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  • ItemEmbargo
    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.
  • ItemEmbargo
    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.
  • ItemOpen 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.
  • ItemEmbargo
    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.
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemEmbargo
    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.
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemEmbargo
    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.
  • ItemEmbargo
    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 YBa2Cu3Oy (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.
  • ItemEmbargo
    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.
  • ItemEmbargo
    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.
  • ItemOpen 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.
  • ItemOpen Access
    On the topological properties of the vibrations of solids
    Peng, Bo; Peng, Bo [0000-0001-6406-663X]
    This thesis presents the study of the topological properties in the atomic vibrations in solids. The state-of-the-art quantum mechanical simulation techniques are presented first to accurately describe the band structures of electrons and vibrational spectra of nuclei for topological analysis. Two concepts are introduced to investigate the non-trivial band crossing in the vibrational spectra. Depending the number of phonon bands and the intrinsic crystalline symmetry, the topological invariant carried by the band crossing can either be an integer number for a two-band subspace or an non-Abelian frame charge when at least three bands are involved. The phonon band crossing formed in a two-band subspace is simply a replica of topological semimetals in electronic systems, although the intrinsic properties of phonons, including the preservation of time-reversal symmetry and the accessibility of the bosonic excitation spectra, indicate that such single-gap topological properties are ubiquitous in phonons. Taking a step further, when three or more bands are included, these intrinsic properties provide unique advantages for phonons to fulfill the requirements for multi-gap topology, rendering phonons as the primary platform to study non-Abelian braiding. The possible experimental signatures are also described and predicted.
  • ItemOpen Access
    Novel radio frequency applications and automated characterisation of silicon quantum dots
    Oakes, Giovanni
    As classical computers have revolutionised the 20th century, quantum computing could do the same in the 21st. For quantum supremacy, quantum error correcting codes will require an estimated million to billions of qubits. Regarding controlling and reading every qubit, classical electronics must be as close to the quantum processor as possible to necessitate active feedback. Silicon quantum dots (QDs) are a promising candidate for quantum computation due to their scalability and co-integration with classical electronics. However, a less researched implementation is the operation of quantum dots as novel circuit components that can be integrated with the quantum processing unit. In this thesis, we focus on novel radio frequency (RF) implementations of silicon quantum dots as charge sensors and frequency multipliers. A major concern with large arrays of silicon QDs is the requirement of complex tuning protocols due to device inhomogeneity, which is currently done heuristically. Therefore, for a fully integrated system, the first step will be to develop a scalable automated procedure to tune large QD arrays. To address this issue, we present an algorithm to extract the two most prominent slopes in a stability diagram, allowing users to control each quantum dot independently and counteract cross-capacitance effects. To extend to larger arrays, we outline how pre-calibrated QDs can act as charge sensors to measure otherwise undetectable transitions due to slow tunnel rates. In this line of research, we demonstrate how a quantum dot connected to a reservoir, known as a single electron box (SEB), can be operated as a sensitive charge sensor, allowing one to measure a neighbouring double quantum dot to the few-electron regime. We then perform state-of-the-art single-shot measurements across a Pauli spin-blockaded transition and obtain a fidelity of 99.2% in 5.6 μs, making this technology a viable competitor to the more commonly adopted single electron transistor (SET). By exploiting the non-linearity of the quantum capacitance, we operate the same device as a frequency multiplier and showcase ideal frequency conversion up to ten times multiplication. This can be useful for high-frequency RF reflectometry set-ups and EDSR pulses for spin manipulation. By driving the system at lower RF frequencies, we detected slow tunnelling events that would have been otherwise unmeasurable. This phenomenon requires better understanding, as it will be invaluable in characterising quantum dots that are physically distant from a reservoir, which will commonly be the case for large 2D arrays. There are ample research avenues to explore the operation of quantum dots as novel circuit components, such as parametric amplification, frequency mixers and multipliers, and charge sensors. In particular, the development of new quantum dot architectures where such circuit components are co-integrated with qubits. Operating such complex arrays will require automated tuning protocols, preferably running in parallel and using low-power devices, such as FPGAs, which can be run locally on the 4K plate. Due to the low power budget, such algorithms must be very efficient, making current deep neural networks unfeasible. This thesis provides valuable insights into the potential use of silicon quantum dots as cryo-CMOS components for quantum computing.
  • ItemOpen Access
    Transition-Edge Sensors for Electron Spectroscopy
    Patel, Kunal; Patel, Kunal [0000-0002-5492-3703]
    Transition-edge sensors (TESs) are highly-sensitive detectors capable of both radiative flux and single photon measurements. TESs have found a number of applications including astronomical photon measurements, neutrino mass measurements and in the search for dark matter. Despite this versatility, the use of TESs as massive particle spectrometers has received remarkably little attention, notably within the field of electron spectroscopy where TESs could provide significant advantages over existing electron spectrometers. In this thesis, I present the first investigation into the capabilities of TESs in electron spectroscopy using a combination of numerical simulations and experimental measurements. Through the use of numerical simulations, I show that TESs are capable of matching the energy resolution of traditional electron spectrometers whilst providing order of magnitude improvements in measurement efficiency. I then describe the design and testing of a TES electron detection system that I used to perform a set of proof-of-principle TES electron measurement experiments. The results of these experiments are presented, showing the successful detection and energy measurement of individual electrons with energies spanning between 0 and 2000 eV with an energy resolution of 3 eV. An important consideration for TES electron spectrometers is their sensitivity to electric and magnetic fields arising from nearby electron optical components. The sensitivity of TESs to magnetic fields has been investigated before, but the impact of strong electrostatic fields on TESs has not been. I show that the application of electric fields up to 90 kV m−1 had no observable effect on the TESs tested, demonstrating that TESs can be operated in strong DC field environments, as may be found in TES electron spectrometers. Having demonstrated the suitability of TESs as electron spectrometers, I then present designs for a set of TESs, made specifically for electron calorimetry with a dedicated electron absorber structure. I show the results of a set of electron absorption measurements used to determine appropriate materials to be used in this structure. I conclude the thesis by summarising the capabilities and benefits of TES electron spectroscopy as well as the challenges that will need to be overcome to realise this technology.
  • ItemOpen Access
    Optimisation of Hall cross devices towards magnetic particle counting
    Herbert, Holly
    The magnetic detection of magnetically labelled disease biomarkers from a sample of bodily fluid presents an interesting architecture for a disease diagnostic device. Many advantages are offered over conventional optical labelling and detection techniques, including reduced background signals and sample pre-processing requirements due to the lack of magnetically responsive material in biological samples, as well as enhanced control over various steps of the assay protocol as magnetic labels may be actuated remotely via the application of magnetic fields. In this thesis, Hall cross sensors are explored for the detection and enumeration of large numbers of magnetic particles for applications in magnetic immunoassays. The response of a Hall cross to a magnetic particle is directly proportional to the stray field of that particle averaged over the active area of the cross. As the active area size increases relative to the particle size, due to the solenoidal nature of the particle’s stray field, this average tends towards zero. As a result, large area Hall sensors suffer from low single particle signals, limiting device resolution. In addition, the Hall response is found to be highly inhomogeneous as a function of particle position, limiting the certainty with which particles may be counted. Hall cross sensors for which the active area size match the particle size produce a much larger response and can be used detect the binary presence or absence of a particle, thus resolving issues with both signal strength and homogeneity. However, dense arrays of individually contactable sensors must be fabricated to detect meaningful numbers of particles, limiting their usefulness. This work focuses on the optimisation of large area Hall cross sensors towards the goal of counting large numbers of magnetic particles simultaneously with improved resolution and measurement uncertainty. It is hypothesised that the inclusion of perforations within the active area of such devices could make the Hall cross insensitive at locations where the stray field components of a landing magnetic particle reduce the overall Hall signal, thus enhancing the response. This concept is explored using COMSOL simulations and it is found that when an array of perforations is added to the active area of a Hall cross and particles land at certain subsets of positions relative to these perforations, both the magnitude of the Hall response and the homogeneity of the response with position are vastly improved. Experimental prototype devices are fabricated from GaAs/AlGaAs at which a 2DEG has formed and the response of perforated devices to arrays of magnetic disks is measured at room temperature, with the aim of demonstrating the same improvement. Good agreement between computational and experimental results is found for the perforated devices, while for the equivalent continuous devices, the measured response suggests that the fabrication of magnetic particles directly on top of the GaAs/AlGaAs resulted in the local, partial depletion of the underlying 2DEG. As such, these devices behave in a manner consistent with having partially formed perforations.
  • ItemOpen Access
    Development of High-Pressure Calorimetry Techniques and Calorimetry Study of the Heavy-Fermion Superconductor CeSb2
    Hodgson, Stephen
    Low-temperature experimentation is crucial to discovery in condensed matter Physics. Novel phenomena often appear at high pressure, which provides unique technical challenges to these measurements. This is especially true for calorimetry studies, for which the ambient pressure technique is predicated on the sample being in quasi-adiabatic conditions. We pioneer a new capability to perform calorimetry studies on samples that are embedded in a thermally conductive medium. With a framework that only requires one lock-in, the 3ω technique goes beyond standard methods by replacing DC thermometry with AC resistance techniques. We focus on the situation of materials at high-pressure in a piston cylinder cell, comparing various possible measurement strategies. Detailed calculations of the thermodynamic variables pave the way for further development and optimisation for future projects studying materials under pressure and beyond. We apply this technique to study the heat capacity of CeSb2 at pressures up to 26kbar, temperatures down to 300mK and fields up to 9T. At ambient pressure, CeSb2 is a Kondo lattice heavy fermion material with a ferromagnetic ground state and various magnetic transitions at low temperatures. Between 6kbar and 16kbar, CeSb2 undergoes a structural transformation that destroys the ferromagnetism, but reveals a new transition to a magnetically ordered state. Further pressure pushes this magnetic transition to a quantum critical point (QCP) and the material becomes an unconventional superconductor. Applying the new calorimetry technique, we discover two magnetic transitions in this phase. We track these transitions as further pressure pushes them together and towards the QCP. The field response of these transitions suggest that both persist close to the vicinity of the QCP which could be the key to understanding the details of its unconventional superconductivity, and understanding superconductivity in heavy fermion materials more broadly.