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Local correlation methods for periodic systems
Get PDFWavefunction-based electronic structure methods, which accurately evaluate the electronic energy of chemical systems, have been increasingly applied to periodic or crystalline materials. Here, the requirement to simulate the in-principle infinite extent of the bulk material significantly exacerbates the computational cost of employing these methods, necessitating the development of reduced-cost algorithms specifically for periodic systems. Domain-based pair natural orbital local correlation (DLPNO) theory has been widely adopted in molecular contexts, and achieves near-linear scaling of computational effort with system size with only modest loss in accuracy by replacing Hamiltonian integrals and excitation amplitudes with low-rank approximations that exploit the inherent locality of electron correlation within non-conducting systems. In this thesis, we extend DLPNO theory to periodic systems, in order to obtain accurate unit cell electronic energies using Møller–Plesset second order perturbation theory (DLPNO-MP2). Using the existing machinery within the TURBOMOLE quantum chemistry package, we present two complementary implementations for periodic DLPNO-MP2, which arise from different choices to account for the infinite summation of lattice images contained within periodic Coulomb integrals. Proof-of-concept calculations, demonstrating the correct convergence to the infinite bulk limit are shown for both schemes, using a range of one-, two- and three-dimensional systems, and further analysis is conducted comparing the rate of convergence and the computational scaling. The pilot approach of one scheme, in particular, already demonstrates linear and sub-linear scaling with respect to supercell size. Localised occupied orbitals are an essential starting input to fully leverage the computational savings afforded through DLPNO theory. This thesis also outlines a novel scheme to generate localised Wannier functions, by adapting the molecular intrinsic atomic orbital approach to periodic systems. Finally, in an effort to demonstrate the computational efficiency afforded by periodic DLPNO-MP2, we apply our approach to study surface adsorption interactions, involving calculations with up to 30000 basis functions within the correlation treatment
Structure–Property Relationships of Near-Infrared Cyanine Dyes: Chalcogen-Driven Singlet Oxygen Generation with High Fluorescence Efficiency
Get PDFWe report the design, synthesis, and optical characterisations of eight novel near-infrared (NIR) cyanine dyes incorporating different chalcogens (O, S, and Se). These dyes exhibited excellent deep-NIR absorption (λmax = 767–833 nm) and emission (λmax = 784–859 nm) profiles. TDDFT calculations matched well the experimental trends and data. All compounds exhibited high extinction coefficients (178,000–267,000 cm–1 M–1) and good fluorescence quantum yields, resulting in high overall brightnesses. Remarkably, the selenium-containing dyes featuring terminal indole and benzoindole-type units exhibited impressive singlet oxygen quantum yields of around 13%, a standout performance in the deep-NIR region. These values are particularly promising and highlights the potential of these dyes for deep-NIR imaging and photodynamic applications
Stranded assets in European agriculture during food system transformations
Get PDFDietary shifts, particularly reduced animal-sourced food (ASF) consumption in high-income countries, risk stranding substantial ASF-related assets. Linking agricultural and economic data to global multi-regional input-output models, we show that ASF assets represent 78% of EU27 + UK fixed agricultural assets, with €158 billion linked to livestock and €100 billion to feed production. We estimate that ASF reductions in EU27 + UK consumption of 9.5%, 60% and 100% could strand 18%, 50% and 77% of these assets, respectively. Current depreciation rates suggest there is generally sufficient time to phase out assets, offering pathways to limit stranding. Policy- and climate-induced stranding risks are intertwined and should both be incorporated into financial modelling as overlapping transition pressures. Given food producers' high exposure to stranding risks cascade throughout supply chains, integrated policy support to repurpose or phase out ASF-related assets is essential to avoid delays in sustainable food system transformations
Aspects of quantum transport in DNA origami nanostructures and in topological Kondo insulator SmB6
Get PDFFor the past seventy years, condensed matter physics has produced ever smaller and more energy efficient information processing devices based on quantum transport processes. In order to continue this trend into the future and also to develop platforms for the emerging industry of quantum computing it is necessary to explore new methods. This thesis details two studies performed with the aim of deepening our understanding of the quantum transport in meso- and nanoscale systems such that they might be used in future information processing. In the first section, I discuss the fabrication, measurement and characterisation of nanogap devices and single molecule electronics. These devices are of strong interest for the role they could play in energy efficient classical computing and as novel qubit platforms. By exploiting DNA origami techniques developed by collaborators, devices can be fabricated with significantly higher yields than current methodologies. The major result of this is the fabrication of high yield nanogaps and a possible Copper porphyrin molecular device. This moves us significantly closer to achieving the deterministic fabrication of devices with multiple functional molecular components. In the second section, I present a theoretical study of the surface states of SmB6, a topological insulator. These materials are of strong interest in spintronics due to their spin polarised surface states. I demonstrate DFT studies that accurately reproduce some of the important experimentally observed features, providing insight into the role of interactions in the material
Performance of a Cryogenically Cooled GaN Inverter
Get PDFThis thesis details the development of a cryogenic GaN inverter. It encompasses transistor-level characterisation, inverter system modelling, and finally the design and testing of a GaN three-phase bridge operating at 77 K (−196 ◦C). This work is motivated by the need for highly efficient and power-dense motor drives for electric aircraft fuelled with liquid hydrogen.At cryogenic temperatures, there is an eight times reduction in the Rds(on) of GaN transistors. In principle, this could be exploited to build highly efficient motor drives with a higher power rating than an equivalent operating at conventional temperatures. The extreme operating conditions introduces the possibility of unexpected failure modes which may limit the performance of an inverter, which negate the benefits of cryogenic operation.The primary research question is therefore: “Are cryogenically cooled GaN inverters suitable for aircraft applications?”To answer this question, a 650 V GaN HEMT’s failure modes and loss mechanisms are examined at cryogenic temperatures. First, high currents in excess of the rated current, are used to determine its Rds(on) characteristics, and to test its functionality and performance when subjected to large thermal loads. The switching behaviour is tested using a double pulse test. The switching energies at cryogenic temperatures are measured to decrease by 27 % at 200 K (−73 ◦C). A simple adaptation to the driver circuit, using −6 V Vgs(off), is found to prevent dvds/dt induced shoot-through that occurred at cryogenic temperatures.Second, a cryogenic GaN inverter is modelled and used simulate an electric aircraft take-off drive cycle. From this it is concluded that if thermally limited, a cryogenic inverter with the same device type and die area is capable of delivering 6.8 times more power than when cooled using non-cryogenic conventional cooling.Finally, a 400 V cryogenic GaN inverter is designed and tested up to 11 kVA output, achieving a peak efficiency of 99.79 %, demonstrating operation at power an order of magnitude higher than any cryogenic GaN converter published to date in the literature. The cause of failure at higher powers is found to be interaction between the parasitic source inductance and the increased did/dt at cryogenic temperatures. Reducing the gate drive impedance was found to reduce oscillatory behaviour at the turn on switching instance, this allowed higher test power to be achieved
Developing space instrumentation for exploring airless bodies in the mid-infrared in support of NASA's Lunar Trailblazer and ESA's Comet Interceptor
Get PDFThis thesis describes work that supports the development of thermal instruments for two key space missions: NASA’s Lunar Trailblazer and ESA’s Comet Interceptor. The focus is on the Lunar Thermal Mapper (LTM), designed for accurate thermal mapping of the lunar surface, and the Modular InfraRed Molecules and Ices Sensor (MIRMIS), intended to study the volatile content of a dynamically new comet.The work presented covers the optimisation of internal temperature measurements, calibration procedures, and overall radiometric performance of each instrument. For LTM, extensive testing and calibration ensure that the instrument meets stringent performance requirements, enabling precise temperature measurements and supporting accurate mineralogical assessments of the lunar surface. Key challenges include accounting for the thermal environment and improving calibration accuracy under light conditions. For MIRMIS, the work described within this thesis develops operational simulations to guide instrument design and ensure that it meets its scientific objectives during high-velocity flybys of comets. The simulation provides a comprehensive end-to-end model, linking operational parameters to scientific outputs. This simulation is crucial for enhancing the involvement of the scientific community in the development of the MIRMIS instrument, ensuring that its design and operations are fully aligned with the mission's scientific goals.This research forms the foundation for future instrument operations, with applications covering mission scenarios of complex thermal environments, thus contributing significantly to the scientific success of both space missions
No evidence that haplodiploidy favors the evolution of eusociality
Get PDFThe potential role of haplodiploid sex determination in promoting the evolution of altruism and eusociality has been the subject of intense debate for over 50 y. Different theoretical models have suggested that haplodiploidy influences relatedness in a way that either does or does not make it easier for altruism to evolve. This debate over the "haplodiploidy hypothesis" can only be resolved with a decisive empirical test that controls for potential phylogenetic bias. Here we critically examine the current state of evidence for an adaptive link between haplodiploidy and eusociality, applying phylogenetically informed methods to ensure that statistical tests reflect independent evolutionary transitions. Using data from 5,678 species, across all major insect orders, we find no evidence that haplodiploidy favors an increased rate of eusocial evolution. We show that this result is robust to: a) different analytical approaches; b) alternative ways of defining both eusociality and haplodiploidy; and c) uncertainty in eusociality assignments. Our analyses suggest that previously reported associations between haplodiploidy and eusociality are likely to have been artifacts, false-positive results primarily driven by a high transition rate to eusociality within the Hymenoptera. This high transition rate could be explained by any factor associated with that group, such as parental care, monogamy, or the possession of a powerful sting
Disinformation, foreign interference and the right to free elections before the ECtHR in Bradshaw and Others v United Kingdom
No full textIn Bradshaw and Others v United Kingdom, the European Court of Human Rights recognised, for the first time, that disinformation and foreign information manipulation and interference engage the right to free elections under Article 3 of Protocol 1 (P1-3) of the European Convention on Human Rights. It further held that states may have a positive obligation to take measures to protect the integrity of electoral processes against such threats where there is a real risk that the ‘very essence’ of the P1-3 right will be curtailed and deprived of its effectiveness. While Bradshaw is an important milestone in the emerging jurisprudence on disinformation and foreign interference, it is susceptible to critique along four lines: (i) it reveals a disconnect between the theory and practice of Convention rights; (ii) it leaves significant uncertainty across the Council of Europe about the existence, extent and fulfilment of states’ obligations to protect electoral integrity against such threats; (iii) the evidentiary threshold set by the Court poses a potentially insurmountable hurdle for future claimants, while the threshold for state compliance is comparatively low; and (iv) the Court missed the opportunity to elucidate the ‘public’ aspect of P1-3 for citizens and states alike
Structures of proteinase 3 and the CD177 receptor complex reveal a major autoantibody epitope
Get PDFGranulomatosis with polyangiitis is a life-threatening systemic vasculitis, characterised by anti-neutrophil cytoplasmic autoantibodies (ANCA) most commonly against proteinase 3 (PR3), a protease expressed intracellularly and on the surface of neutrophils. Most cell surface PR3 is bound to the receptor CD177; however, the molecular mechanism of the interactions is not well understood. Here, we present crystal structures of CD177 in complex with PR3 and unliganded CD177. We describe a mainly hydrophobic binding interface between PR3 and CD177, involving the first two Ly6/uPAR (LU) domains of CD177. These form a globular structure which is connected to downstream domains via a flexible linker. Using a panel of PR3-ANCA-positive patient samples, we show that a significant proportion of ANCAs target the CD177-binding site of PR3 in these samples. Structure-guided mutation of the CD177-binding site on PR3 is effective in reducing PR3-ANCA binding. The results demonstrate that the CD177-binding surface of PR3 harbours a major PR3-ANCA epitope, and that the extent of binding to this surface varies between different patients