Sentef, Michael
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Item-typ:Veröffentlichung, Generation of multipartite entanglement in coupled-cavity arrays as a resource for quantum technologies(2023-10-11); ; ; Entanglement is an essential part of quantum mechanics and a resource for most quantum technologies. While entangled two-particle Bell states are well established, the generation and classification of entanglement in multipartite systems is much more difficult to achieve. At the same time, multipartite entanglement is the key to unlock the full advantage provided by the exponentially large Hilbert space of quantum systems. This thesis explores possibilities of entanglement generation with a specific focus on applications in quantum technologies. In systems of coupled microcavity arrays, coherent optical pulses are considered to directly drive the quantum system into multipartite entangled target states. The building block of such coupled-cavity arrays consists of a single mode cavity with a two-level emitter, whose interaction is described by the Jaynes–Cummings model. In order to gain insight into the system, a novel representation of its eigenspectrum is presented, which shows the eigenenergies and the composition of the corresponding eigenstates. Based on this, a numerical scheme, which is the central achievement of this thesis, is developed that allows to determine precise excitation parameters to generate entanglement. Finally, this scheme is used to show the generation of multipartite entanglement in the form of W and phased Dicke states with high fidelity.Dissertation342 221 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Thermal Transport Mechanisms of Phonons, Diffusons, Electrons, and Ions in Functional Materials(2026-02-13); ; Calisen, GordonThe design of advanced functional materials relies on a fundamental understanding of heat transport processes, which are governed by multiple thermal carriers including phonons, diffusons, electrons, and mobile ions. Each of these carriers interacts with the underlying atomic structure through mechanisms closely related to lattice dynamics and anharmonicity. In this thesis, I systematically investigate thermal transport behavior in solid-state materials—ranging from two-dimensional crystals to layered and bulk compounds—with a focus on identifying the distinct contributions from phononic, diffusonic, electronic, and ionic channels. Employing a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, I explore how intrinsic structural features and anharmonic interactions influence thermal conductivity. The findings provide insights into carrier scattering processes, transport anisotropy, and temperature-dependent conductivity trends. Firstly, I focus on the impact of phonon transport on thermal properties by employing the two-channel model, which distinguishes between propagating phonon modes and diffusive (locally confined or non-propagating) vibrational modes. In Chapter 3, I examine the potential utilization of the diverse crystal structure found in Cu2Te for on/off thermal conductivity switching. In Chapter 4, I find that the nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity and a non-standard temperature dependence. In Chapter 5, I investigate the thermal properties of Ag8SnSe6. Despite being a crystal, Ag8SnSe6 exhibits an exceptionally-low and nearly temperature independent lattice thermal conductivity. Secondly, another key heat carrier, electrons, is introduced and investigated. In Chapters 6, I focus on the effects of high order phonon scattering and electron-phonon scattering on thermal properties in two-dimensional 2H-TaS2. In Chapters 7, the early concept of a “phonon-glass-electron-crystal” for enhancing the thermoelectric figure of merit (ZT) is explored theoretically in layered Ge-Se crystals, where phonon transport exhibits wave-like behavior. Finally, I employ a hybrid approach combining the Green-Kubo formalism and molecular dynamics based on machine-learning potential to predict the thermal conductivity of Li3OCl. I systematically analyze the contributions of atomic vibrations, ion transport, and vibrationsions couplings to its thermal conductivity. It is found that as temperatures increase, Li3OCl transitions into a quasi-liquid state, resulting in a substantial ion migration that contributes non-negligibly to thermal conductivity. These findings are expected to enhance the fundamental understanding of thermal properties in solids and promote the practical applications of functional materials.Dissertation35 52 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Theoretical modelling of semiconductor nanolasers: On the influence of many-body effects on the optical properties of semiconductor nanostructures(2025-12-10); ; ; The rapid rise in global data centre energy consumption poses a critical challenge in the age of digitalisation and artificial intelligence. A promising strategy for improving energy efficiency involves the use of on-chip integrated photonic components to enable high-speed and low-loss optical links and photonic computing platforms. Realising this vision requires integrable laser sources on the nanoscale. This dissertation develops a comprehensive theoretical framework for semiconductor nanolasers, with a particular emphasis on the influence of quantum many-body effects on the optical properties of low-dimensional gain media. This work is structured to consist of three interdependent chapters. The first chapter investigates a metallic-cavity nanolaser incorporating multiple quantum wells. A fully quantised electromagnetic field formalism is employed and Quantum Laser Equations are derived on the quadruplet-level. These equations enable access to key observables including the input-output characteristics, coherence time, and second-order correlation function at zero delay. Theoretical predictions are shown to be in substantial agreement with experimental data from a silver-coated MQW nanolaser device. A spectral lineshape anomaly is identified: A transition from a Lorentzian to a Gaussian emission profile at the lasing threshold, attributed to intrinsic non-linear effects and partial mode-locking in the open cavity system. The second chapter establishes a microscopic model of monolayer molybdenum disulfide, a transition metal dichalcogenide, as a gain material. Based on a tight-binding approach, material-specific dipole and Coulomb matrix elements are derived and energy renormalisations via the screened-exchange Coulomb-hole approximation are introduced to account for the influence of screening due to the presence of excited carriers. Absorption spectra are generated using the Semiconductor Bloch Equations, demonstrating the importance of screening effects in shaping the optical response. The impact of the Brillouin zone sampling density on computational efficiency and spectral accuracy is systematically analysed and a minimum carrier density for the expectation of gain is quantified. The third and final chapter combines insights gained in the previous two and proposes a theoretical nanolaser device consisting of a molybdenum disulfide monolayer integrated with a photonic crystal cavity. A material-oriented doublet-level formulation of the Quantum Laser Equations is developed to manage the multiscale dynamics, spanning femtosecond Coulomb processes to nanosecond lasing behaviour. The theory predicts electron-hole-plasma-based lasing at room temperature, characterised by an S-shaped input-output curve, hole burning at the K- and K′-valley, and spectral clamping. These features are indicative of lasing driven by plasma gain at high carrier densities and emerge naturally from the material-oriented quantum-optical treatment of the device. Together, the three chapters establish a consistent, predictive, and microscopically realistic theoretical framework for semiconductor nanolasers. The developed models combine quantum-optical treatments with device-level properties focused on the utilised gain media, enabling a detailed understanding of the interplay between many-body physics and stimulated emission in nanoscale systems. These results contribute to the design of next-generation nanophotonic light sources and offer a pathway towards energy-efficient, on-chip integrated lasers suitable for future optical links and photonic computing platforms.Dissertation48 64 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Method development for the DFT calculation of charge-assisted surface reactions in a periodic model(2024-06-03); ; ; Many important processes of great technological importance can be modeled by properly designed charged systems. A common example is photocatalysis, where photon absorption promotes electrons from the valence band of a semiconductor photocatalyst to its conduction band, generating electrons and holes. Density functional theory is widely used for investigating the properties of defects in bulk and on the surface of solids. However, Local or semi-local approximations in the density functional may cause incorrect occupation of defect states and incorrect formation energies. These methods also underestimate the localization of defect states, missing the formation of small polarons. I show that, by making the atom- and angular-momentum-dependent parameters of the Lany-Zunger polaron correction also coordination-dependent, it is possible to correctly describe charge trapping in small polaron states on the anatase (101) and rutile (110) surfaces at a low computational cost. I implemented Komsa and Pasquarello correction scheme for charged models in a user-friendly code and extended the method to handle mediums with anisotropic dielectric tensors as well as cases where the extra charge is localized at multiple sites. In slab models with a large vacuum between the layers, a posteriori charge correction methods may not be adequate, and self-consistent correction may be needed to eliminate the spurious effects. I introduce a self-consistent potential correction method, co-developed by myself, that is capable of dealing with those cases. Finally, I use the developed framework to investigate the photocatalytic CO oxidation on the anatase (101) surface. For the restoration of the pristine surface, I propose a mechanism to eliminate the surface oxygen vacancies by including electron-scavenging oxygen molecules in the gas phase. With the proposed mechanism, it is possible to achieve a complete catalytic cycle for the oxidation of CO over the anatase (101) surface.Dissertation136 332