Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
175 results
Search Results
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 Nanoscale magnetic resonance spectroscopy with nitrogen-vacancy centers in diamond(2021) Paone, Domenico; Wrachtrup, Jörg (Prof. Dr.)Stickstoff-Fehlstellen (NV-Zentren) in Diamant bilden interessante Quantensysteme, welche für Quanten-Sensing Protokolle genutzt werden können. In der vorliegenden Arbeit, werden NV-Zentren genutzt, um einzelne Molekülsysteme auszulesen und supraleitende Proben lokal zu charakterisieren. Zusätzlich werden Methoden entwickelt, um die Spineigenschaften der NV-Zentren zu optimieren, welche dann Einfluss auf das Sensorikverhalten des Systems haben.Item Open Access Entwicklung laserspektroskopischer Methoden zur Analyse der Verdunstungseigenschaften von Brennstofftropfen(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2021) Werner, Stefanie; Riedel, Uwe (Prof. Dr. rer. nat.)Die steigenden Emissionen des klimaschädlichen Treibhausgases CO2 durch die Verbrennung von fossilen, endlichen Energieträgern müssen möglichst schnell und nachhaltig reduziert werden. Ein vielversprechender Lösungsansatz zur Reduzierung der Schadstoffemissionen bei der Verbrennung liegt in dem Einsatz von alternativen und erneuerbaren Brennstoffen. Als Energieträger bieten sich auf Grund ihrer hohen Energiedichte vor allem flüssige Brennstoffe an. Diese werden typischerweise durch Druckzerstäubung in die Brennkammer eingebracht, verdunstet und dann mit dem Oxidationsmittel vermischt und verbrannt. Die Verdunstung der kleinen Brennstofftropfen des sogenannten Sprays ist von entscheidender Bedeutung für den Gesamtverbrennungsprozess in Verbrennungsmotoren und Gasturbinen. Im Allgemeinen bestimmt die Verdunstungsrate die Verbrennungsrate. Daher sind Modelle notwendig, die eine genaue Vorhersage der Brennstoffverdunstung ermöglichen. Zur Validierung dieser Modelle werden quantitative Messungen unter genau definierten Randbedingungen benötigt. Da die Prozesse in technischen Brennkammern sehr komplex sind, werden Experimente zur Tropfenverdunstung häufig mit linearen, monodispersen Tropfenketten durchgeführt, um die Kopplung zwischen den verschiedenen Effekten zu minimieren. Durch die geringe Größe der Tropfen (typischerweise wenige hundert Mikrometer oder weniger), erfordert die experimentelle Untersuchung eine hohe räumliche Auflösung. In dieser Arbeit wurden quantitative, laseroptische Messtechniken mit hoher räumlicher Auflösung zur experimentellen Untersuchung der Tropfenverdunstung an monodispersen Tropfenketten entwickelt. Mit den Messtechniken wurden Validierungsdaten für die Verdunstungseigenschaften von verschiedenen Brennstoffen bestimmt. Konzentrationsmessungen von verdunsteten Kohlenwasserstoffen wurden unter Verwendung von Infrarot-Laserabsorptionsspektroskopie und laserinduzierter Fluoreszenzspektroskopie (LIF) durchgeführt. Tropfenketten wurden mit einem Tropfenkettengenerator erzeugt, welcher vertikal in einem Strömungskanal installiert wurde. Die untersuchten Brennstoffe waren Cyclohexan, iso-Octan, n-Heptan, n-Pentan, 1-Butanol und Anisol. Der Strömungskanal wurde mit einer laminaren Luftströmung bei verschiedenen Temperaturen (313 K - 430 K) durchströmt. Da die untersuchten Tropfen einen Durchmesser in der Größenordnung von 120 bis 160 µm hatten und die Konzentrationsgradienten nahe der Tropfenoberfläche groß waren, war eine hohe räumliche Auflösung der Messtechniken erforderlich. Die Absorptionsmessungen wurden mit der Infrarotstrahlung eines HeNe-Lasers bei λ = 3,39 µm durchgeführt, um die CH-Streckschwingung der Kohlenwasserstoffe anzuregen. Die für die Quantifizierung der Brennstoffkonzentrationen benötigten Absorptionsquerschnitte wurden in einer beheizten Gaszelle für Temperaturen von 300 K - 773 K bestimmt. Die räumliche Auflösung im Strömungskanal betrug < 50 µm über eine Länge von 2 mm (Halbwertsbreite). Durch die Zylindersymmetrie und gute Stabilität der Tropfenketten konnten zeitliche Mittelungs- und Tomografieverfahren angewandt werden. Hierdurch konnten radiale Konzentrationsprofile an mehreren Positionen im Strömungskanal erhalten werden. Aus dem Anstieg der Dampfkonzentration an verschiedenen Messpositionen konnte die Verdunstungsrate bestimmt werden. Die Verdunstungsraten wurden in Abhängigkeit von der Mantelstromtemperatur (313 K - 430 K), der Tropfengeschwindigkeit (8 m/s - 23 m/s), der Tropfenerzeugungsfrequenz (12 kHz - 75 kHz) und dem Tropfenabstand (300 µm - 685 µm) gemessen. Im untersuchten Temperaturbereich steigt die Verdunstungsrate des Brennstoffs linear mit der Temperatur an. Die Reihenfolge der Brennstoffe in Bezug auf die Verdunstungsrate entspricht den Siedepunkten der einzelnen Brennstoffe. Da technische Brennstoffe häufig eine Mischung mehrerer Komponenten sind, ist die Untersuchung von Brennstoffgemischen von großem Interesse. Daher wurde ein Messverfahren entwickelt, um binäre Gemische zu untersuchen. Das Verfahren wurde verwendet, um eine Mischung aus Cyclohexan und Anisol zu untersuchen. Zwei Messtechniken - laserinduzierte Fluoreszenz (LIF) und Infrarot Absorptionsspektroskopie - wurden verwendet, um beide Spezies zu messen. Um λ = 3,39 µm ist der Absorptionsquerschnitt von Cyclohexan um etwa den Faktor 8 größer als von Anisol. Im untersuchten Fall war die Konzentration aufgrund des höheren Dampfdrucks ebenfalls deutlich größer. Daher konnte das Infrarot-Absorptionssignal praktisch ausschließlich Cyclohexan zugeordnet werden. Anisol hat bei Anregung bei λ = 266 nm eine sehr gute Fluoreszenzquantenausbeute, während Cyclohexan keine Fluoreszenz zeigt. LIF ermöglicht daher die Quantifizierung von Anisol (oder anderen Aromaten) ohne Interferenz durch Kohlenwasserstoffe. Es wurde ein Messverfahren entwickelt, welches Halationseffekte vermeidet, die typischerweise in planaren LIF-Experimenten an Tropfenketten auftreten. Kalibrationsmessungen, die im gleichen Strömungskanal durchgeführt wurden, ermöglichten die Quantifizierung der verdunsteten Anisolkonzentrationen. Die räumliche Auflösung betrug 80 µm. Ähnlich wie bei den Einzelkomponentenmessungen wurden Verdunstungsraten bestimmt. Wie aufgrund des niedrigeren Dampfdrucks zu erwarten, ist die Verdunstungsrate von Anisol niedriger als die von Cyclohexan. Die Verdunstungsrate von Cyclohexan in der binären Mischung stimmt gut mit den Einzelkomponentenmessungen überein. Das entwickelte Messverfahren ist sehr vielversprechend für weitere Untersuchungen an Mehrkomponentenmischungen. In dieser Arbeit konnte damit erstmals mit hoher räumlicher Auflösung die Verdunstung von Brennstoffkomponenten mittels Absorptionsspektroskopie in der Nähe von Brennstofftropfen untersucht werden. Zusätzlich wurden in Kombination mit laserinduzierter Fluoreszenzspektroskopie Messungen an binären Mischungen durchgeführt. Damit steht ein wertvoller Datensatz zur Validierung von numerischen Simulationen zur Verfügung.Item Open Access Towards an underdamped thermodynamic uncertainty relation(2020) Fischer, Lukas P.; Seifert, Udo (Prof. Dr.)A recent result of stochastic thermodynamics is the so-called thermodynamic uncertainty relation (TUR). This relation, appearing in the form of an inequality, bounds the precision of fluctuating currents by the entropic costs that are required to drive the non-vanishing mean of the observable. As a consequence, the relation enables the access to parameters that are not accessible in an experimental setting via the precision of a experimentally accessible observable. For instance, it was possible to bound the efficiency of molecular machines by means of their measurable moments of motion. Albeit being generalized and modified to more general terms and dynamics, the putative generalization of the thermodynamic uncertainty relation to underdamped dynamics where the inertia is not negligible remains a puzzling problem. Although there are convincing indications for the overdamped TUR being valid for underdamped dynamics as well in some systems, a straightforward application can also lead to violations of the bound. This thesis summarizes the efforts towards an underdamped generalization of the thermodynamic uncertainty relation and shows challenges and chances that come along by generalization of the TUR. To this end, the intriguing limitations of the TUR in the underdamped domain are explored and discussed. For instance, the TUR is inherently broken for finite times where the evolution is governed by ballistic dynamics due to the inertia being present. Furthermore, it is possible to improve the precision beyond the overdamped bound in presence of velocity dependent forces such as the Lorentz force induced by a magnetic field. Beyond the limitations of the TUR in the underdamped regime, this thesis gives a thorough analysis of the proof that leads to the TUR in the overdamped regime and discusses the obstacles which have to be overcome to find the sought-after proof that is valid for underdamped dynamics. The method is illustrated by deriving thermodynamic bounds that are, however, not as transparent and often not as tight as the original TUR. Finally, a conjecture for a generalized TUR is presented which is based on the precision of free diffusion and holds for all times. The corresponding bound converges to the overdamped TUR in the appropriate limit and tightly bounds the precision, even in the ballistic regime. Being based on free diffusion this conjecture also puts the interpretation of the original TUR in a different perspective.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 Polarized neutron reflectometry study of complex magnetism and hydrogen incorporation in thin-film structures(2022) Guasco, Laura; Keimer, Bernhard (Prof. Dr.)In this thesis, we present the study of the structural and magnetic properties of simple metals and complex oxide thin films by means of polarized neutron reflectometry. The nuclear and electronic properties of thin films were modified via two routes, namely via hydrogen incorporation, in the case of niobium systems and complex oxide layers, and via depth modulated hole doping, in the case of manganite heterostructures.Item Open Access Thermodynamic uncertainty relation for stochastic field theories : general formulation and application to the Kardar-Parisi-Zhang equation(2022) Niggemann, Oliver; Seifert, Udo (Prof. Dr.)Item Open Access Real-space spectroscopy of interacting quasiparticles in exotic semimetals(2022) He, Qingyu; Takagi, Hidenori (Prof. Dr.)Item Open Access High quality graphene for magnetic sensing(2022) Herlinger, Patrick; Smet, Jurgen (Dr.)In this thesis, we investigated the reliable fabrication of high quality graphene and its use as Hall transducer material. Charged impurities and random strain fluctuations were identified as main culprits that deteriorate the electrical properties of graphene devices. It was shown that these extrinsic sources of disorder can be reduced through optimized device processing steps as well as the use of a proper substrate material for graphene such as hexagonal boron nitride (hBN). This insulating material is atomically flat and possesses a very low intrinsic density of charged impurities. By performing Raman spectroscopy and electrical transport measurements, both without and with applied magnetic field, on a large number of different types of graphene devices, it was demonstrated that the encapsulation of graphene between hexagonal boron nitride thin films is the best way to obtain high quality graphene devices. However, even for these hBN-encapsulated devices, we still observed a notable sample-to-sample variation of the electrical properties. Therefore, we developed a post-processing technique that allows us to improve the electrical properties of such devices both significantly and reliably. Since our technique is applied after device fabrication, we could also demonstrate its beneficial effect by comparing one and the same device before and after treatment. We then assessed the application of such high quality graphene as Hall transducer material. The dependencies on and between all relevant operating parameters were explored. This allowed us to develop a deep understanding and empirical model for graphene Hall elements, including the interplay between thermal and 1/f noise in these devices. All key performance indicators for Hall sensors were measured and their typical values reported. For comparable device dimensions, hBN-encapsulated graphene Hall elements were found to have the potential to become a strong competitor to existing materials that are used in today's commercial Hall sensors. Unfortunately, the large-scale fabrication of hBN thin films still remains an unresolved challenge for the industrialization of large area, high quality graphene Hall elements. Also, the Si CMOS integration demands further development. Even though the application of graphene in Hall devices is promising, as shown in this work, this use case alone does likely not justify the significant efforts and investments we expect to be necessary to industrialize the fabrication of high quality graphene devices. Instead, these efforts and costs must be shared by developing a common technology platform for 2D materials that can address several commercially attractive applications where graphene or another 2D material offers superior performance as well. We hope that the insights provided in this work can help to accelerate this process.