutralizer and optimizing the counter electrode's orifice size. This enhanced system achieves over 40% particle transmission from solution to the X-ray interaction region. The He-ES system uses a 3D-printed nozzle to reduce N2 and CO2 usage compared to traditional electrospray while ensuring stable sample delivery. It enhances particle delivery efficiency tenfold for 26 nm-sized biological particles and decreases gas load in the interaction chamber by 80%. Lastly, the CHeES system uses a coaxial 3D-printed nozzle to accommodate a broader conductivity range up to 40 000 µS/cm-eight times higher than traditional systems, and to lower background noise using He-ES technique. In tests at the European XFEL, the CHeES system notably lowered background noise by more than threefold in helium mode. My findings indicate improvements in transmission efficiency, background noise reduction, and sample versatility in SPI experiments, potentially enhancing both data quality and quantity. These advancements could yield higher-resolution structures and expand the scope for studying diverse biological and material science samples. My research has broader implications for structural biology, as obtaining higher-resolution structures is crucial for understanding the atomic structure of proteins and other biomolecules.Item Open Access Motile bacteria in complex environments(2024) Lohrmann, Christoph; Holm, Christian (Prof. Dr.)