Theses - Physics

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Open Access
Ultrahigh resolution surface phononics
Liu, Boyao; Liu, Boyao [0000-0001-9329-3428]
Phonons are one of the most important quasiparticles in condensed matter physics; they influence a wide range of physical properties ranging from thermal and electrical conductivity to the propagation of sound. Phonons are also crucial to emerging research fields such as quantum information and superconductivity. This thesis mainly focuses on the measurement of surface phonons using the helium spin echo (HeSE) technique. In Chapter 1, the concept of phonons is introduced. An overview of phonon measurement techniques is also presented, followed by a more detailed introduction of HeSE. Since the energy resolution of HeSE is in µeV regimes, much higher than other techniques, phonon linewidths can be extracted. This demonstrates HeSE's unique superiority because phonon linewidth is a property which is crucial in many research field related to phononics. Phonon linewidths are typically influenced by three different mechanisms, which are phonon-phonon interaction, defect-phonon interaction, and electron-phonon interaction, which are all discussed in this thesis. Chapter 2 provides a theoretical framework for phonons by using a simplified model of a monatomic chain. The theoretical derivation in this chapter uses time-independent perturbation theory, which aims to simplify existing results from the literature. It is shown that crystal anharmonicity is related to phonon-phonon interaction. It is also proved that phonon-phonon interaction will make phonon energy and phonon linewidth change linearly as a function of temperature. Both trends are demonstrated using HeSE phonon measurement data on a Ni(111) surface. A proof is also given to show that phonon lifetime is inversely proportional to phonon linewidth. Chapter 3 presents a direct experimental measurement of how crystal defects broaden the linewidths of Rayleigh wave (RW) mode phonons on a Ni(111) surface. Defects are found to contribute a temperature-independent component to the linewidths of RW phonons on a Ni(111) surface. Chapter 3 also characterised the increase in phonon scattering with both surface defect density and phonon wave vector. A quantitative estimate of the scattering rate between phonon modes and surface line defects is extracted from the experimental data. In Chapter 4, RW phonon linewidths on a Ru(0001) surface are studied. It is found that, contrary to most phonon measurements, the linewidths of RW phonons decrease with temperature below about 400 K. This is due to the interaction between electrons and phonons. A quantitative model combining phonon-phonon interaction, defect-phonon interaction, and electron-phonon interaction is used to explain the data. Chapter 5 is about the interpretation of the intensity, or height, of peaks in HeSE phonon spectra. The logarithm of the height of diffuse elastic peaks in HeSE spectra is found to decrease linearly as a function of temperature. The phenomenon is attributed to Debye-Waller attenuation. Theoretical analysis has also been performed to analyse the temperature dependence of the phonon peak intensity. The ratio between phonon peak height and elastic peak height versus temperature has been found to give the Bose-Einstein distribution of the phonon. Moreover, electron-phonon coupling constants have been extracted from the data. Chapter 6 presents the newly-designed spin manipulation system of HeSE. Combining two new spin precession solenoids, three RPSu power supplies, and a switch system, the new spin manipulation system improves the performance of the Cambridge HeSE instrument. The new system can be used for beam profile measurements, surface diffusion measurements, and surface phonon measurements. This thesis finishes with Chapter 7 discussing a conclusion of the preceding chapters and an outlook of potential research projects that can emanate from them.
Open Access
Interaction of Light and Conjugated Polymers in Plasmonic Nanogaps
Xiong, Yuling; Xiong, Yuling [0000-0003-1820-3722]
Conjugated polymers, characterized by a backbone of alternating double and single bonds, exhibit unique optical and electronic properties due to their delocalized electrons. These properties make them suitable for various applications, including organic transistors, electrochromic displays, and flexible electronics. Integrating conjugated polymers into plasmonic nanocavities has unlocked electrochromic plasmonic coloration with nano-scale resolution display potential and has served as a platform for nanoscale mechanism characterization. This thesis focuses on the electrochromic nanoparticle on mirror (eNPoM) geometry, which consists of conjugated polymer-coated gold nanoparticles positioned on a gold mirror. This structure has shown potential as a switchable plasmonic structures with bistable colors scalable from single nanoparticles to centimeter-scale films. Despite its promise, enhancing color performance for commercial viability remains a challenge. To address this, my research extends across various conjugated polymers to widen the color switching range and introduces a novel polymerization method using a co-solvent system, enabling a more extensive incorporation of polymers into the eNPoM framework. During optimization, I observed phenomena such as reversed color switching in dark-field (DF) and fluctuations in surface-enhanced Raman scattering (SERS) spectra in eNPoM systems with shell thicknesses below 5 nm. These observations point to previously unidentified aspects of the physics underlying the polymer-plasmonic nanocavity interface. Further analysis revealed that the reversed color switching under DF corresponds to the optical anisotropy and orientation of polymer chains near metallic interfaces, offering a novel approach to study material interfaces. Moreover, I observed significant fluctuations in SERS spectra within eNPoM systems, suggesting the possibility of SERS detection down to the few or single molecule level, likely facilitated by the formation of pico-cavities. Through statistical analysis of the spectral features of these fluctuating SERS signals in air, combined with multi-Gaussian peak fitting analysis on the electrochemistry of single-molecule SERS, I have gleaned insights into the dynamic interactions between the polymer and the gold surface. This approach holds promise for unveiling transient structural fingerprints and deepening our understanding of redox transition doping mechanisms.
Embargo
Charge Transport Physics in Coordination Nanosheets (CONASHs)
Wu, Tian
Metal-organic frameworks (MOFs) have emerged as a versatile class of materials with intriguing properties, driven by their modular structure and tuneable functionalities. The insulating nature of most MOFs has hindered their applications which require highly conductive MOFs. A specific type of MOF distinguished by its two-dimensional (2D) π-conjugated structure called coordination nanosheet (CONASHs) exhibits significantly enhanced conductivity. This study delves into the unique charge transport characteristics of CONASHs. The investigation focuses on two exemplary coordination nanosheets, copper-benzenehexathiol (Cu-BHT) and nickel-based benzenehexathiol (Ni-BHT), presenting a detailed exploration of their charge transport properties. A comprehensive temperature-dependent transport study of Cu-BHT shows its remarkable conductivity exceeding 1500 S/cm at low temperatures and metallic transport nature. The integration of magneto-transport and thermoelectric characterizations reveals insights into its ambipolar charge transport behaviour. The simultaneous extraction of electron and hole mobilities contributes to a deeper understanding of its complex charge transport physics. Electrolyte gating transistors (EGTs) based on Cu-BHT demonstrate a gate-tuned metal-to-insulator transition. This study unveils a tuneable charge density, leading to a five-order-of-magnitude change in conductivity. The observed transition from ambipolar to unipolar transport under positive gate voltage illustrates the controllability of conducting channels, establishing EGTs as a potent tool for manipulating charge-dependent properties in coordination nanosheets. The investigation extends to Ni-BHT, a non-porous coordination nanosheet with a conductivity up to 112 S/cm. The exploration of temperature-dependent conductivity reveals an Efros–Shklovskii variable-range hopping transport, and the anomaly between Hall effect and thermoelectric coefficients is attributed to abnormal Hall effect in strongly disordered systems. The introduction of an EGT based on Lithium Perchlorate/Poly(ethylene oxide) electrolyte allows for the tunability of polarity, showcasing its potential for the manipulation of electronic states in CONASHs. This study establishes coordination nanosheets as a promising avenue for materials engineering, leveraging their unique π-conjugated structure for enhanced conductivity. The controllable tuning of charge transport properties through electrolyte gating opens up exciting possibilities for applications in electronic devices, underscoring the broader significance of investigating charge transport in CONASHs.
Open Access
Development of applications of liquid crystalline elastomers from Materials to Applications
Guo, Hongye
Liquid crystalline elastomer (LCE) is a class of "smart polymer" with constituent molecules called mesogens, exhibiting orientational ordering. These mesogens can self-assemble into many types of ordering and transition to being isotropic reversibly subjected to external stimuli. In the last four decades, this field has seen many advancements: the first generation of polysiloxane LCEs successfully introduced this wonderful material to the world; the second generation of LCEs deployed much more robust and easier synthesis procedure, bringing in researcher from many other disciplines; the third generation of LCE utilised the bond-exchange behaviour in vitrimers to produce exchangeable LCE (xLCE), which enabled post-polymerisation alignment, welding and recycling of LCE. Stemming from such unique network structures and a wide range of synthetic chemistries, LCEs have a variety of remarkable properties. LCEs are renowned for the "soft elasticity", which is the ability to be elongated without increasing the stress; as well as the enormous viscoelastic dissipation originating from the rotation of internal nematic orders. LCEs are also famous for the reversible actuation properties which can be triggered by a wide range of external stimuli. LCEs have promising practical applications such as artificial muscles, smart textiles, sensors and soft robotics. This thesis aims to contribute to the field of LCE in the following three key aspects: (1) developing new LCE materials with novel synthesis routes and network structures, in order to find new or improved material properties; (2) investigating new fundamental physical effects associated with LCE in order to find new application areas; and (3) Designing and constructing demonstrator devices to illustrate the great potential of LCE materials. **New materials** First, novel side-chain LCEs based on the robust thiol-acrylate click reaction were developed. The effects of mesogen structures on the LCEs’ phase transition behaviours and network structures were investigated. The unusual features were the semi-crystalline nature of elastomers with non-polar mesogens, and the clear role of side-by-side rod dimerisation of polar mesogens leading to the higher smectic layer spacing. When stretching beyond the full alignment, the smectic structures were found to evolve in two ways: forming the coarsened Helfrich-Hurault zig-zag layer texture, or the large-scale stripe domains of uniform layer rotation in the systems with lower order parameter and the associated layer constraints. This then led to a new composite liquid crystalline elastomer, combining main-chain nematic and side-chain smectic together, which resulted in micro-phase separated regions of nematic and smectic ordering in the macroscopically homogeneous elastomer. Thermal phase transitions of both phases coexisting in the material were detected by calorimetry, and the nematic/smectic structure investigated by X-Ray scattering. The tensile stress–strain data revealed the key effect of such a multi-phase composite, where the nematic fraction adds ductility while the smectic fraction increases the modulus and mechanical stiffness. Thus, mechanical properties of this material type can be optimised by varying the composition. **New physical effects** Next, the effect of LCE viscoelastic impact damping was investigated, in comparison with a reference silicone rubber. This project focused not only on the energy dissipation, but also on the momentum conservation and transfer during the collision, because the latter determines the maximum force exerted, which is responsible for damaging the target and/or the impactor. To better assess the momentum transfer, I compared the collision with a very heavy object and the collision with a comparable mass, when some of the impact momentum is retained in the target receding away from the collision. A method to estimate the optimal thickness of an elastomer damping pad was proposed for minimising the energy in impactor rebound. It has been found that thicker pads introduce a large elastic rebound and the optimal thickness is therefore the thinnest possible pad that does not suffer from mechanical failure. This anomalously high vibration/impact damping in nematic poly-domain LCEs has also been assumed to be the cause of their anomalously high pressure-sensitive adhesion (PSA). The mechanism behind enhanced PSA was investigated by manufacturing generic LCE coated adhesive tapes with varying cross-linking densities. Industrial standard adhesion tests were performed to characterise LCE adhesion behaviours, which helped to reveal the strong dependence of adhesion strength on contact time. The long saturation time (sometimes exceeding 24 hours) is caused by the slow relaxation of local stress and director orientation in nematic domains after pressing against the surface. This mechanism was further confirmed by a freshly pressed and annealed tape reaching the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film. Based on these findings, a class of much stronger, multi-use amine-acrylate LCE adhesive materials was developed. These adhesives exhibit low tackiness at room temperature; however, upon heating and annealing, they can be activated, enabling effective deployment. The LCEs were formulated to have tunable glass transition temperatures which is crucial for the low tackiness at ambient temperature. All formulations showed high adhesion strength (peel force) in the nematic region (1.0 to 1.6 N/mm) and low peel force in the isotropic region. Furthermore, the adhesive materials demonstrated the capability for reuse in more than five heating and cooling peeling cycles and have shown remarkable contamination tolerance to sand, oil, and dirt. Moreover, these adhesive materials displayed adhesion strengths (lap shear) that comparable to those of traditional PSAs, reaching up to 3 MPa, with a clean detachment. **New device** Finally, a spontaneous heliotracking prototype device was designed and constructed based on the differential light-induced actuation of LCE. The design drew inspiration from nature itself with many living organisms responding to light stimulus and track the light source. The synthesis of the actuator material involves a robust thiol-acrylate "click" polymerisation, while the addition of indocyanine green (ICG) dye imparts the sensitivity to broad-spectrum and near-infrared light. Highly reproducible thermal and photo-induced linear actuation was demonstrated. The device is based on a freely pivoting payload platform held in place by several linear LCE actuators around the 360° circumference. The side of the device, when exposed to light, has the actuators contracting and tilting the platform towards the light source. As the light source was moving around the device, the platform tilt followed, always exposing the payload face to the light; in the dark, the device recovers its neutral position.
Open Access
The tin-vacancy centre in diamond: a coherent spin-photon interface for quantum network nodes
Stramma, Alexander Moritz
Quantum networks require spin-photon interfaces capable of generating entanglement between a stationary qubit and flying photons. So far, such an interface with a large emission rate of indistinguishable photons and excellent spin properties has been missing. Group IV colour centres in diamond offer strong emission into the zero-phonon line, an inversion symmetric structure and integration into nanophotonics. Especially the tin-vacancy centre in diamond has both excellent optical properties and long spin coherence times at elevated cryogenic temperatures. High-fidelity quantum control, however, has remained elusive. In this thesis, we introduce a new platform: deterministically strained tin-vacancy centres in a thin diamond membrane. The crystal strain allows microwave control of the spin state with a gate fidelity of 99.36(9) % at aligned magnetic fields with highly spin-selective optical transitions. Dynamical decoupling protocols are used to protect the spin coherence for up to 5.7(11) ms. We identify the properties of the spin bath, allowing us to understand material engineering challenges for next-generation devices. The crystal strain suppresses phonon-induced dephasing processes, enabling coherence times of up to 223(10) μs at 4 K, a record-value for group IV colour centres. For a low-strain tin-vacancy centre in a diamond nanopillar device, we show polarisation of more than 65 % of a strongly coupled 13C nuclear spin. We find that optically induced laser-detuning independent dephasing limits the gate fidelities of the all-optical stimulated Raman drive, confirming microwave control as the most viable route for quantum control of the spin-photon interface. Lastly, we report on the development of a versatile open optical microcavity platform for quantum materials. The combination of high-fidelity quantum control shown in this thesis and achievements in related works, like fibre-packaged diamond waveguides and an intrinsic strongly coupled nuclear spin, renders the tin-vacancy centre as a prime candidate for quantum network nodes.
Open Access
New Insights on the Evolutionary Mechanisms of Local and Distant Galaxies
Baker, William; Baker, William [0000-0003-0215-1104]
Galaxies can be roughly characterised by falling into two distinct populations, blue, star-forming, disc dominated and red, quiescent elliptical bulge dominated systems. These populations are defined via multiple scaling relations such as the star-forming main sequence, Schmidt-Kennicutt relation and Molecular Gas Main Sequence. But what halts star-formation in the disc galaxies and causes them to turn into quiescent bulge dominated galaxies? This stopping of star-formation is the process of galaxy quenching. Exploring this is the target of my thesis. In the first part of my thesis I use a methodology of partial correlation coefficients and random forest regression to disentangle multiple inter-correlated galaxy properties enabling me to determine the intrinsic drivers of some of the key galaxy observables. I will start in Chapter 1 with an introduction to the field and the reasoning behind the research. Chapter 2 then introduces the methodology used and explains partial correlation coefficients and random forest regression. Then I start in Chapters 3 and 4 by exploring the drivers of the star-forming main sequence, the relationship between star-formation rate (SFR) and stellar mass ($M_*$), finding that it is simply a by-product of the more fundamental Schmidt-Kennicutt relation, between SFR and molecular gas mass ($M_{H_2}$), and the Molecular gas main sequence, between $M_{H_2}$ and $M_*$, on both global and spatially resolved scales. I show that this is the case throughout most of cosmic time. In Chapters 5 and 6, I then expand my analysis to investigating gas-phase metallicities and the real drivers of the mass-metallicity relation (MZR) and fundamental metallicity relation (FMR) on both resolved and global scales. I find evidence for a resolved version of the FMR and also that local metallicity depends on both local and global galactic properties, hence the global FMR is not simply a by-product of the local version. I also find that dilution alone cannot describe the FMR. I discover that gas-phase metallicity does actually depend on stellar mass and that this is not simply a proxy for the dynamical mass or gravitational potential. I suggest that the explanation for the MZR is via the effects of integrated metal production. In Chapter 7 I investigate the drivers of stellar metallicity for star-forming and passive galaxies in order to investigate quenching explicitly. I find that for star-forming galaxies the stellar metallicity is driven by stellar mass as expected, but that for passive galaxies it is driven by black hole mass, showing the effects of integrated AGN feedback in quenching the galaxies via starvation. In Chapter 8 I utilise the power of the new JWST to explore the properties of one of the earliest galaxies. I use an approach of spatially resolved photometry to explore the properties of the galaxy's disc and core components. I then use SED modelling to infer the stellar population properties and compare to more local analogues to explore the galaxy's role as a possible progenitor.
Open Access
Analysing First Passage Time Distributions of Large Ill-Conditioned Energy Landscapes
Woods, Esmae; Woods, Esmae [0000-0001-9614-0865]
In this thesis we develop theory and associated computational tools to investigate the kinetics of competing pathways on multifunnel energy landscapes. Multifunnel landscapes are associated with molecular switches and multifunctional materials, and are expected to exhibit multiple relaxation time scales. Energy landscapes, consisting of local minima connected by transition states, can be represented by a kinetic transition network. Each local minimum is represented by a node in the network, and two nodes are joined by an edge if a transition state directly connects their associated minima. Each edge has a forwards and backwards branching probability, and a waiting time is associated with each node. This provides a continuous time discrete state Markov chain that represents the network. Our focus is on investigating, understanding, and improving algorithms to compute first passage time (FPT) distributions. First, we introduce new insights into FPT distributions, including showing how the distribution depends on initial conditions, and how features can be assigned to specific kinetic traps. When the state space gets too large, finding the full FPT distribution becomes computationally expensive. We introduce partial Graph Transformation, a network reduction tool that conserves the mean first passage time, and approximately preserves the full first passage time distribution. When there are significant differences in the fastest and slowest transition timescales in the network, the system becomes ill-conditioned. In practical terms, the separation of timescales increases as the system temperature is reduced, which leads to loss of precision in linear algebra computations. We introduce a method to reconstruct the FPT distribution in the ill-conditioned regime, by combining accurate treatment of mean first passage time computations with the reliable short time parts of the FPT distribution from linear algebra approaches. We test our theoretical and computational developments on two model landscapes, and a Lennard-Jones cluster.
Open Access
Tuneable homogeneous Bose fluids close to and far from equilibrium
Dogra, Lena
This thesis describes experiments with an ultracold weakly interacting Bose gas of 39 K in a 3- dimensional homogeneous optical trap. We expound three phenomena in different regimes from adiabatic equilibrium to ‘true’ far-from-equilibrium. We first outline a cooling effect driven by three-body recombination, which has traditionally been associated with heating and adverse effects. Three-body loss in a partially condensed three-dimensional homogeneous Bose gas, should, under certain conditions, even purify the sample, i.e. reduce the entropy per particle and increase the condensed fraction. The mechanism is a purely ideal gas effect, with the only role of weak interactions being to ensure thermalisation. We show that this ideal gas effect is robust to the influence of weak interaction energy and suggest realistic experimental conditions for the observation of the cooling and purification. Second, we describe our simultaneous observation of first and second sound, a phenomenon that is central for the concept of superfluidity. Sound waves occur close to equilibrium, as a linear response to weak perturbations. With liquid helium, the existence of two different sound modes at the same wavelength has been studied, where the modes are a pressure wave as in air, and a temperature wave. While it is known that Landau’s famous two-fluid model successfully captures the behaviour of the almost incompressible helium, we study the two modes in the opposite limit of a compressible Bose gas. By exciting center-of-mass oscillations of our homogeneous gas at different frequencies, we find two resonances out of which only one persists above the critical temperature. We further explore the microscopic structure of the two modes, which shows density oscillations dominated by, respectively, thermal and condensed atoms for first and second sound, in agreement with Landau’s hydrodynamic theory. Finally, in a normal gas above the critical temperature, we explore the crossover between the hydrodynamic and the collisionless regime by varying the interaction strength. The third subject of this thesis is turbulence, a paradigmatic example of a far-from-equilibrium phenomenon. For far-from-equilibrium states, (equilibrium) thermodynamic descriptions do not apply even locally, and it is a major challenge to find similarly concise macroscopic descriptions, in a variety of contexts also including glasses and active matter. Here we use the same excitation scheme as for our study of first and second sound, to now drive the Bose gas strongly. Under continuous drive, a turbulent cascade steady state is established, which is characterised by a power-law momentum distribution and is sustained by a constant energy flux. We find that the exponent of the distribution is essentially independent of the details of the system, and establish experimentally the cascade amplitudeand the transported energy flux as state variables that are related through a relation that is independent of the forcing and the dissipation, as well as the history of the system, in analogy to equilibrium equations of state. Finally, we show that the equations of state for a wide range of densities and interaction strengths can be scaled onto each other, giving a universal dimensionless equation of state that provides benchmarks and challenges for the theory and could also be relevant for other turbulent systems.
Open Access
Development of graphene growth surfaces for III-V semiconductors
Zulqurnain, Muhammad; Zulqurnain, Muhammad [0000-0002-6612-049X]
Modern electronic and optoelectronic device industry is shifting from conventional rigid and bulky devices to smarter, flexible, transparent, economical, extra efficient and multifunctional devices. III-V semiconductor materials are an integral part of these devices due to their superior optical properties and electron mobilities as compared to silicon. However, the use of III-V semiconductor materials is currently reserved for high specification use cases. Two key challenges, which limit their application to emerging technology products are cost and integration. One approach to minimise the cost is to adopt thin films of these materials which can be released from its substrate and heterointegrated with silicon or transferred to flexible substrates. This is of particular interest for energy conversion devices. However, the fabrication of high quality large area thin films at lower cost is still a significant challenge. This work focuses on the development of the emerging technique of remote epitaxy, which exploits an atomically thin two-dimensional (2D) material as an interface layer between a III-V growth substrate and an epitaxial film. For example, 2D material such as graphene coated on GaAs substrates allows the registry information of the underlying substrate to permeate through the graphene and facilitate the formation of exact copy of single crystal growth template. Therefore, the grown layer would replicate the crystal orientation of the underlying substrate. The critical advantage of this technique is that the film is only bonded to the graphene with a Van der Waal’s bond, allowing it to be readily released from the growth substrate non-destructively and subsequently bonded to an alternative flexible substrate to fabricate optoelectronic devices. In this work, we fabricate GaAs thin films epitaxially on graphene coated substrates. I have shown that the CVD graphene grown on Cu, wet transferred to a GaAs substrate can be used as an interface layer for the growth of single crystal epitaxial GaAs films and subsequent exfoliation. We observe wet transfer of graphene leads to the formation of a native oxide layer at the graphene/substrate thus widening the gap between graphene and the substrate. This hinders the remote interaction from the substrate. To mitigate the problem of oxide layer formation we exposed graphene to an Ar-ion beam to create pinhole defects. This allows the desorption of native oxides at elevated temperature and the nucleation of GaAs at defect sites followed by lateral overgrowth. The epilayer is exfoliated from the growth substrate revealing the nucleation of the epilayer through pinholes. We also explore the possibility of semi-dry transferred CVD graphene to avoid native oxide growth at the graphene interface, reducing the need for defect seeding of the epitaxial layer. The processes demonstrated in this work would significantly reduce the cost of fabricating thin films and pave the way for their industrial scale adoption.
Embargo
Predicting the structure and performance of dye-sensitized solar cells by computational methods.
Devereux, Leon
Dye-sensitized solar cells (DSCs) are a photovoltaic technology based around light-harvesting dye molecules bound to thin semiconductor films of high surface area. Many of the highest-performing DSCs to date incorporate multiple dyes that harvest light from different regions of the solar spectrum in a complementary manner – these are known as cosensitized DSCs. However, finding dyes that are well-suited for cosensitization is a long and costly experimental process when carried out through trial and error in a laboratory. To help direct experimentalists towards promising candidates, the main project of this thesis harnesses ideas from data-driven materials discovery to develop an entirely computational pipeline that predicts boosts in performance of dye pairs when cosensitized. It does this by identifying partner dyes that show the most complementary absorption characteristics to sets of well-known or high-performing starting dyes, systematically sifting candidates from a large database of optically active compounds. It then uses density functional theory (DFT) simulations to compute key structural, electronic and optical properties of the selected pairs of dyes, which are used as inputs to models that predict short-circuit current density (J_SC) and open-circuit voltage (V_OC), two key device performance parameters. The predictive models for J_SC and V_OC of singly-sensitized devices are developed further from existing models used in previous works, and are also expanded to the cosensitized case for the first time. 11 starting dyes were passed through the pipeline (six organic and five organometallic), leading to 22 dyes in total being modelled at the DFT level as 11 pairs. The accuracy of predicted J_SC and V_OC for single sensitizers was tested against existing experimental references. Notably, half of the J_SC predictions were within 20% error or less of experimental values whilst others had greater discrepancies, the sources of which are discussed in detail. These results are significant given the choice of structurally dissimilar dyes here – this accuracy is on par with previous computational studies that focussed only on sets of structurally analogous dyes. From the predictions of cosensitized devices containing the complementary dye pairs, two standout cells were those containing **SQ2**+**LD2** dyes and **YD2**+**VKXB** dyes, which gave +13% and +12% boosts to J_SC relative to their singly-sensitized counterparts, respectively. A secondary computational project was also carried out in collaboration with previous experiments of DSC dye monolayer growth over time. Whilst complete dye monolayers have been studied extensively, their behaviour as they grow is less well understood, despite its importance for DSC fabrication. X-ray reflectometry (XRR) had been used by a collaborator to investigate monolayer thicknesses and densities as they grow under different conditions in the DSC fabrication process. This author trained a neural network to perform rapid, deterministic fitting of 360 experimental reflectivity curves in high-throughput fashion. The DSC dye layer parameters predicted by this machine-learning model were compared to those from a human-assisted fit with standard software (such fitting being orders of magnitude slower to carry out). The neural network predictions had high accuracy for instances where monolayers adhered to the assumptions of the Parratt model used to fit reflectivity curves, but poorer accuracy during periods of faster change in thickness, suggesting dynamic behaviour of dye ensembles that warrants further investigation. Thus, the neural network acted as a supporting tool to identify where to focus further experimental DSC investigation, which is the overarching theme connecting the two projects of this thesis. Chapter 1 provides a literature review of DSC function, the structure-property relationships of their component materials, and pre-existing computational methods that predict DSC performance. Chapter 2 provides a technical background to the density-functional theory (DFT) methods used throughout much of this work. Chapter 3 presents the design-to-device pipeline methodology developed in this work. Chapter 4 displays and discusses the results of this pipeline as applied to six well-known or high-performing organic dyes and their six complementary partner dyes identified. Chapter 5 similarly presents results for five ruthenium-based dyes and their cognate organic partner dyes that were identified by the pipeline. Chapter 6 provides a background to XRR and neural networks, before presenting the training of neural network and evaluating its performance in reproducing fitted layer parameters from the experimental XRR data described above. Chapter 7 discusses the conclusions of this work and how further research may be enabled.
Embargo
Jahn-Teller distortions in layered nickel oxides
Nagle-Cocco, Liam; Nagle-Cocco, Liam [0000-0001-9265-1588]
This thesis presents the results of my work on Jahn-Teller distorted transition metal oxides. The Jahn-Teller distortion is a key factor in important phenomena and applications of materials with octahedrally-coordinated transition metal ions such as batteries, high-temperature cuprate superconductivity, and colossal magnetoresistance. This work advances both the methodology for parameterising Jahn-Teller-distorted octahedra in crystal structures, via calculation of the Van Vleck modes, and sheds new insight onto the Jahn-Teller distortion in layered nickel oxides LiNiO₂ and NaNiO₂. A new method for calculating the van Vleck modes is presented which, in contrast to the established method in the literature, does not assume octahedra are angularly-undistorted and hence allows for the acquisition of information on octahedral shear. A Python package, VanVleckCalculator, is presented which is available on GitHub for public use. This code is used in the analysis presented throughout this thesis. Variable-temperature studies on the crystal structure of NaNiO₂, using synchrotron diffraction as a probe of the average structure, in conjunction with EXAFS and neutron total scattering as probes of the local structure, are presented. The neutron total scattering data is studied via modelling of large supercells. The analysis yields insight on the nature of the Jahn-Teller transitions. I find a displacive Jahn-Teller transition in NaNiO₂, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO₃ showing an order-disorder transition. *Ab initio* molecular dynamics calculations using density functional theory, which were performed by a collaborator, has been used to obtain many crystal structures at various temperature points, on LiNiO₂ and NaNiO₂. These are studied in-depth using VanVleckCalculator with the results for NaNiO₂ indicating a displacive transition, consistent with the experimental work. The work on LiNiO₂ shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO₂. Finally, variable-pressure studies on NaNiO₂ using both neutron and x-ray synchrotron diffraction are presented. It is shown that in the range from ambient pressure to ~6 GPa, the Jahn-Teller distortion is reduced in magnitude while also remaining cooperative to higher temperatures (i.e. *dT*_JT/*dP* > 0). At higher pressures, it is also shown that there is a transition from the monoclinic to rhombohedral structure which begins but is not observed to complete, potentially indicating a dome-shaped phase diagram.
Restricted
Plasmonic Biosensing via Surface-Enhanced Raman Spectroscopy
Niihori, Marika
[Restricted]
Controlled Access
EXCALIBRATE Bayesian calibration for data-intensive astrophysical experimentation
Roque, Ian Laurent Van; Roque, Ian [0000-0003-4874-9371]
The detection of minute radio-frequency signals from the primordial Universe are thought to contain fundamental information on the evolution of the first luminous sources. Such breakthroughs however are hindered by the unprecedented levels of sensitivity and calibration needed to confidently distinguish these millikelvin-level signatures from galactic foregrounds and instrument systematics. In this work we detail the development of a calibration methodology that expands upon the Dicke switching procedure introduced for microwave-frequency devices and apply it to contemporary experiments targeting early time periods such as the Dark Ages, Cosmic Dawn and Epoch of Reionisation. Included are the designs and practical considerations for a receiver unit housing numerous calibration standards, a compact microcontroller unit, portable vector network analyser and Peltier-based thermal management system for deployment with the REACH radiometer experiment in the South African Radio Astronomy Observatory. Following this, we detail a first-of-its-kind Bayesian calibration algorithm named EXCALIBRATE which offers unparalleled speed and mobility, allowing for the characterisation of the radiometer in the same environment as observational measurements. Datasets taken at various points of the receiver development are evaluated with EXCALIBRATE which achieves calibration accuracies of about 1 kelvin or less. Upon numerous adjustments to both the physical receiver unit and our code, we demonstrate that the polynomial approximation for calibration parameters used by EXCALIBRATE may not be an appropriate model for continued advancement towards a calibration accuracy less than ten millikelvin. Such results may call into question, the analysis and interpretation of experiments that rely on polynomial models for calibration parameters such as the EDGES experiment, which has reported the first possible detection of an absorption profile from the Cosmic Dawn. In light of this, we derive a mathematical framework for an alternative method to solve for calibration parameters as singular values at each frequency point and conclude with further suggestions for increasing the sensitivity of the radiometer.
Embargo
Advances in understanding protein-lipid interactions through biophysical and bioelectronic approaches
Kadgathur Jayaram, Akhila
Intrinsically disordered proteins (IDPs) are those that to do not possess a well-defined three dimensional structure. As a result, they show great flexibility when it comes to adopting a suitable conformation based on the target of interest. However, due to the nature of the free energy landscape with respect to protein folding, they can misfold and transform into toxic oligomeric or fibrillar species. In particular, this thesis focuses on a subset of IDPs, namely amyloid-beta 40 and 42, α-synuclein and Fused-in-Sarcoma. These proteins are implicated in the development and progression of neurodegenerative disorders such as Alzheimer’s Disease, Parkinson’s Disease, Amyotrophic Lateral Sclerosis and Frontotemporal Dementia respectively. The phenomenon of aggregation is seen to be the hallmark connecting the three IDPs, with multiple factors either accelerating or inhibiting the process. Currently, there is no cure for these disorders, therefore it becomes imperative to better understand the mechanisms behind their aggregation, so as to enable the development of therapeutics. This thesis focuses on understanding how these proteins interact with lipid molecules. Lipids form the majority component of the cell membrane. They are not just involved in maintaining the structural integrity of the plasma membrane, but are also involved in more complex processes such as signalling, regulation and intracellular transport. As the IDPs mentioned above are known to interact in the cytoplasmic space, it is interesting to characterise their interactions with lipids not just from a mechanistic standpoint, but also in terms of understanding their influence on subsequent aggregation or membrane disruption. To quantitatively characterise these protein-lipid interactions, we utilise a variety of biophysical approaches such as monitoring of aggregation kinetics, circular dichroism, transmission electron microscopy, microfluidic diffusional sizing and confocal microscopy. In our studies, we have aimed to incorporate natural membrane lipids in addition to the conventionally used synthetic lipids in order to increase the biological significance of our findings. We have also conceptualised a novel polymer-based bioelectronic device as a proof-of-concept to analyse membrane perturbations arising from the interaction of α-synuclein with anionic lipid membranes.
Embargo
High Yield Fabrication of Monolayer WSe₂ Mechanical Resonators and a Study of Their Loss Mechanisms
Pitts, Michael
As a result of the advances in the field of quantum computing, there is increasing demand for quantum transducers to enable coupling between different quantum systems over long distances. Monolayer tungsten diselenide (WSe₂) mechanical resonators have great potential as an answer to this demand: possessing large zero-point motion, being optically active due to its direct bandgap, and its ability to host quantum emitters. In order for monolayer WSe₂ resonators to reach their potential it is necessary to develop methods of improving their quality factors. The first step on this road is understanding the dominant mechanical loss mechanisms in monolayer WSe₂ mechanical resonators. In this thesis, I describe the method I have developed to fabricate fully suspended monolayer WSe₂ mechanical resonators using a high yield fabrication method based on gold exfoliation of WSe₂, polycarbonate (PC) transfer, and a delicate chloroform cleaning. With this I am able to produce thousands of resonators with low contamination and high room temperature quality factors, of up to 1050 for 5 µm diameter resonators and 500 for 2.5 µm diameter resonators. I demonstrate my use of a reflection based Michelson interferometer detection scheme to detect thermal motion of monolayer WSe₂ resonators at room temperature and how we are able to take direct measurements of resonator masses to determine cleanliness. I also discuss my use of UV laser thermomechanical driving to allow measurement of resonators at cryogenic temperatures of 4 K. Using these methods we are able to measure 42 resonators with 2.5 µm diameter and 19 resonators with 5 µm diameter at room temperature across a range of frequencies resulting from different levels of pre-tension induced during fabrication. I then fit a model of quality factor based on bending losses and find a good agreement with the data providing strong evidence that bending losses dominate mechanical losses in layered material (LM) resonators at room temperature, and that dissipation dilution will work as a method of improving quality factors. Finally I describe my investigation into mechanical loss mechanisms at cryogenic temperatures through the use of phononic shielding. I compare the results of measured quality factors from experiments done on 2.5 µm diameter and 5 µm resonators samples on TEM grid membranes to 4 µm diameter resonator samples utilising phononic shielding through the use of phononic crystal membranes. I find that implementing phononic shielding results in a fivefold enhancement of the maximum quality factor observed, increasing from Q₅ = 17,000 for 5 µm diameter resonators to Q₄ = 94,000 for the phononically shielded 4 µm diameter resonators. To the best of my knowledge, this represents the highest recorded quality factor for monolayer WSe₂ resonators in the literature today. Finally I discuss how this data indicates that phonon tunnelling losses appear to play a dominant role in mechanical losses in LM resonators at cryogenic temperatures and that phononic shielding may allow quality factors to be increased further. The development of a fabrication and detection method capable of producing and measuring large numbers of monolayer WSe₂ resonators opens up new possibilities for the systematic study of mechanical losses in LM resonators. Additionally, the ability to fabricate fully suspended resonators has created the opportunity to study new resonator geometries, such as ’membrane in the middle’ cavities and further research into phononic crystal supported resonators. Furthermore, the evidence indicating that bending losses dominate LM resonator mechanical losses at room temperature and phonon tunnelling losses play a role at cryogenic temperatures opens up the possibility of developing techniques to reduce mechanical losses of LM resonators, allowing further increases in quality factor in future.
Embargo
Computer vision and deep-learning tools for automatic data extraction from chemical reaction schemes
Wilary, Damian
This thesis focuses on the application of computer vision and deep-learning techniques to the development of an artificial intelligence tool for image-mining chemical reaction schemes. In particular, the aim of this work is the development of an autonomous, high-throughput tool for converting visual data into machine-readable formats. Data extracted in this manner can be used for the creation of databases and provides insight into the chemical domain through a big data approach, which facilitates the chemical discovery process. The extraction process is also complementary to the more common data mining workflows based on natural language processing, since data from reaction schemes are often inaccessible through text. Chapter 1 reviews the current state of research in the area and its limitations. It places the work presented in this thesis in the context of previous developments while also describing more broadly the motivation behind it. Additionally, the broader context of chemical reaction database generation and recent research efforts are discussed. Chapter 2 provides background on the machine-learning and computer vision techniques used, emphasizing the unsupervised learning and deep learning paradigms that underlie most of this research. Following the introduction, chapters describing results are presented. These describe the present research in-depth and its evolution along the path of creating an intelligent and autonomous tool for image mining from chemical reaction schemes. Chapter 3 describes the first published work on ReactionDataExtractor v. 1.0, which is an early instance of the chemical extraction tool based on a combination of unsupervised machine learning and symbolic artificial intelligence approaches that allow extraction from simple reaction schemes. Chapter 4 describes a second published work on the more mature ReactionDataExtractor v. 2.0. system, which has a much more robust architecture and a sophisticated design that effectively lifts several limitations of the earlier version. Chapter 5 presents the final improvements and integrations performed on the pipeline to enable chemical patent data mining. It concerns the integration of ReactionDataExtractor into a PDF data mining tool and the Markush structure recognition. Finally, Chapter 6 summarises the contributions made by this thesis as well as the current state and limitations of the framework. It also proposes avenues for future work.
Open Access
Probing the First Stars with the 21-cm Signal: Theory, Methods, and Forecasts
Gessey-Jones, Thomas; Gessey-Jones, Thomas [0000-0002-4425-8746]
This thesis explores the potential for probing the properties of the first stars, in particular their mass distribution, using the cosmological 21-cm signal. We begin in Part I by introducing the first stars, 21-cm cosmology, and the link between them (Chapter 1), as well as describing in detail the semi-numerical 21-cm signal simulation code 21cmSPACE, used throughout this thesis (Chapter 2). Part II then presents a series of studies into different mechanisms by which the first stars and their mass distribution can impact the 21-cm signal. Chapter 3 considers the Lyman-photon mediated impacts of the first stars, including the Wouthuysen-Field effect, Lyman-α heating, and Lyman-Werner feedback. Chapter 4 investigates how cosmic rays emitted via the supernovae of the first stars can heat the intergalactic medium and imprint distinctive signatures on the 21-cm signal. And finally, Chapter 5 shows that variations in X-ray binary emissivity and abundances enhance the sensitivity of the 21-cm signal to the mass distribution of the first stars. Together, these three chapters construct a comprehensive theoretical model of the impacts of the first star mass distribution on the 21-cm signal. Next, in Part III, we perform a joint analysis of current 21-cm data sets and X-ray background measurements to determine the constraints these observations place on the potential properties of superconducting cosmic strings. Alongside providing insights into an alternative candidate for the first luminous objects in the Universe, this analysis demonstrates the methodology we hope to use to constrain the properties of the first stars using future 21-cm data sets. Part IV then details the headline results of this thesis, forecasts for the prospective constraints on the first star mass distribution from the REACH and SKA-Low 21-cm signal experiments (Chapter 7). Both experiments are found to be able to constrain the mass distribution at > 3σ at their projected sensitivities. Chapter 8 concludes the research content of this thesis with a discussion of a limitation of the Bayesian forecasting techniques employed in Chapter 7 and proposes a novel methodology for fully Bayesian forecasts which addresses this. Lastly, in Part V, we summarize the key conclusions of this thesis and discuss the further research directions these findings motivate.
Controlled Access
Multiphysics modelling of geometrically-complex flow metering devices
Coveney, Damian
The simulation of complex multiphase flows has been the subject of extensive research for decades, owing to the wide range of industrial and natural processes involving the interaction of two or more fluids, particularly in the presence of variable geometries. This is essential for multiphase flow metering, which allows for production optimisation in oil fields for example. In this work, a novel approach for the numerical simulation of problems involving free-surface flows interacting with geometrically-complex rigid bodies is presented. This is implemented as an augmentation of a standard projection algorithm for the solution of the variable density incompressible Navier- Stokes equations. Additional physics, such as interfacial tension, are represented as body forces in the momentum equations. The procedure is simple to code and negates the use of specialised mesh generation or mesh-quality optimisation techniques. This method was implemented within an existing software framework (AMReX) which solves the governing partial differential equations for the flow while also including the capability for adaptive mesh refinement with optimal parallel scaling. The method is comprehensively assessed for accuracy and scalability by means of an extensive range of validation benchmarks. Subsequently, the code was used to simulate complex multiphase flow phenomena inside a prototype industrial flowmeter, demonstrating excellent agreement with experimental observations and showing that the framework can be used to inform the engineering design of flow control devices. Finally, the flexibility of the software was exploited to incorporate a computational model of an electrical capacitance tomography sensor for the purposes of multiphase flow measurement and monitoring. The approach was shown to accurately predict quantities of interest in a fully three-dimensional multiphase flow, allowing for investigations into the feasibility of using electrical sensors in real-time multiphase flow metering systems. This lays an excellent foundation for continually developing the software by incorporating additional physical models for flow phenomena of interest.
Open Access
The Interplay of Magnetism and Topology in Topological Insulators
Devlin, Niall
The conductive helical edge states in topological insulators (TIs) have been lauded over the last decade as a means towards the development of low-energy electronic and spintronic devices. Unfortunately, development of TI devices have been hampered by issues such as impurities and difficulty in fabrication, meaning that they are no closer to replacing traditional semiconductor platforms than when they were discovered over a decade ago. However, while the low carrier mobility of edge states in TI devices may always preclude their use in the electronics industry, the non-trivial topological nature of these edge states mean they are also a novel playground for the development and observation of exotic physics and emergent phenomena. Indeed, in recent years focus has transitioned from finding utility in bare TI devices to investigating combination of topological protection with other effects, such as magnetism and superconductivity. In particular, the introduction of magnetism into TIs leads to a variety of unique phenomena and exotic quasiparticles not observed in conventional material systems, such as the quantum anomalous Hall effect (QAHE) and Majorana fermions. The study and development of devices based on such phenomena are not only interesting from the perspective of fundamental physics, but also propose practical applications in spintronics and quantum computing. This thesis presents work on the interplay between magnetism and topology and discusses the technological significance of such an interaction. Early stage research into the introduction of magnetism into TIs focused on doping TIs with magnetic adatoms, or engineering magnetic insulator/TI interfaces to induce magnetism into the TI edge states through a proximity effect. However, as was the case with bare TI devices, fabricating devices based on such platforms is not without difficulty. Inhomogeneities in the concentration of magnetic adatoms in doped samples, and the very weak interaction between the TI and magnetic insulator in proximity based devices mean that interesting effects and phenomena are only observable at temperatures on the range of 10's of millikelvins. In recent years, the discovery of intrinsic magnetic TIs has attracted an intense amount of research activity, as they provide a novel platform to investigate both topology and magnetism in van der Waals materials without suffering from many of the shortcomings of magnetically doped TIs or TI/magnetic insulator heterostructures. Furthermore, the antiferromagnetic coupling between layers gives rise to interesting layer dependent phenomena, where the electronic structure of samples is dependent on the parity of the number of layers, i.e. whether there are an even or odd number of layers. Literature has mainly focused on $\text{MnBi}_{2}\text{Te}_{4}$, whose order along the $\hat{z}$ axis means that it can host the QAHE and axionic insulator state. However, the family of intrinsic magnetic TIs is extensive and can host different magnetic configurations. In particular, in this thesis we have investigated in-plane magnetisation as a means to engineer flat-bands in the energy dispersion relation of topologically non-trivial materials. When considering antiferromagnetic interlayer coupling, we uncover an interesting dependence of the electronic dispersion and the local density of states on the parity of the number of layers. Furthermore, we demonstrate that magnetic textures at the surface of magnetic topological materials lead to spin-polarised flat-bands. In addition, the infinite mass quasiparticles occupying these flat-band states are strongly localised around magnetic domain walls. The means of engineering flat-bands developed in this thesis may have great technological significance in electronic and spintronic applications. For instance, we propose that the system discussed in chapter \ref{chap:AFMTI_dws} could be used in re-configurable magnetic memory, however they may also prove useful in the investigation of exotic physics and emergent phenomena. It is well known that the high density of electronic states in flat-bands leads to many-body interactions gaining greater importance in the overall dynamics of a system. As a result, flat-band systems can exhibit strong electronic correlations, which can result in the emergence of interesting phenomena such as non-BCS superconductivity, and other non-Fermi liquid phases, and charge or spin fractionalization. While the investigation of strongly correlated physics is beyond the scope of this thesis, the results presented here nevertheless demonstrate that magnetic TI systems are novel playgrounds for the investigation of emergent phenomena that could advance our understanding of fundamental physics in condensed matter systems.
Embargo
Discovery and Characterisation of Long-Period Exoplanets with TESS and CHEOPS
Tuson, Amy
In this thesis, I present my work on the discovery and characterisation of long-period transiting exoplanets. Detecting exoplanets via the transit method is inherently biased towards short-period planets. Due to the nature of its observing strategy, the Transiting Exoplanet Survey Satellite (TESS) is particularly susceptible to this detection bias. To increase the number of long-period planets, I use TESS ‘duotransits’ — planet candidates with two observed transits separated by a large gap, typically two years. From the two non-consecutive transits the orbital period is unknown, but there exists a discrete set of allowed period aliases. With the CHaracterising ExOPlanet Satellite (CHEOPS), I perform targeted follow-up of TESS duotransits to determine their true periods and confirm them as long-period planets. I begin by describing the specialised pipeline that I created to discover TESS duotransits. My pipeline reads in TESS lightcurves, detrends them using a time-windowed sliding mean, runs a box least squares (BLS) transit search and outputs duotransit candidates. The process was optimised to discover duotransits suitable for CHEOPS follow-up by using injection-recovery tests and selecting BLS parameters based on typical duotransit properties. After running my pipeline on ∼ 40,000 stars, I discovered five duotransit systems that have since been observed by CHEOPS: HD 5806, TOI-5678, TIC 182992572, TIC 130843507 and HD 185619. For the four that have had their orbital period confirmed so far, I performed a joint TESS and CHEOPS analysis to derive each planet’s properties. I also present the TESS and CHEOPS discovery of two warm sub-Neptunes transiting the bright K-dwarf HD 15906, as reported in a publication which I led. In total, I contributed to the discovery of 19 long-period planets as a core member of the CHEOPS Duotransit Program. All of these new discoveries have orbital periods longer than 20 days, radii smaller than 5 R_⊕ and host stars brighter than a Gaia magnitude of 12. These small planets on long-period orbits around bright stars are amenable to detailed characterisation studies, for example radial velocity follow-up to measure their masses or transmission spectroscopy to probe their atmospheres. That makes them particularly valuable for understanding how exoplanet properties change as a function of stellar irradiation and improving our understanding of planet formation and evolution.

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