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Shape optimization of a thermoelastic body under thermal uncertainties
Full text linkWe consider a shape optimization problem in the framework of the thermoelasticity model under uncertainty. The uncertainty is supposed to be located in the Robin boundary condition of the heat equation. The purpose of considering this model is to account for thermal residual stresses or thermal deformations, which may hinder the mechanical properties of the final design in case of a high environmental temperature. In this situation, the presence of uncertainty in the external temperature must be taken into account to ensure the correct manufacturing and performance of the device of interest. The objective functional under consideration is based on volume minimization in the presence of an inequality constraint for a quadratic shape functional. Exemplarily, we consider the -norm of the von Mises stress and demonstrate that the robust constraint and its derivative are completely determined by low order moments of the random input, thus computable by means of low-rank approximation. The resulting shape optimization problem is discretized by using the finite element method for the underlying partial differential equations and the level-set method to represent the sought domain. Numerical results for a model case in structural optimization are given
Coherent photons and coherent spins in a GaAs quantum dot
Full text linkThis thesis explores droplet-etched GaAs quantum dots as potential candidates for spin-photon interfaces.
The first part of the thesis focuses on the optical coherence of the emitted photons. We confirm the emission of single photons from a single quantum dot with a high single-photon purity characterised by of . A Hong-Ou-Mandel experiment reveals a two-photon interference visibility of for consecutively emitted photons, proving high photon indistinguishability. A more striking experiment is to analyse the indistinguishability of photons emitted further apart in time, ideally infinitely apart. Such an experiment can be realised by interfering photons emitted from separate quantum dot sources with uncorrelated noise environments - an experiment that has previously led to visibilities just above . Using GaAs quantum dots, we determine a high two-photon visibility of for photons emitted from remote sources, indicating that the noise environments in the quantum dot devices are exceptionally low. This high visibility allows us to perform a final experiment, entangling two streams of photons from different sources - a proof-of-principle demonstration of the probabilistic generation of a Bell state.
The second part of the thesis focuses on the spin coherence of an electron spin in a GaAs quantum dot. We implement an all-optical spin control scheme and combine fast optical pulses with flexible microwave spin manipulation. We find short electron-spin coherence times (ns), a consequence of strong interactions between the central electron spin and the host nuclei. This interaction is enhanced when the electron spin is manipulated at Rabi frequencies matching the Larmor frequencies of the nuclei. Exploiting this interaction, we implement two nuclear cooling schemes and increase the coherence time to ns and µs, respectively. This constitutes a 155-fold increase in the coherence time and a reduction of nuclear-spin fluctuations comparable to an effective temperature of µK (starting from K). Moreover, we implement refocusing pulses and show that the electron-spin coherence can be extended using dynamical decoupling pulses up to µs with the Carr-Purcell-Meiboom-Gill sequence.
The combination of highly coherent single photons and fast control of a long-coherent spin constitutes ideal properties for a spin-photon interface. The enhanced spin coherence time is much larger than the radiative lifetime of the exciton (ps) and the time required to rotate the spin (ns), enabling the emission of hundreds of spin-photon entangled states within the coherence time
Sleep and Cognitive Functioning in Adolescent Psychiatric Outpatients and the Moderating Role of Physical Activity
No full textBackground: This master thesis is concerned with the moderating effect that physical activity has on the cognitive performance of adolescents with sleep as well as psychological disorders. Furthermore, a correlation between sleep, physical activity and cognitive performance will be explored and the effects of physical activity and sleep health on cognitive performance will be determined.
Methods: The study featured a total of 17 participants, 4 male and 13 female, aged 12-18, which were deemed eligible due to having a sleep disorder as well as one other psychological disorder. This study relied on cross-sectional data, which captures a moment in time of the participants’ habit. The participants completed entry questionnaires to determine the composite sleep health score (CSH score) and wore Actigraph GT3X+ on their wrist to measure their level of physical activity.
Results: There were significant correlation results between physical activity and cognitive performance. There was no correlation found between sleep and physical activity. In this cross-sectional study, physical activity did not show a significant effect on cognitive performance. There was also no significant result found for physical activity in a moderating role, as the effects found cannot be attributed to physical activity.
Discussion: As there were no significant results found, more research looking into this topic should be done. There were several flaws within the study, one major problem was the insufficient number of participants and their lackluster amount of physical activity
Nonmonotonic Velocity Dependence of Atomic Friction Induced by Multiple Slips
Full text linkThe transition from single to multiple atomic slips, theoretically expected and important in atomic-scale friction, has never been demonstrated experimentally as a function of velocity. Here we show by high-resolution friction force microscopy on monolayer MoS2/Au(111) that multiple slips leave a unique footprint—a frictional velocity weakening. Specifically, in a wide velocity interval from 10 to 100 nm/s, friction surprisingly decreases. Model simulations show a similar nonmonotonic behavior at velocities in quantitative agreement with experiment. Results suggest a velocity-corrugation phase diagram, whose validity is proposed more generally
Patients’ perspective on function after total knee arthroplasty: analyses of patient-reported outcome measures
Full text linkPatient-reported outcome measures (PROMs) are validated questionnaires completed by patients. They are essential to assess what is meaningful to patients and provide insight into the patients’ perspective. One in five patients with total knee arthroplasty (TKA) is dissatisfied with the result of their surgery. To better understand the underlying reasons, we began to collect PROMs routinely from our patients scheduled for TKA. They complete a series of PROMs preoperatively, at four months, and annually for up to five years postoperatively. This overall PROMs collection project was the basis for the entire thesis. Four sub-projects are embedded in it.
The first project involved the High-Activity Arthroplasty Score (HAAS), a questionnaire that measures a wider range of functional abilities, particularly in more active patients. As a validated German version was not yet available, we aimed to translate and cross-culturally adapt the HAAS. After forward and backward translation, we examined the psychometric properties in 52 patients 12 months after primary TKA. The German version of the HAAS showed good validity and reliability. It can be easily self-administered and is recommended to measure high-intensity activities in patients after TKA.
One motivation for collecting routine PROMs was a new generation of implants, customised individually made (CIM) TKA. They were introduced to address anatomical variability with the aim of restoring individual anatomy and potentially improving patient satisfaction and function. In the second project, we conducted a matched-pair analysis based on a propensity score matching on 85 CIM and 85 off-the-shelf (OTS) TKA. Follow-up was at four months, one year and two years. We found high patient satisfaction with no differences between patients with CIM and OTS TKA. Both implant systems improved subjective and objective function, pain and health-related quality of life. Patients with CIM TKA showed superior results with regard to demanding activities.
In the third project, we assessed the responsiveness of different PROMs in patients with TKA. Responsiveness is the ability of a measure to detect change over time and was determined by effect size (ES), standardised response means (SRM), area under the receiver operating characteristics (ROC) curve (AUC), floor and ceiling effects, and hypothesis testing. We analysed data from 309 TKA at four months, one year and two years follow-up. We compared the Knee injury and Osteoarthritis Outcome Score (KOOS), KOOS-12, Forgotten Joint Score (FJS-12), HAAS and EQ-5D-3L. We demonstrated high responsiveness, which varied between the different measures. The KOOS-12 and FJS-12 showed the largest internal and external responsiveness, although ceiling effects occurred with the KOOS-12.
Finally, in the fourth project, we examined the relationship between preoperative expectations and postoperative satisfaction and other PROMs in TKA patients. We analysed data from 193 TKA at baseline, 4 months and 12 months. Patient expectations were measured using the Hospital for Special Surgeries Knee Replacement Expectation Survey (HSS-KRES). Preoperative expectations were high but did not correlate with postoperative satisfaction or any of the pre- or postoperative PROMs. While it is worth educating patients about realistic expectations, high patient expectations do not always seem to be a warning sign.
In summary, PROMs are essential to better reflect the patient perspective. PROMs help to focus on patient priorities and increase patient involvement in the treatment process. This is especially true for TKA procedures, which are commonly performed but still have a substantial number of dissatisfied patients. We believe that our research is a valuable contribution to further promote the potential of PROMs and their use in everyday medical practice
Exploring bound states of NbSe₂ van der Waals superconductor probed by tunneling spectroscopy
Full text linkThis thesis investigates the properties of the unconventional superconductor NbSe₂, focusing specifically on the origins of sub-gap excitations observed in NbSe₂-based van der Waals heterostructures. The work is structured as follows: following the introduction in Chapter 1, Chapter 2 provides the theoretical background of superconductors (SC), quantum dots (QD), and hybrid QD-SC devices, including the concept of forming Andreev bound states. Additionally, Chapter 2 covers tunneling spectroscopy techniques and the properties of van der Waals materials, specifically superconducting NbSe₂. Chapter 3 describes the fabrication processes of the devices and the experimental setup, detailing mechanical exfoliation, the dry stacking technique for forming heterostructures, and the electronics and cryogenic systems used for measurements. Chapter 4 explores the macroscopic properties of NbSe₂, such as the critical transition temperature and critical magnetic field values, and presents tunneling spectra data with fitting to investigate the nature of superconductivity in NbSe₂. Chapter 5 presents experimental results and a discussion of subgap states observed in NbSe₂-based tunnel junctions. Finally, Chapter 6 summarizes the thesis and suggests possible directions for future experiments
Einblicke: die Struktur endoskopischer Sichtbarkeit
Full text linkDiese Dissertation untersucht endoskopische Verfahren und Techniken aus medienwissenschaftlicher und wissensgeschichtlicher Perspektive. Sie fragt danach, welches Sehen durch den Einblick in den lebendigen menschlichen Körper ermöglicht wird und welche Bilder des Körperinneren dabei produziert und wie diese verständlich gemacht wurden und werden. Zudem ist die Untersuchung ein Beitrag zur umfangreichen Vorgeschichte elektronischer Bilder, zu der auch die Entwicklung der Videoendoskopie in der zweiten Hälfte des 20. Jahrhunderts gehört
Rewriting security: the (private) security industry and the gendered and racialised formations of prevention
Full text linkThis thesis analyses the agenda of Preventing and Countering Violent Extremism (P/CVE) and addresses the noteworthy absence of academic literature that explores the link between P/CVE and the private security industry, underlining that power manifests on a continuum between the public and private. Contributing to feminist security studies and adopting a feminist analysis which deconstructs “security” as an ontological entity, I use the terminology of a public-private continuum to problematise the binary public-private distinction by acknowledging a fluidity and connection between the sectors. Crucially, by highlighting P/CVE’s agility as both a security and development agenda, I demonstrate how P/CVE and its logic of prevention upholds, legitimises, and expands upon existing racialised and gendered power structures. Through an analysis of the context of Kenya, the thesis significantly contributes to unravelling the entanglement of security and development and highlights how security is adapting, even shapeshifting, through prevention. I argue that this adaptation is a justification and foundation for existing security strategies which further obscure racialised and gendered security strategies and practices as benign, blurring the line between what is seen “public” and what as “private”, what is made visible and what stays invisible