Theses - Physics

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Automatic data interpretation from scientific literatures in the portable document format with information-extraction tools for the advancement of materials discovery
Zhu, Miao
This thesis addresses the development of scientific literature mining software tools to automatically extract metadata, text, images, and chemistry information from scientific literatures that are in PDF (portable document format), a widely used format in academic and scientific communities, to accelerate the data-driven materials discovery. The nature of PDF files presents specific challenges for data extraction. Typically, no semantic tags are usually provided in a PDF file that is not designed to be edited or its data interpreted by software. This creates a barrier to efficiently accessing and utilising the wealth of information they contain, especially in domains like materials science and chemistry where data extraction and analysis are crucial. In the materials discovery domain, efficiently accessing and analysing chemical and property data is of paramount importance. More crucially, it emphasises the need to understand and exploit the relationships between these data points to enable data science in areas such as data-driven materials discovery, where identifying correlations between different materials and their properties can lead to significant scientific breakthroughs. By addressing these challenges, this thesis contributes to enabling data science in areas like data-driven materials discovery. It demonstrates how extracting and analysing data from scientific literature can provide new insights and accelerate the process of discovery in this field. The software tools developed as part of this thesis represent significant technical innovations. They are designed to navigate the complexities of PDF files and extract valuable data with precision. This makes the tools not only valuable for researchers in materials science but also potentially applicable in other scientific domains where data extraction from literature is a key part of the research process. Chapter 1 discusses the fundamental and history of portable document format and the current literatures on data extraction from portable document format for materials discovery. Chapter 2 introduces relevant methodologies and frameworks used throughout the thesis. Chapter 3 introduces PDFDataExtractor, a highly automated data and information extraction toolkit that can automatically detect layouts of scientific literatures that are in portable document format, from which semantic information can be extracted and interpreted to reconstruct the logical structure of articles to a machine-readable format. This tool was tested on a self-created evaluation set and key metadata are extracted with nearly 60% precision. Chapter 4 descries a complete workflow for the extraction of image, scheme, and figure with corresponding caption from scientific literatures and supplementary information that are in portable document format. This workflow also extracts chemical and property data by channelling results to ImageDataExtractor1. ImageDataExtractor1 is a toolkit that extracts quantitative data from microscopy images. Chapter 5 details a bespoke workflow for the extraction of metadata, text, image, chemistry information and property data from physics literatures. Chapter 6 concludes the work and discusses future research opportunities.
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Multi and Hyperspectral Imaging of Early Detection of Disease
Taylor-Williams, Michaela
Multi and hyperspectral imaging (MSI/HSI) techniques provide spatial and spectral information and can be applied to detect and understand changes in biological tissue that occur with disease. This thesis has evaluated the application of MSI/HSI to two disease applications: analysing nailfold capillaries to aid in the evaluation of systemic scleroderma, and enhancing cancer detection in the gastrointestinal tract during endoscopies. The nailfold capillaries are the smallest blood vessels in the body, and deformations in these capillaries are indicators of systemic scleroderma, a rheumatic disease. A hypothesis was formed correlating these deformations with changes in oxygen levels. Two multispectral systems, capable of imaging the nailfold capillaries were designed: one based on LED illumination, and the other a snapshot detector-based imaging system. These systems were tested with microfluidic blood phantoms, which simulated varying oxygenation levels. Developing microfluidic blood phantoms proved critical for evaluating system performance. Phantoms with microfluidic depths used horse blood that was chemically oxygenated and deoxygenated to variable blood oxygenation levels. The LED-based system was subsequently used for imaging healthy nailfold capillaries in a proof-of-concept demonstration. Concurrently, the multispectral snapshot camera system was incorporated into a clinical trial at the University of Manchester. Data from the nailbed capillaries of healthy controls as well as those with systemic sclerosis were evaluated and classification was achieved. Early detection of cancer in the gastrointestinal tract can lead to curative intervention, but contrast for early lesions is low. To evaluate the potential for MSI methods to address this challenge, design optimisation was performed using pre-existing hyperspectral data from endoscopies conducted in the gastrointestinal tract (oesophagus and colon). An open-source Python-based toolbox for spectral band optimisation was developed to analyse datasets in order to design the optimal imaging bands and finalise the spatial layout of multispectral filter arrays that could be deployed in clinical settings. Disease detection accuracy was optimised by selecting subsets of spectral bands and integrating machine learning methods, such as k-nearest neighbour (kNN) classification and support vector machines (SVM). The maximum classification accuracies occurred using a kNN classifier and were 0.848 and 0.999 for the oesophagus and colon, respectively. SVM performed reasonably well with accuracies of 0.811 and 0.997 for the oesophagus and colon; while spectral angle mapping classification was a good classifier of colon tissue (accuracy of 0.995), it performed poorly on oesophageal tissue (accuracy of 0.245). The toolbox was also deployed to design filters capable of imaging blood oxygenation in tissue, to improve the detection and understanding of cancers where hypoxia plays a role. This research demonstrates the promising diagnostic capabilities of spectral imaging in measuring blood oxygenation.
Restricted
Automatic construction of a photocatalytic materials database using natural language processing
Isazawa, Taketomo
[Restricted]
Open Access
Extending the Reach of Searches for Staus, Charginos and Neutralinos with the ATLAS Experiment at the Large Hadron Collider
Jones, Dominic
This thesis describes new approaches to extend the sensitivity reach of searches for three types of supersymmetric particles; staus, charginos and neutralinos, using the 139 fb⁻¹ of data collected with the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. Searches for three different production modes of these particles are considered, with new approaches for each that highlight different methods for improving sensitivity. Firstly, a search for direct stau production, in which staus decay to tau-leptons and neutralinos, is presented. The search makes use of Boosted Decision Trees to classify the signal from the Standard Model backgrounds and in doing so significantly improves the sensitivity compared to a previous search using the same dataset. No significant differences between the observed data and the Standard Model predictions are found but new, world-leading limits on direct stau production are set. Models with mass degenerate left and right-handed staus are excluded at 95% confidence level for stau masses between 80 GeV and 500 GeV, with the low mass reach closing a previous gap in sensitivity between previous searches performed by the ATLAS experiment and searches conducted by experiments at the Large Electron Positron collider. This search also provides the first sensitivity at the ATLAS experiment to the right-handed stau only interpretation, for which models with right-handed stau masses between 100 GeV and 350 GeV are excluded for a massless neutralino-one at 95% confidence level. Secondly, new search channels for the production of charginos and neutralinos with highly compressed mass spectra are explored. In particular, a re-interpretation of a previous search for new physics performed at ATLAS in a final state with an energetic jet and large missing transverse momentum is compared to a sensitivity study of a vector boson fusion channel. The re-interpretation is found to have sensitivity to previously uncovered regions of parameter space; for a simplified wino/bino interpretation models with neutralino-two masses of 140 GeV are excluded at 95% confidence level, under the assumption that signal theoretical uncertainties have a negligible impact. The vector boson fusion channel is assessed to have comparatively limited sensitivity with differences in the signal modelling between this study and previous work on this channel being identified. However, it is shown that the use of machine learning in this channel can provide a significant gain in the sensitivity. Thirdly, a statistical combination of previous ATLAS searches for chargino pair production in three different topologies is performed for the first time, leading to a notable enhancement in the overall sensitivity.
Open Access
Star-forming Galaxies and Quenched Systems throughout Cosmic Time
Sandles, Lester; Sandles, Lester [0000-0001-9276-7062]
This thesis delves into the investigation of star-forming galaxies and quenched systems at high redshifts, exploring their evolution and properties throughout cosmic time. Firstly, I utilise the foreground lensing of massive galaxy clusters in the Hubble Frontier Fields to probe the high-redshift evolution of the main sequence of star-forming galaxies. I use the BEAGLE SED-fitting code to derive stellar masses, SFRs and redshifts from galaxies within the ASTRODEEP catalogue. I fit a fully Bayesian hierarchical model of the main sequence over redshifts 1.25 < z < 6 while explicitly modelling the outlier distribution. My results agree with an increase in normalisation of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas onto dark matter halos. We additionally measure the slope and intrinsic scatter of the star-forming main sequence. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 μm) is insufficient to constrain the stellar mass and SFR over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. Whilst this filter set represented (prior to the launch of *JWST*) the best sampling of high-redshift galaxy SEDs out to z > 3, I show that measurements of the main sequence to low masses and high redshifts were still strongly dependent on the priors employed in SED fitting (as well as other fitting assumptions). In the first field targeted by JADES, the statistics were not large enough to extend the full main sequence analysis to *JWST*-based datasets. I therefore continued to study high-redshift star-forming and quenched galaxies with smaller projects, more suitable for the first deep set of spectroscopy obtained by JADES. Dust, often one of the most poorly constrained parameters in SED fitting, drives the motivation for my second project. As part of the JADES survey, utilising data obtained with the *JWST*/NIRSpec Micro-Shutter Assembly, I directly explore dust attenuation in the star forming galaxy population. This is achieved by analysing Balmer decrement measurements for a sample of 51 galaxies at redshifts 4 < z < 7. Leveraging 28-hour long exposures and the efficiency of the prism disperser (but also using information from the medium-resolution gratings), I was able to probe directly the low-mass end of the galaxy population, reaching stellar masses as low as 10⁷ M$_\odot$. I find that the correlation between Balmer decrement and stellar mass is already established at these high redshifts, indicating a rapid build up of dust in moderately massive galaxies at such early epochs. The lowest-mass galaxies in our sample (1 - 3 x 10⁷ M$_\odot$) display a remarkably low Balmer decrement of 2.88, consistent with case B recombination and little or no dust. I further compare the Balmer decrement to continuum-derived star-formation rates, finding tentative evidence of a correlation, which likely traces the underlying connection between SFR and the mass of cold gas. Finally, based on deep *JWST*/NIRSpec spectroscopy, I report the discovery of a quiescent galaxy at z = 2.34 with a stellar mass of only 9.5 x 10⁸ M$_\odot$. This is the least massive quiescent galaxy found so far at high redshift. I use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8 - 1.7 Gyr (dominated by systematics). The galaxy displays a colour gradient (redder towards the edge), consistent with outside-in, environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, I identify a galaxy overdensity near the location of the target (5-σ above the background level at this redshift). The overdensity contains three spectroscopically confirmed, massive, old galaxies. The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2 (in agreement with previous works). In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to the overdensity’s centre. This suggests the low-mass galaxy was quenched by environment, making it the earliest evidence for environment-driven quenching to date.
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Machine learning force fields for elemental sulphur
Carare, Vlad
The sulphur phase diagram is one of the most complex ones of all elemental systems, competing with that of carbon. The flexibility of the bonds allows for a variety of motifs: rings of 5 or more atoms in various conformations, short diradical chains and thousands-atoms long polymers to name a few; which give rise to a plethora of structures: molecular & polymeric crystals of many different symmetries, amorphous solids and molecular & polymeric liquids. Modelling transitions between such phases is a challenging task, out of the reach of any current force fields (which are too inaccurate) or quantum mechanical methods (which are too slow and expensive). However, following the footsteps of similar work done on silicon, phosphorus and carbon, surrogate machine learning models mimicking quantum methods at a fraction of the cost could achieve this feat. In this work we propose several such models, prompted by the continuous evolution of the field, and benchmark them on a series of static and dynamic tests. We successfully describe the ambient condition solid phase, melting, polymerisation and depolymerisation of sulphur: an achievement out of reach of any previous method. We also dedicate considerable effort to investigating the liquid-liquid phase transition recently reported in the experimental literature [1]. This consists of a change between low and high density liquid forms heralded by a jump in density and alterations in radial distribution functions and Raman spectra. While a simple analytical model to explain the transformation was proposed in a recent publication [2], a quantum-mechanically accurate exposition of the microscopic phenomena is desirable. Our models surpass previous length and time constraints and allow the simulation of liquid sulphur for up to hundreds of nanoseconds for thousands of atoms, which enable the reaching of thermal equilibrium and the obtaining of meaningful and precise measurements of the structure factors, cluster sizes and coordination statistics. We are able to characterise the two phases: one consisting of an almost even fraction of polymers and small rings and the other comprising mostly of tightly-packed polymers. Another important contribution of this thesis is the in-depth display of the process of building a general potential for such a complex system. We showcase several methods for creating a relevant dataset, through: manual selection, iterative training and automated selection; which could prove useful for the community. Furthermore, we investigate the effect of magnetic dipole moments on small sulphur clusters and condensed liquid phases, and put forward the first general machine-learning potential trained on spin-polarised data.
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Thermoelectric Properties of Organic Polymers
Zhu, Wenjin
Thermoelectric devices present an enticing solution for harnessing waste thermal energy from industrial processes and converting it into electrical power. Due to their cost-effectiveness and potential in flexible electronic applications, organic thermoelectrics have attracted considerable attention. Nonetheless, further enhancing their thermoelectric performance remains a great challenge. Solutions have been proposed, encompassing molecular structure design, uniaxial alignment, and advanced doping techniques. This dissertation explores the relationship between the enhanced thermoelectric properties and the structure of organic polymers. It delves into both innovative new materials systems and established typical materials. First it introduces the background and basic experimental techniques in Chapters 1-2. Then it delves into the function of uniaxial alignment and ion exchange doping to optimize the thermoelectric properties of organic polymers in Chapter 3. Uniaxial alignment achieves anisotropic charge transport by orienting the polymer backbones, which facilitates charge movement along backbones. Ion exchange doping has demonstrated superiority over traditional molecular and electrochemical doping methods, increasing charge carrier densities. By integrating these two techniques, we've observed marked improvements in the thermoelectric attributes of some typical conjugated polymers such as poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and diketopyrrolopyrrole (DPP)-based polymers. In Chapter 4, we report a new model system for better understanding the key factors governing their thermoelectric properties: aligned, ribbon-phase PBTTT doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods we study the effect of controlled incorporation of tie-chains between the crystalline domains through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity, that is not accompanied by a reduction in Seebeck coefficient nor a large increase in thermal conductivity. We demonstrate respectable power factors of 172.6 μW m^-1 K^-2 in this model system. Our approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance. Furthermore, this thesis explores the relationship between molecular structure and thermoelectric properties of some specially designed novel polymers in Chapters 5-6. This includes Poly-3-hexyl-thiophene (P3HT)-based random co-polymers, P[(3HT)1-x-stat-(T)x] containing different proportions of unsubstituted thiophene units (x ranging from 0 to 0.36). It also includes naphthalene diimide (NDI)-based copolymers modified by substituting side chains, incorporating Selenium atoms, and incorporating Fluorine atoms. Finally, Chapter 7 of this thesis provides a summary and offers an outlook, highlighting how our methodology is broadly applicable across various semicrystalline polymers, which can potentially and substantially improve the thermoelectric figure-of-merit in these emerging materials. Furthermore, the techniques discussed in this thesis show great promise for expanding the applications of organic thermoelectric devices in potential future industrial scenarios.
Open Access
Liquid Crystalline Elastomers as Renewable Functional Materials: From Chemistry to Application
Gablier, Alexandra
Liquid Crystalline Elastomers (LCEs) are thermosets that belong to the family of “smart plastics”. They combine the softness and elasticity of elastomers, with the orientational order properties of liquid crystals (LCs), resulting in the ability for these materials to undergo large reversible deformation when subjected to external stimuli. This unique ability to actuate and perform work without mechanical parts has positioned LCEs as attractive materials for applications in fields such as soft robotics, sensors, surface coatings, and tissue engineering. The mechanical performance of LCEs and their capacity for large-amplitude, reversible actuation depend on the underlying chemistry and the alignment of the LC components in the network. However, achieving specific, complex, macroscopic-scale 3D structures with a predetermined actuation behaviour remains a challenge with conventional alignment techniques. The introduction of dynamic covalent chemistry into the networks of LCEs (to form xLCEs) was a significant breakthrough in the field, promising enhanced material processing and sample alignment. However, at the outset of this PhD, the understanding and control over the exchange dynamics of xLCEs was still lacking, stemming in part from a need for a broader library of materials with a greater variety of dynamic properties. Additionally, the field suffered from a lack of LCE applications geared towards addressing real-world problems. This thesis hence aims to contribute to the advancement of the field of LCEs in three key areas: (1) investigating the mechanics of xLCEs to establish fundamental principles, (2) exploring novel network chemistries, and (3) applying the knowledge gained to develop new and practical applications. First, I build on existing bond-exchange reactions to establish wider knowledge about factors controlling the material flow on a macroscopic scale in dynamic covalent polymer systems (vitrimers). I notably demonstrate that the bond exchange reaction activation energy is a poor predictor of material flow at high temperatures, with the network elastic modulus and the concentration of reactive functions for the bond exchange having a dominant impact on flow behaviours. This enhanced understanding provides design principles for controlling material dynamic properties in xLCEs. Second, I expand the library of exchange and network chemistries available for xLCE materials. Through the use of an epoxy-thiol reaction, I introduce a new network chemistry for an established covalent exchange reaction (transesterification). The reaction is simple, utilises mild conditions, cheap starting materials, and results in true elastomer xLCE materials with a wide range of material properties accessible through the system’s modular character. I show that the LC isotropic transition temperature, the material flow at high temperature from bond exchange, and the LC mesophase can all be controlled and tailored through a simple variation of the network composition. The expansion of material properties available broadens the range of possible outcomes for transesterification-based xLCE. Another new type of network with dynamic covalent properties that is introduced in this work is a poly(thiourethane) xLCE system. Such an xLCE thermoset network, containing dynamic covalent thiourethane bonds, is strengthened by physical crosslinks (H-bonding), resulting in a unique set of material properties such as enhanced strength and a remarkably high ductility at room temperature. The material obtained is the first example of an xLCE that can be reprocessed using industrially ubiquitous methods such as injection moulding and extrusion. Lastly, an example of use of LCEs towards a real world problem is investigated, namely through the use of LCEs as a to-scale Braille soft continuum interface for dynamic Braille devices. I demonstrate that the complex and numerous moving parts of a dynamic Braille device could be replaced by a single sheet of LCEs embossed with small actuating bumps. A simple moulding procedure produces a surface patterned with at-scale Braille bumps, as a result of a precise and complex internal organisation within the elastomer sample that emerges during polymerisation (as is evidenced by theoretical modelling). Unlike in previous attempts to use LCEs for Braille technology, the millimetre-scale protruding features are generated out of the bulk of the material, resulting in structural integrity, and high resistance to compression force. The localised bump-to-flat reversible actuation occurs on a timescale of seconds. The potential of this development for application into a complete Braille dynamic display are discussed. These findings open new lines of research in multiple directions for the field, in the hopes of advancing knowledge and bringing LCEs one step closer to commercialisation.
Embargo
Interplay of Spin and Photophysics in Luminescent Open-Shell Molecular Semiconductors
Gorgon, Sebastian; Gorgon, Sebastian [0000-0002-1361-1973]
Luminescent organic radicals are an emerging class of molecular semiconductors which exhibit many unique properties attractive for optoelectronic and spintronic devices. In this thesis, we employ optical and spin-based probes to reveal the dynamics of photogenerated excitons in a selection of novel tris(2,4,6-trichlorophenyl)-methyl (TTM)-based radicals. The first three chapters present a motivation, relevant theory and methodology. In Chapter 4, we focus on the intrinsic properties of luminescent doublet (*S*=1/2) states. We find evidence of intermolecular charge transfer excitations which drastically alter the emission spectrum and lifetime in thin films. In Chapter 5, we investigate solid state intermolecular interactions between radicals and triplet (*S*=1) states on closed-shell materials, and show their management can lead to improvements in Organic Light Emitting Diode (OLED) performance. For the first time we observe cycling between the triplet and doublet manifolds, and direct energy transfer on sub-nanosecond timescales. In Chapter 6, we present the first organic molecules which can reversibly access a quartet (*S*=3/2) excited state. This is achieved by engineering strong exchange coupling between resonant radical and triplet manifolds in covalently linked structures. The resulting high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to the energetically accessible radical state. In Chapter 7, we extend these results to a luminescent biradical structure which supports a quintet (*S*=2) excited state. The light-induced cycling through this state drastically increases the strength of the exchange coupling between the two radical spins, and leads to a long-lived ground-state polarisation. The findings and models developed in this thesis open a path to few functionalities for open-shell semiconductors, as outlined in Chapter 8, ranging from improved light emission to molecular quantum information science.
Open Access
Deterministic spin and photon control with a symmetry protected colour centre
Parker, Ryan
Quantum networking, whereby quantum mechanical entanglement is distributed through a network and used as a vector for information transfer, is an ambitious emerging technology. It requires a stationary, compute, qubit at a node in the network to interface with a flying, photonic, qubit for information distribution where these two qubits have different requirements to be technologically relevant. The stationary qubit needs long-lived internal degrees of freedom that can be coherently manipulated and optically interfaced. Whereas, the flying photonic qubit should interact minimally with the environment, to prevent decoherence from disrupting information distribution throughout the network. These two sets of independent requirements for the stationary and flying qubits impose strong engineering constraints on the underlying physical system underpinning the quantum network, and no one physical system has demonstrated a scalable solution that meets both sets of requirements. In this thesis, the negatively charged tin vacancy centre (SnV) in diamond is presented as a viable, symmetry protected, platform for quantum networking applications. The stationary qubit in the network is formed by the SnV centre's intrinsic optically addressable spin-1/2 qubit. This thesis accesses the coherence of the SnV centre's spin-manifold for the first time and is subsequently leveraged to achieve multi-axis coherent control of the SnV centre's spin at Ω/2π = 4.5(1) MHz Rabi frequencies and with 82(5)% π/2-gate fidelities. Leveraging this control reveals long-lived internal degrees of freedom yielding an inhomogeneous dephasing time of T$_{2}^*$ = 1.4(3) µs. Access to an ancillary quantum register, formed of proximate nuclear spins, is also shown, which further provides a resource for quantum networking by enabling quantum memory operations and quantum state storage to be available during network activity. The flying photonic qubit interaction channel takes the form of the optically addressable, spin-selective, transitions of the SnV centre. This thesis demonstrates, for the first time, isolation of coherent photons from the SnV centre with 99.7$_{-2.5}^{+0.3}$% purity and 63(9)% indistinguishability. The symmetry protected nature of the SnV centre's spin manifold enables up to 10⁶ identical photons to be generated per entanglement attempt before optical coherence is lost. Full quantum control of the optical transition is further demonstrated, yielding a 77.1(8)% fidelity optical π-pulse in 1.71(1) ns. Thus, the presence of both a robust photon-photon interaction and controllable optical channel is demonstrated for quantum networking applications leveraging the SnV centre. The stationary SnV spin qubit and the flying photonic qubit are combined in a single high-efficiency packaged platform, yielding a 57(6)% collection efficiency and the observation of 5-photon states. A giant optical non-linearity conditional on the spin qubit's state is used for information transmission in a two-node directional network. Further, the presence of an ancillary quantum memory register is extended through the use of the intrinsic spin-1/2 ¹¹⁷Sn register of the ¹¹⁷SnV centre. This novel resource, discovered in this thesis, enables a high-efficiency photonic interface to interact directly with the ¹¹⁷Sn nuclear spin degrees of freedom. Thus, nuclear initialisation to 98.6(3)% fidelity and single-shot optical nuclear spin readout with 80(1)% fidelity are achieved in an all-optical control protocol, significantly reducing the overhead per qubit needed for quantum repeater nodes. Therefore, this thesis presents the SnV centre in diamond as a novel resource for quantum networking. The symmetry of the centre enables robust coherence of both the stationary spin-qubit and the flying photonic qubit that is insensitive to nano-photonic integration. This high intrinsic coherence positions the SnV centre for class-leading integration into Purcell enhanced cavity systems. Such integration would then facilitate near unity collection efficiencies and control fidelities, thereby enabling fault-tolerant quantum networking with a single, low overhead, platform.
Open Access
Ultrafast Raman Scattering in Plasmonic Nanocavities
Jakob, Lukas
When bound to metals, molecular vibrations play a key role in sensing, catalysis, molecular electronics and beyond, but investigating their coherence and dynamics is difficult as pulsed experiments prove very challenging. In this thesis, I study vibrations of 1-1000 molecules in a plasmonic nanocavity when driven by picosecond pulsed lasers out of the linear regime. This unravels new non-linear effects such as room-temperature vibrational pumping, giant optomechanical spring shifts, collective molecular vibrations, accelerated decay of vibrational coherence, and the generation of correlated photon pairs. In plasmonic nanocavities, optical fields are enhanced 100-fold and focused to a nanometre-thin gap. Vibrations of molecules placed in the cavity interact strongly with the optical resonances, forming a coupled optomechanical system. Using pulsed laser illumination, I find that surface-enhanced Raman scattering can significantly increase the phonon population above the thermal equilibrium. This vibrational pumping leads to non-linear anti-Stokes scattering observable at room temperature. Further, the optomechanical coupling induces a red-shift of the vibrational energy by >100 cm⁻¹ and broadening of the Raman line at high peak laser powers (optomechanical spring shift). These non-linear effects are strongly enhanced by the excitation of collective molecular phonon modes. Further experiments show that Stokes-induced anti-Stokes scattering exhibits strong cross-frequency photon bunching. These correlated Stokes – anti-Stokes photon pairs show non-classical behaviour and could be used for applications in quantum computing and communication. To study the dynamics of molecular vibrations, I use time-resolved incoherent and coherent anti-Stokes Raman scattering. Developing a new single-photon lock-in detection technique, it is possible to simultaneously record the decay of the vibrational population and vibrational dephasing for each nanocavity. The vibrational dephasing is found to strongly accelerate depending on the exciting laser intensity. Understanding these modified vibrational dynamics on plasmonically-active substrates is crucial for improving surface-enhanced catalysis of chemical reactions and heat transfer in molecular electronics.
Open Access
Emergent Critical Phases in Strongly Correlated Low-Dimensional Magnetic Systems
Deng, Shiyu; Deng, Shiyu [0000-0002-0507-2009]
This thesis delves into the realm of condensed matter physics, where the discovery of novel quantum states, from unconventional superconductors to non-trivial metallic behaviours, has led to promising applications and intriguing questions regarding the underlying physics. Understanding these non-trivial phenomena demands a combined theoretical and experimental effort. While the exact mechanisms remain under ongoing investigations, it is widely acknowledged that emergent phenomena often arise in the presence of strongly correlated electrons with reduced dimensions, in many cases in proximity to quantum critical points. This thesis contributes to the exploration of a relatively unexploited and highly fertile collection of van der Waals magnetic insulators known as transition metal phosphorous trichalcogenides, denoted as *TM*P*X*₃ (*TM* = Mn, Fe, Ni, *X* = S, Se). These compounds have proven to be ideal examples where structural, magnetic and electronic properties evolve into novel states when their dimensionality is tuned with a clean and controllable parameter, pressure. At ambient pressure, they are two-dimensional van-der-Waals antiferromagnets with strongly correlated physics. Recent experimental findings have unveiled pressure induced dimensionality crossover, crystalline structure change, insulator-to-metal transitions and the emergence of novel magnetic phases and superconductivity. Solving high-pressure structure models, particularly in terms of interplanar stacking geometry, has posed challenges due to the nature of van der Waals materials, which often exhibit mosaicity in single crystals or strong preferred orientation in powder samples. To elucidate the relationships between structural transitions, magnetism and electronic properties, this thesis employs a random structure search using first-principles calculations at high pressures and Density Functional Theory (DFT) + Hubbard U studies. FePS₃ has been chosen as the stereotype compound within the family and has been investigated thoroughly. The coexistence of the low- and intermediate-pressure phases has been carefully examined and explained with theoretical models. Additionally, novel high-pressure phases with distinctive dimensionality and possible alternative options for interpreting the origins of metallicity have been predicted. The validity of the methodology can be extended to other compounds within the family. The thesis also presents a comprehensive high-pressure synchrotron X-ray study of FePSe₃ using both single crystal and powder samples at the Diamond Light Source. Although FePSe₃ shares a similar intraplanar configuration with FePS₃, it exhibits differences in interplanar stacking at both ambient and elevated pressures. Pressure-induced superconductivity has only been reported in the FePSe₃ so far, occurring at 2.5 K and 9.0 GPa. Despite challenges in defining the crystalline structure models at high pressure, this work provides definitive crystallographic insights into the phases that emerge under pressure. Additionally, magnetic phases have been explored using powder samples within a specially designed pressure cell, with results obtained at the Institut Laue Langevin.
Embargo
Device physics of perovskite light-emitting diodes
Sun, Yuqi; Sun, Yuqi [0000-0002-8471-3010]
Metal halide perovskites have emerged as next-generation semiconductors for light emission, owing to their bright luminescence, excellent charge-transport properties, ease of processing and tunable bandgap. In less than a decade, the external quantum efficiency of perovskite light-emitting diodes (PeLEDs) has increased rapidly to over 25%, which is comparable with that of organic LEDs. The behaviour of PeLEDs is determined by the device processes including charge injection and transport, recombination and light extraction. Understanding the physics governing these processes is significant for controlling the performance of PeLEDs. This thesis focuses on the photonic and electronic device physics that controls the performance of PeLEDs. In the first study, we identify that non-radiative losses in bulk perovskites and interfaces reduce the performance of PeLEDs. To address this, we design a novel multifunctional molecular additive to control the optoelectronic, structural and morphological properties of perovskite films. This approach efficiently suppresses non-radiative loss pathways in bulk perovskite films and interfaces in the devices, thus achieving improved device performance. However, the efficiency of PeLEDs is severely limited by light extraction. We then discuss light extraction losses in different models by considering the photon recycling effect. Based on this understanding, we develop a strategy to improve light outcoupling in PeLEDs by enhancing the photon recycling in perovskites. In the last study, we identify that the unique device characteristics of PeLEDs necessitates a more profound understanding of their device physics. We then develop an archetypical device structure and employ drift-diffusion modelling to explore the working principles and unique device physics of PeLEDs.
Open Access
Modulated Magnetic Field Effects, Molecular Design, and Indigoids: A Mechanistic Study of Singlet Fission
Walton, Jessica
Singlet fission has the potential to significantly enhance the efficiency of photovoltaic light harvesting of silicon solar cells beyond the Shockley-Queisser limit. The progress of this technology has been hindered by the limited selection of suitable molecules that can undergo singlet fission, and the methods we use to screen new materials. This thesis is constructed in two parts: the investigation of two indigoids in their candidacy for singlet fission, and the use of an alternative method, modulated magnetic field effects in photoluminescence (modMPL), as a potential screening tool for materials. After relevant theoretical and experimental background is discussed in Chapters 2 and 3, Chapter 4 presents an alternative method for investigating singlet fission: modMPL. We employ this highly sensitive technique to examine thin films of the well-studied singlet fission system, TIPS-tetracene. This technique reveals complex lineshapes describing the spin physics in great detail. ModMPL is a rapid, low-degradation technique, that greatly enhances the screening of new potential materials. In particular, it allows comparison of sample morphologies and their impacts on singlet fission dynamics in a way that is not currently available with conventional screening techniques. A discussion of how to simulate and understand modMPL lineshapes is included in Chapter 5. Secondly, we make use of ultrafast transient absorption spectroscopy to investigate two indigoids, a novel aza-cibalackrot (Chapter 6) and thienoisoindigo (Chapter 7). Both derivatives of indigo dyes, they are highly attractive candidates for singlet fission due to their superior photostability, high extinction coefficient, and ideal predicted triplet energy. We explore these new, versatile, potential molecular families for their singlet fission capability. Furthermore, we discuss an alternative molecular design principle for creating singlet fission candidates with greater photostability, which may then be applied to other molecular families in the search for singlet fission chromophores.
Open Access
Light Coupling to Plasmonic Nanocavities
Elliott, Eoin
The work reported in this thesis concerns how light can be coupled to plasmonic nanocavities, increasing its electric field by orders of magnitude. Variations of a popular NanoParticle (NP) on Mirror (NPoM, or patch antenna) structure were used, which localizes visible light in 3 dimensions to an area of contact between the NP and a Self-Assembled Monolayer (SAM), within a defined facet area. This enables strong Surface-Enhanced Raman Scattering (SERS) from the molecules at the facet. The energy deposited in the NP through laser irradiation was exploited to perform all-optical thermal measurements of SAMs in stable junctions. The deficiencies in our understanding of light coupling to such nanocavities are highlighted through this work. The Quasi-Normal Modes (QNMs) of lossy plasmonic nanocavities were investigated across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. We show that the gap thickness *t* and refractive index *n* are spectroscopically indistinguishable, accounted for by a single gap parameter $G=n/t^{0.47}$. Simple tuning of mode resonant frequencies and strength is found for each QNM, including important ‘dark’ modes, revealing a spectroscopic “fingerprint” for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. Selection rules based on QNM symmetry are extracted, and differences in mode brightness are accounted for by Poynting analysis on the scattered field. These insights are then applied to a range of NPoM measurements to explain the findings, and a spectroscopic method of imaging ‘dark’ modes is achieved.
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Phase Engineering and Optical Property Tuning of Transition Metal Dichalcogenides
Lim, Juhwan
This thesis investigates the cation-assisted crystallographic phase transitions and optical property modifications of two-dimensional transition metal dichalcogenides (2D TMDs) using optical techniques. The thesis begins with an introduction to the context of the research (Chapter 1), followed by an overview to the key materials and theoretical concepts in Chapter 2. In Chapter 3 we introduce in the key experimental methods used in the work. Chapter 4 examines the mechanism of semiconducting hexagonal (1H, 2H) to metallic tetragonal (1T, distorted 1T) phase transition reactions in 2D TMDs during chemical intercalation of lithium cations, employing real-time optical visualization. We directly quantify the dynamics of the phase transition in micrometer-sized TMD flakes with diffraction limited resolution. In addition, we complement the results with *ex-situ* Raman and photoluminescence measurement. We show this reaction to be a charge-limited surface driven intercalation reaction. After establishing our ability to probe reaction dynamics *in-situ* via optical microscopy, in Chapter 5, we explore the effect of optical excitation on this phase transition reaction. We demonstrate that illuminating the material with photons having energy above the band gap accelerates the transition of 2H to 1T phases by more than two orders of magnitudes. This finding also enables a novel and rapid spatial photo-redox phase patterning within mono- and few layered 2D-TMDs. We then demonstrate the improved performance of a phase-engineered photodetector based on mono-layer 1H-MoS2 using this method. We compare chemical lithiation to electrochemical lithiation to develop a detailed mechanistic picture of this process. Based on our findings, we propose a universal route for chemical cation intercalation and phase engineering of TMDs based on redox-potential matching. This is supported by our demonstration of the same phase transition using a newly synthesized solvent of polycyclic aromatic hydrocarbon with lithium and sodium, which significantly shortens the reaction time from several days to just a few minutes, and replaces the highly pyrophoric chemical n-butyllithium, which has been used in this process for the past five decades. This advanced phase engineering method can be applied to a various type of TMDs, such as powder, crystal, and thin flakes, and offers a promising pathway for scalable production of phase-engineered TMDs. Finally, in Chapter 6 we conduct chemical treatments using the cation-(bis(trifluoromethane) sulfonimide) system on MoS2 grown by metal-organic chemical vapour deposition (MOCVD). By altering the surrounding chemical nature of the cation, we were able to maintain the phase of MoS2 in its semiconducting hexagonal, and only enhance the radiative recombination intensity. This effect is particularly enhanced by adding additional prior treatment using sulfides, which passivate sulfur defects. In conclusion, through the utilization of various optical characterization methods, we have explored the versatile role of cations in phase engineering and luminescence properties for TMDs.
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Big data, bigger magnets, and tiny high-temperature superconducting cuprates
Hickey, Alexander
This work focuses on high magnetic field studies of the high-temperature cuprate superconductor YBa₂Cu₃O_y (YBCO) and associated data processing. YBCO is of much interest due to its high critical temperature (>90 K) and complex phase diagram containing many exotic phases. This work makes use of high magnetic fields (>30 T) to suppress the superconductivity and access under-explored parts of this phase diagram. The region of particular interest in this work is around 12 % hole doping where there is the charge density wave (CDW) region. The CDW is an ordered electronic state which has a large overlap with the high-temperature superconducting dome. In this region and on its periphery there are non-ohmic, quantum critical, and phase competition behaviours. To explore and disentangle these various effects, many high-field measurements have been taken over a range of temperatures, magnetic fields, applied currents, and hole dopings. The main focus of this work is developing and applying data processing methods to collate and explore these measurements to the fullest. Through the collection of many analysis steps and their visualisation into a single pipeline, synergies appear allowing consistent treatment across large datasets. From this, contour maps can be produced allowing mapping of the phase diagram using a variety of metrics. From the application of these techniques, three areas of enquiry stand out. Firstly, the superconductivity in YBCO extends to unexpectedly high applied magnetic fields at the lowest temperatures, and this is examined by studying the non-ohmic behaviour to understand the vortex dynamics. Secondly, the extent of the CDW in YBCO seems to vary on measurement technique indicating changing dynamics with temperature. Finally, the end of the CDW dome has been associated with quantum criticality and is further explored with magnetotransport measurements.
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Electrical transport and superconductivity in doped quantum critical ferroelectrics
Liu, Shuyu
This thesis (Electrical transport and superconductivity in doped quantum critical ferroelectrics) presents the results of research into the electrical transport and superconductivity found in the neighbourhood of quantum phase transitions in carrier-doped ferroelectrics and paraelectrics. The materials of interest are doped SrTiO3, located close to the quantum critical point at ambient pressure and doped ferroelectric BaTiO3, which may be tuned to its quantum phase transition with hydrostatic pressure. In the project of carrier-doped SrTiO3, we observed unconventional resistivity varying as the square of the temperature at ambient pressure, which is not thought to be attributed to conventional Fermi-liquid electron-electron scattering. The results of resistivity measured under high pressure are presented which indicate a potential relation to quantum criticality. A theoretical model is presented based on the idea that electrons can scatter from fluctuations of the polarization-squared field which are also known as 'energy' or 'two-phonon' fluctuations. The model seems to find quantitative agreement with the high-pressure resistivity measurements as well as recently published thermal conductivity data. We also further investigated the enhanced superconductivity of carrier-doped SrTiO3 around the quantum critical point by high-pressure measurements in samples of varying charge carrier densities. Our data provided further evidence in support of the so-called hybrid-polar-mode mechanism of superconductivity. Although the second candidate, carrier-doped BaTiO3, is far away from the quantum critical point, we used a high-pressure moissanite anvil cell to tune oxygen-reduced specimens near to the quantum phase transition with carrier densities similar in range to those in superconducting SrTiO3. We succeeded in making semiconductive and metallic BaTiO3 samples with ferroelectricity retained. We did not detect clear evidence of superconductivity of oxygen-reduced BaTiO3 so far at the particular pressure points investigated.
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Precise radial velocities and simultaneous magnetic flux estimates from intensity spectra
Lienhard, Florian
The Radial Velocity (RV) community has made tremendous leaps forward in the past decades detecting and characterising ever smaller and lighter exoplanets. In recent years, this trend has been broken as planet-induced RV signals smaller than 1 m/s are drowned out by the stars' activity. The detection of Earth analogues causing an RV effect of about 10 cm/s is, therefore, out of reach at the time of writing. A few avenues are being explored to resume the trend to detect ever lighter planets. These include improving (a) instruments, (b) observation strategies, (c) RV extraction techniques, (d) the monitoring of stellar activity, and (e) the stellar activity models. This thesis is subdivided into three interconnected topics. First, I present my contribution to problem (c). I implemented a technique called Least-Squares Deconvolution (LSD) to estimate precise stellar RVs. Instead of using a template-based mask, I inferred the average stellar line profile based on laboratory data and extracted the RV from this profile. I analysed the dependence of the RVs on the quality thresholds and found a suitable optimisation scheme. We call this method the Multi-Mask Least-Squares Deconvolution technique (MM-LSD), and I have made it publicly available on GitHub. MM-LSD can be a valuable tool if observations are spread out over time and have not been reduced with the same pipelines or CCF masks, as can be the case in archival data. I expect the multi-mask approach to be adopted in tandem with the CCF technique, which will then provide more stable RVs, reducing method-induced RV variations. These variations are not the aim of the current modelling efforts focused on mitigating stellar-induced RV variations and are essential to eliminate. The flexibility and transparency of the MM-LSD pipeline enable one to extend it easily. For the second part of my PhD, I have implemented a magnetic flux estimation technique built on MM-LSD. This extension is aimed to contribute to solving the problem (d) above. I modelled the Zeeman effect in intensity spectra for which it can be parameterised in a way suitable for the LSD approach. Through this method, the information contained in thousands of lines can be harnessed simultaneously. This approach suppresses noise within the spectra and leads to the averaging out of many other effects affecting the absorption lines. The approach and the results are published in Lienhard et al. (2023). The extracted indicator shows higher correlations with the RVs than any classical indicators and is thus a very promising tool for mitigating stellar activity in solar-type stars. Lastly, I led the observation campaign for TOI-1774 carried out with the HARPS-N spectrograph. For this star, we initiated a collaboration with CHEOPS to share the data and run a joint analysis on the photometric (CHEOPS) and RV data (HARPS-N). I tested the two approaches above on this target, estimating the planetary mass to 7.14 +/- 2.08 Earth masses and the radius to 2.836 +/- 0.036 Earth radii. The orbital period of this planet was known from the transits observed by TESS and is equal to 16.71 days. Lastly, I assessed the probability of the existence of other planets in the system.
Open Access
Ferromagnetic dynamics in coupled systems
Patchett, James
In this work, ferromagnetic thin films coupled strongly to other physical subsystems (magnetic or otherwise) are studied, and the effect this coupling has on their static and dynamic properties is investigated in order to understand both the ferromagnets themselves, and the systems they are coupled to. This work investigates recent claims that spin-triplet superconducting Cooper pairs can be generated at superconducting Nb-fullerene interfaces in thin-film heterostructures by looking for evidence of an increase in magnetic damping below the Nb transition temperature in an adjacent permalloy (Py) layer. From these measurements, it is proposed that the experimental evidence purported to show evidence of a spin-triplet population can instead be understood as a signal from the vortex population within a spin-singlet superconductor. It is shown how it is possible to apply group theory to the dynamic modes of a ferromagnet, or multiple coupled ferromagnets, obeying the linearised Landau-Lifshitz-Gilbert (LLG) equation. From this, the effect of symmetry on the expected resonance spectrum of antiferromagnetically coupled magnetic moments is investigated. Features such as anticrossings and mode degeneracies are shown to be understandable from symmetry arguments, and this is demonstrated experimentally via measurements of the ferromagnetic resonance spectrum of two synthetic antiferromagnets: one bilayer with close to identical ferromagnetic layers, and another bilayer with layers with disparate properties. Features of the magnetoresistance behaviour in CoFeB single-layers and synthetic antiferromagnetic CoFeB/Ru/CoFeB nanowires adjacent to heavy-metal Pt layers are reported. It is shown how symmetry arguments can be applied to understand features of the magnetoresistive signal and observe a current dependent uniaxial magnetoresistive signal at high current densities which is attributed to the onset of auto-oscillations within the exchange magnon population of the nanowires.

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