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The discovery of graphene and its remarkable properties have given rise to an increased interest of the scientific community towards the field of carbon-based electronics. In order to overcome the limitation of graphene's metallic character, several routes have been considered, such as chemical decoration, substrate-induced bandgap opening or doping with boron and/or nitrogen atoms, by creating a hybrid graphene-h-BN two-dimensional material. Surface-assisted synthesis offers the possibility of creating organic analogues of graphene with high electron mobility and tunable bandgaps. Two-dimensional molecular networks or polymers can be obtained through bottom-up synthesis methods, where the building blocks - small organic molecules - have the potential of tuning the desired properties of the covalent molecular network. The field of synthesizing hybrid boron nitride-carbon 2D covalent molecular networks, that can create a graphene-like 2D material, via surface reactions is still incipient, while their electronic properties remain yet unexplored. Moreover, no studies of the synthesis of chiral covalent 2D molecular networks, which can allow their usage in enantiospecific applications have been reported up to date. This thesis is focused on synthesizing carbon-boron nitride 2D molecular networks using surface assisted reactions, namely the Ullmann coupling reaction of brominated borazatruxenes as well as the B3N3 ring closure reaction of isodiaminodiborane precursors. The electronic properties and assembly of borazatruxene, a new class of BN-doped polycyclic aromatic hydrocarbon, and its comparison with truxene, its organic counterpart, shows that the inclusion of the BN ring in borazatruxene increases its electronic bandgap. A hierarchical mechanism based on chiral homodimer units of 3Br-borazatruxene towards the formation of 1D chains and 2D H bonded networks is discussed. This mechanism is key for homochiral segregation into large chiral domains, which allow the formation of chiral borazatruxene-based covalent molecular networks. The obtained 3Br-borazatruxene H-bonded networks present low symmetry, which allowed the formation of small pores able to trap atoms or small molecular species. Theoretical predictions show that up to two Na atoms could be trapped within these pores, inducing hybrid Na-C bands with anisotropic dispersion. Subsequently, the on-surface Ullmann coupling of 3Br-borazatruxene precursors on Au(111), starting from the H-bonded proto-stage discussed above is investigated. We obtained the first chiral 3Br-borazatruxene macrocycles on Au(111), whose electronic properties are investigated by scanning tunneling spectroscopy dI/dV measurements. In order to improve the quality of the network, we employ the gradual dehalogenation process of this precursor on Au(111), along with the homodimer-based low symmetry of the H-bonded network, leading to the description of a two-stage Ullmann reaction of 3Br-borazatruxenes. The final part of the thesis is focused on the B₃N₃ ring closure reaction of isodiaminodiborane (IDADB) on Au(111), as a new surface-asissted reaction for obtaining B₃N₃-C molecular networks. Temperature-dependent XPS studies have shown the successful formation of closed B3N3 rings by investigating the chemical fingerprints of B, N and C 1s and via STM imaging investigations of the formed covalent assemblies. Building on the experimental results, a mechanism of the surface-assisted BN ring closure of isodiaminodiborane on Au(111) is proposed.

The structure of glasses that cover a large range of compositions in the (MgO)x- (Al2O3)y-(SiO2)(1-x-y) system was investigated by neutron diffraction and 27Al magicangle spinning nuclear magnetic resonance (NMR) spectroscopy. Site-specific information was thereby gained on the composition-dependent local structure of the Si, Al and Mg atoms. The results were interpreted with the aid of a structural model developed for M-aluminosilicate glasses (M = alkali or alkaline Earth metal), which assumes that Al atoms can take on more than one structural role. For compositions with R = x/y > 1, the glass structure consists primarily of SiO4 and AlO4 tetrahedral units, which are linked through bridging oxygen (BO) atoms to form an aluminosilicate network. The Mg2+ ions either (i) associate with non-bridging oxygen (NBO) atoms, that is, an oxygen atom that is connected to only one Si- or Al-centred tetrahedral unit; or (ii) stabilise the formation of Al-centred tetrahedral units by balancing the negative charge associated with an AlO4 unit. For compositions with R < 1, there are an insufficient number of Mg2+ ions to stabilise all the Al3+ ions in tetrahedral units. In the R < 1 regime, the formation of Al-centred tetrahedral units, therefore, requires some fraction of the Al3+ ions to behave in a similar way to Mg2+ ions. These Al3+ ions are not part of the aluminosilicate network and reside in sites that have five or six nearest-neighbour oxygen atoms. In situ high-pressure neutron diffraction experiments were performed on calcium aluminosilicate (CAS) glasses at pressures up to 17.5(5) GPa. In their as-prepared form, the CAS glasses primarily consist of a tetrahedral aluminosilicate network, with Alcentred tetrahedral units stabilised by Ca2+ ions. A pair-distribution function (PDF) analysis of the diffraction data revealed the complete or almost complete conversion of Al-centred units from tetrahedral to octahedral, accompanied by a shift in the average Al-O distance from rAl-O ˜ 1.760(10) Å at ambient conditions to rAl-O = 1.894(10) Å at 14.4(5) GPa. The Al-O coordination environment shows little change upon further compression to 17.5(5) GPa. In comparison, the average Si-O distance remains 1.611(10) = rSi-O = 1.634(10) Å for pressures P = 14.4(5) GPa, reflecting the persistence of SiO4 tetrahedra. For the highest pressure of 17.5(5) GPa, The average Si-O distance increases to rSi-O = 1.643(10) Å, which could mark the onset of the conversion of Si-centred units from tetrahedral to octahedral. An investigation of the CAS glasses recovered from 8.2(5) and 17.5(5) GPa showed that their structure did not recover to its as-prepared form and highlighted substantial differences between the structure of CAS glasses before, during and after compression. A relationship between the average Al-O coordination number and the densification of aluminosilicate glasses was established by comparing the results from this study with those available in the literature. The structure of crystalline and amorphous materials from the Na1+aAlaGe2-a(PO4)3 (NAGP) system with a = 0, 0.4 and 0.8 was investigated by neutron diffraction PDF analysis. The results for the crystalline materials conform to the expected crystal structure, wherein corner-sharing PO4 and GeO6 or AlO6 polyhedral units are linked to form a 3-dimensional network with Na+ ions residing in interstitial cavities. The corresponding glasses contain a significant proportion of sub-octahedral Ge- and Alcentred units, which are not present in the crystal structure. The differences between the glass and crystal structure means that substantial structural reorganisation must be undertaken during the crystallisation process. The measured Al-O and Ge-O coordination numbers were used to assess the network connectivity in NAGP materials and how this evolves during the early stages of crystallisation. A structural model for the alumina-free glass was proposed, which centres on the formation of Na2P6GeO18 super-structural units within a tetrahedral GeO2 network. New assemblies were developed for the Paris-Edinburgh press to access high pressure - high temperature (high P-T) conditions. These high P-T assemblies were designed for either (i) in situ neutron diffraction on the PEARL diffractometer at ISIS; (ii) in situ X-ray diffraction on the I15 beamline at the Diamond Light Source (DLS); or (iii) laboratory-based experiments at the University of Bath. The materials used to construct the high P-T assemblies were tailored to suit their application. Calibration experiments were used to assess their performance. The high P-T assemblies developed for PEARL were used to study the structure of crystalline GeO2 up to a temperature of 2166(173) K at 9.9(5) GPa. The assemblies developed for X-ray diffraction experiments were used to demonstrate the functionality of the high pressure apparatus for I15. The high P-T assemblies developed for laboratory based experiments were used to characterise the behaviour of different furnace materials. A high P-T processing technique was established with a priority placed on the accurate determination of the sample conditions. The technique was used to produce a set of permanently densified GeO2 glasses.

Graphitisation of diamond and SiC surfaces to produce high-quality epitaxial graphene was developed and investigated using surface sensitive techniques, namely X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), and X-ray photoemission electron microscopy (XPEEM). The transfer of epitaxial graphene grown by catalytic graphitisation has been achieved and studied using Raman spectroscopy. Above 930 ◦C, the diamond (111) surface undergoes a (2×1) reconstruction with three domains, confirmed by XPS and LEED. Previously acquired angle-resolved photoemission spectroscopy (ARPES) measurements are affirmed by modern density-functional theory (DFT) modelling along the KΓK reciprocal space direction which demonstrates the π-band rising above the Fermi level, indicative of a metallic surface state. Heating the diamond surface above 1000 ◦C produces graphene that co-exists and strongly interacts with the (2 × 1) reconstruction, evidenced by the emergence of Dirac cones along the KgMKg direction in both previously acquired ARPES and DFT modelling. The temperature required to graphitise the diamond (111) surface is catalytically reduced to ∼500 ◦C in the presence of a thin iron overlayer. The purity and crystallography of the iron is vital in producing epitaxial graphene with minimal defects. Real-time electron emission spectroscopy (REES) allowed the detachment, transport, and re-crystallisation of carbon from the diamond surface into graphene to be monitored for a linear temperature ramp to 685 ◦C. A heavily boron-doped diamond was catalytically graphitised at 640 ◦C. Angle-resolved XPS and Raman measurements reveal that boron is transported through the iron and forms a boron-doped graphitic structure with a boron content of ∼5 % and p-type characteristics. Patterned graphene is fabricated directly on the 6H-SiC (0001) surface using catalytic graphitisation. LEEM, XPEEM, and Raman spectroscopy mapping confirm that graphitised regions adhere perfectly to the catalyst pattern with a step edge < 50 nm. An acid-free delamination transfer technique using a polyvinyl alcohol scaffold was developed in order to move graphene, catalytically-grown on SiC, onto silicon dioxide. This improved transfer heralds an order-of-magnitude improvement in the post-transfer defect density of graphene when compared to acid-etch transfer techniques, as well as significantly reducing polymer residues and contamination. Raman spectra with the characteristic graphene Raman peaks (D, G, and 2D) have been measured for the first time on catalytically-grown graphene from diamond.

Quantum theory challenges our intuitions of causality, as manifest in the existence of Bell non-local correlations. The latter seem to falsify Reichenbach's common cause principle and evade causal explanations within the classical causal model framework developed in recent decades. But could quantum correlations be explained in causal, albeit quantum causal terms? Even if one embraces a revised causal relation, challenges of our intuitions seem to persist - for instance, 'quantum indefinite causal order' of events being conceivable and intensely studied. This thesis hopes to contribute to our understanding of how causal reasoning can be maintained, in a rigorous manner, in light of quantum theory. Inspired by the work of Allen et al. [Phys. Rev. X 7, 031021 (2017)], a definition of quantum causal relations is given in terms of influence in underlying unitary transformations. The notion of quantum causal structure that ensues is then explored in three, closely related directions. First, a quantum causal model framework is presented that generalises classical causal models and allows causal explanations of quantum processes, assuming that a definite causal order does exist. Amongst other things, notions of 'quantum conditional independence' are presented and generalisations of core classical theorems, such as the d-separation theorem, are derived. Second, the thesis studies how causal structure of unitary transformations can be understood in terms of their compositional structure. The results here reveal how causal structure is closely associated with the interplay between direct sums and direct products of Hilbert spaces. An extension of quantum circuit diagrams is introduced to visualise the found 'causal decompositions' of unitaries. Third, a generalisation of quantum causal models to cyclic causal structure is proposed, to analyse processes that feature indefinite causal order. The idea is illustrated through analysing well-known examples of such processes and some first results are presented.

This thesis concerns the study of Mercury's surface and exosphere using the Mercury Imaging X-ray Spectrometer (MIXS) onboard BepiColombo, focussing on predicted X-ray fluorescence observations of different regions of Mercury and achieving the limits of what MIXS will be capable of over the course of the mission. X-ray fluorescence (XRF) spectroscopy is a commonly used technique for elemental analysis of a target material; however, it is typically reserved for a laboratory setting with a man-made X-ray source. Due to Mercury's proximity to the Sun, Solar X-rays can act as the X-ray source. This becomes less practical at greater distances from the Sun due to the flux following the inverse-square law. Because of this, XRF has only been used on missions to the inner solar system such as the Moon and Eros. The Mercury Imaging X-ray Spectrometer (MIXS), which launched on board the ESA/JAXA BepiColombo mission to Mercury in October 2018, uses microchannel plate optics to focus incoming X-rays onto a DEPFET micropixel detector. This allows for improved spatial and spectral detections over previous detectors, as well as imaging in X-ray wavelengths. Chapter 3 aims to investigate how the improvements to the spatial and spectral detections can be fully utilised when observing dayside fluorescence at Mercury. To do this, a program initially created by Bruce Swinyard to model X-ray fluorescence from the Lunar surface is adapted to instead reflect the Mercurian environment. X-ray fluorescence spectra for both MIXS-C and MIXS-T are produced for a range of terrane compositions present at Mercury during different solar conditions. Using the intensity of the detected fluorescence for the lighter elements such as sodium, magnesium, aluminium, and silicon it is possible to determine the type of geochemical terrane being observed by MIXS-C based on the fluorescence detected. In Chapter 4 expands on what MIXS was primarily designed to achieve by investigating the possibility of observing X-ray fluorescence from the sodium exosphere of Mercury. A lowdensity gaseous target is used in place of the solid planetary surface that has been considered previously. Because of this MIXS-C will be the primary detector considered in this chapter due to its far superior effective area than that of MIXS-T. This high effective area is required to detect such a weak signal. Initial predicted detections give unfavourable spectra and require a high intensity X-class flare along with a prolonged observation time to obtain a significant detection. Two different observational orientations are considered to enhance the detections of X-ray fluorescence from exospheric sodium by focusing on high sodium density regions of the exosphere. Taking the limitations of the observations into account such as observation time and pointing angle, the detected fluorescence is too low to be considered significant. This result and its implications are discussed further. Chapter 5 returns to surface-based fluorescence emission, but instead now focussing on fluorescence on the nightside surface of Mercury. Using an energetic electron flux background from Ho, et al., (2011) in the place of solar X-ray flux, predictions of the detected X-ray flux are made for both MIXS-C and MIXS-T for several observation times and electron flux intensities. Following this a comparison between dayside and nightside fluorescence observations is conducted, with particular emphasis on terranes found close to the cusp regions of Mercury's magnetosphere as that is where Particle Induced X-ray Emission (PIXE) events have been observed previously. It is found that PIXE events produced increased fluorescence flux for heavier elements such as calcium and iron, whereas solar insuded fluorescence is better suited to lighter elements such as sodium and magnesium. The results of this comparison, and the impact they will have on the BepiColombo mission as well as our understanding of Mercury as a whole are discussed futher. Despite being one of the terrestial planets, Mercury is one of the planets in the solar system that is currently the least understood. The work in this thesis aims to fully explore the possibilities of detections that MIXS is capable of to maximise its scientific output upon its arrival at Mercury in December 2025.

In this project I studied the collective behaviour of dense swarms of rodshaped particles with heterogeneous properties. I consider confined systems as well as unbounded domains with periodic boundary conditions and (ir-)regular obstacles of various nature. All results that I provide are based on my own molecular dynamics based code and can be used in various cases of collective behaviour such as bacterial motion, artificial active particles (swarm robotics), animal interaction, for example, flocking of birds or schooling of fish or even crowd control. Ch. 1 provides an introduction to active matter. I give a brief explanation of this phenomena and provide various examples that naturally arise in living and artificial systems. I also discuss various models of active matter, including the one that is used in the following chapters, and their pros and cons. In Ch. 2, I thoroughly discuss the model for heterogeneous active matter I present it as an extension for an existing method to simulate homogeneous self-motile particles. I also provide the numerical background for the simulation code as well as some solutions to particular problems that come out in computer simulations of confined heterogeneous active matter. The results will be submitted in the form of a computational package in the near future. Ch. 3 introduces self-propelled rods moving in a periodic (quasi-) 2D channel. I start with conventional active systems where all particles are identical. I compare them to systems where rods have heterogeneous properties i.e. every active particle has its own hardness and/or self-propellant force picked from a given distribution. I study how this introduced heterogeneity affects the resulting distribution of active matter in the confinement comparing to homogeneous systems. The results of this study have been published in Physical Review E [1]. In Ch. 4, the statistical properties of the homogeneous active matter are given using the mean square displacement and the so-called giant density fluctuations metric. This part of the thesis shows how a variety of behaviours emerges in confined systems of self-motile rods. The main finding here is that all patterns of motion observed in such systems can be arranged according to the corresponding values of the metrics above. Conclusions and the perspective of all unanswered questions are given in Ch. 5. Whereas the area of active matter has been developed for more than 20 years, many problems still have to be solved. This chapter provides a potential direction of further active matter development as well as a summary of the thesis.

Atom interferometric sensors can enable extremely precise measurements of inertial motion and external fields by manipulating and interfering atomic states using pulses of laser light. However, like many experiments that require the coherent control of a quantum system, the interaction fidelity is limited by inhomogeneities in the control fields. Variations in atomic velocity and laser intensity lead different atoms to experience different interactions under the same pulse, reducing the interference fringe contrast, introducing bias, and limiting the sensitivity. We present the theoretical design and experimental demonstration of pulses for atom interferometry which compensate inhomogeneities in atomic velocity and laser intensity. By varying the laser phase throughout a pulse and choosing an appropriate fidelity measure to be maximised, pulses are optimised by adapting optimal control techniques originally designed for nuclear magnetic resonance applications. We show using simulations that optimised pulses significantly improve the fidelity of interferometer operations and verify this experimentally using Raman transitions within a cold sample of 85Rb atoms. We demonstrate a robust state-transfer pulse that achieves a fidelity of 99.8(3)% in a ∼ 35 µK sample and obtain a threefold increase in the fringe contrast using a full sequence of optimised pulses. Many of the pulse shapes found by optimal control are simple and symmetrical, and we show that certain symmetries are integral to error compensation. By systematically exploring the dependence of these solutions on the model and optimisation parameters, we demonstrate a stability which underlines the general applicability of optimised pulses to a range of interferometer configurations. Finally, we introduce and computationally analyse a novel theoretical approach to improve the sensitivity of large-momentum-transfer (LMT) interferometers, whereby "biselective" pulses are optimised to track the changing resonance conditions encountered in extended pulse sequences that are designed to increase the measurement sensitivity. When conventional pulses of steady phase are used, the interference contrast decays rapidly as extra pulses are added because of the change in resonance. Using numerical simulations, we show that bi-selective pulses maintain interaction fidelity throughout extended pulse sequences, allowing significant increases in the sensitivity that may be obtained using LMT.

We have performed experiments to probe the ground state dynamics of YFe2Ge2, NbGeSb and NbSiSb. An unusually large Sommerfeld coefficient (γ ~ 100 mJmol−1K−2) has been observed in the d-electron system YFe2Ge2. It also shows an anomalous power law temperature dependence of the electrical resistivity (ρ = ρ0 + AT3⁄2) which indicates Fermi liquid breakdown, possibly connected to its vicinity to a quantum critical point. The materials NbXSb (X = Ge/Si) have been theoretically predicted via initial density functional theory (DFT) calculations to harbour Dirac/Weyl-like features in their electronic band structure. Quantum oscillations were measured in the range 5-18 T using the de Haas-van Alphen effect. The observed quantum oscillation frequencies have been compared to DFT calculations of the electronic structure. Several Fermi surface sheets predicted by DFT have been observed and their quasiparticle masses have been measured. The context of these measurements with regards to the electronic structure in YFe2Ge2 has been discussed. It was found that the quasiparticle effective mass enhancement of the detected Fermi surface sheets can account for only ~40% of the mass enhancement previously observed from specific heat measurements. Quantum oscillations were measured over the range 3-18 T using the Shubnikov-de Haas effect in NbXSb. Many frequencies have been observed in NbSiSb which also shows an extremely large magnetoresistance. We have considered the origin of parts of the quantum oscillation spectrum at high fields and briefly discussed this in relation to magnetic interactions and magnetic breakdown. One clear frequency has been seen in NbGeSb, which also shows an unusual negative magnetoresistance at low fields. Quantum oscillation frequencies in both materials have been compared to DFT calculations. The Berry phase has been estimated for the observed Fermi surface sheets in both materials, suggesting a topological nature in these materials.

In this thesis we study two fairly different aspects of gravity: vacuum decay seeded by black holes and black hole thermodynamics. The first part of this work is devoted to the study of black holes within the (higher dimensional) Randall- Sundrum braneworld scenario and their effect on vacuum decay rates. We argue that, in close parallel to the 4-dimensional case, the decay rate is given by the difference in areas between the seeding and remnant black holes. We follow a brane approach to study the effective equations on the brane and focus on the tidal solution given by Dadhich et al. We solve numerically the equations of motion of a Higgs- like scalar field and obtain its decay rate. We then compare it to the Hawking evaporation rate and find that black holes of certain masses are likely to trigger vacuum decay. Finally, we study decay in the absence of a black hole and determine that, in close analogy to the 4-dimensional case, it is the presence of the black hole that enhances vacuum decay rates. The second part of this thesis discusses the thermodynamics of charged, rotating, accelerating AdS black holes. We impose sensible physical restrictions to the black hole metric and translate them into bounds of the black hole parameter space. We discuss the implications of having an exothermic term in the definition of enthalpy. We then focus on critical black holes, i.e. spacetimes in which at least one of the sides of the black hole's rotation axis has a conical deficit of 2π. Finally, we consider the Penrose process for neutrally charged critical black holes and discuss about the definition of efficiency in this process.

This thesis concerns the use of density functional theory (DFT) to determine muon stopping sites in crystalline solids. New tools for carrying out these calculations are introduced and these techniques are demonstrated through the results of calculations on the skyrmion-hosting semiconductors GaV₄S₈ and GaV₄S₈ and the heavy-fermion metals URu₂Si₂ and CeRu₂Si₂. The results of three studies on significantly different magnetic systems are presented, where in each case the interpretation of the results of muon-spin spectroscopy (μ⁺SR) experiments is aided by knowledge of the muon site. The results of μ⁺SR measurements on the iron-pnictide compound FeCrAs are presented and indicate a magnetically ordered phase throughout the material below T_N =105(5) K. There are signs of fluctuating magnetism in a narrow range of temperatures above T_N involving low-energy excitations, while at temperatures well below T_N a characteristic freezing of dynamics is observed. Using DFT, a distinct muon stopping site is proposed for this system. The results of transverse-field (TF) μ⁺SR measurements on the molecular spin ladder compound (Hpip)₂CuBr₄, [Hpip=(C₅H₁₂N)] are reported. Characteristic behaviour in each of the regions of the phase diagram is identified in the TF μ⁺SR spectra. Analysis of the muon stopping sites, calculated using DFT, suggests that the muon plus its local distortion can lead to a local probe unit with good sensitivity to the magnetic state. Finally, the results of μ⁺SR measurements on the charge density wave system 1T-TaS₂ are presented, which show three distinct phases versus temperature. The critical exponents for each of these phases are compared with the predictions of quantum spin liquid models. Using DFT, a quantum delocalised state for the muon between the TaS₂ layers is proposed, which is used in conjunction with its associated hyperfine interactions to determine the coupling of the muon to the diffusing spinons.

Star formation can be characterised by the presence of accretion disks and outflows that originate from the inner regions of these disks. Despite playing a crucial role in the star formation process, the mechanism by which jets are formed is unknown. The jets appear to originate from the very inner regions of protoplanetary disks, making it difficult to observe thejet-launchingregionbyconventionalmeans. Inordertoachievethehigh-angularresolutionrequiredtoobservetheinnerdiskitisnecessarytoobservewithopticalinterferometry. In this thesis I aim to answer the question of how astrophysical jets and winds are launched from the inner regions of circumstellar disks by observing with spectrally dispersed interferometry in the near-infrared wavelength regime. In this thesis I present near-infrared, K-band VLTI/AMBER and VLT/CRIRES observations of the Herbig B[e] star MWC297. I interpret velocity-resolved images across the Brγ line, aswellas thederivedtwo-dimensionalphotocentredisplacementvectors, andfitkinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AUscales. Thevelocity-resolvedchannelmapsandmomentmapsrevealthemotionof the Brγ-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. The Brγ photocentre shifts trace a rotation-dominated velocity field, where the blue- and red-shifted emissions are displaced along a position angle of 24◦±3◦ and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals are modeled using a disk-wind model with a poloidal velocity of∼20kms−1. Simulations show that adding a poloidal velocity component causes the perceived system axis to shift, offeringapowerful newdiagnostic for thedetectionof non-Keplerianvelocity fields. In addition to the K-band data, I present analysis of AMBER spectro-interferometric data for MWC297 in the H-band, spectrally and spatially resolving multiple different Brackett series lines. I use the differential phase data to construct photocentre displacement vectors for each of the Brackett series lines, which show that all the lines in the H-band trace a similar velocity field. I construct a global kinematic model for the whole H-band, with the results showing that the H-band Brackett series lines originate from a compact disk wind region, with a poloidal velocity of∼220kms−1. I also present AMBER and CHARA interferometry data along with CRIRES spectroscopy data (R = 100000) of the Herbig Be star MWC147. The continuum emission is fitted with aninclinedGaussianandaringwitharadiusof0.60mas(0.39au),whichiswellwithinthe expected dust sublimation radius of 1.52 au. No significant change is detected in the measured visibilities across the Brγ line, indicating that the line-emitting gas is located in the same region as the continuum-emitting disk. The differential phase data is used to construct photocentre displacement vectors across the Brγ line, revealing a velocity profile consistent with a rotating disk. The AMBER spectro-interferometry data is fitted with a kinematic model of a disk in Keplerian rotation, where both the line-emitting and continuum-emitting components of the disk originate from the same compact region close to the central star. The presence of line-emitting gas in the same region as the K-band continuum supports the interpretation that the K-band continuum traces an optically thick gas disk. I present spectro-interferometric observations of the Brγ emission from the the T Tauri star CW Tau. I construct photocentre shifts from the GRAVITY differential phase data, which indicate motion along a PA of 120.5◦ which is close to the previously observed jet axis for this object. This suggests that the velocity field traced by the Brγ emission of CW Tau traces a high-velocity outflow close to the radius where the disk is truncated by the stellar magnetic field. Each of the individual studies in this work introduces a new insight into the jet-launching region of young stellar objects. I use observations with high-spatial and high-spectral resolution to place strong physical constraints on the morphology and velocity fields of the jet-launching regions in young stars. The technical achievements presented in this thesis demonstrate the effectiveness of spectro-interferometry as a tool for studying the dynamic physical processes associated with star formation.

In recent years, efforts to understand electron transport at the molecular scale have intensified, driven by the desire to understand the quantum nature of electrical conductance at such length scales and by the need to design molecular-scale devices for switching, sensing and energy harvesting. The aim of this thesis is to investigate theoretically electrical properties of molecules placed between nanogap electrodes. Such structures can be realised using mechanically-controlled break junctions, which nowadays is a widely adopted experimental technique. The method used in this thesis is based on density functional theory (DFT), which is implemented in the SIESTA code, and involves combining DFT with quantum transport calculations using Greens functions. Chapter 2 presents a brief introduction to the theoretical concepts of DFT which has been developed to describe the electronic properties taking into account the full atomistic details of the systems. Chapter 3 presents solutions to the Green's function used for infinite and semi-infinite chains and introduces the transmission coefficient equations which forms the theoretical basis of the GOLLUM quantum transport code. The theoretical work carried out in this thesis focusses on the electrical properties of gold|molecule|gold junctions, in which a single molecule (or perhaps a small number of molecules) is placed between gold electrodes since experimentally, this is the most common choice of electrodes. The first topic I investigated is the conductance of break-junctions containing imidazole and benzimidazole series of molecules. In chapter 4, I present a combined experimental and theoretical study of the electrical conductance of series. The conductance of these molecules is measured using a mechanically controlled break junction and compared with density functional theory calculations. The theoretical results are in broad agreement with experimental reveal the decrease in electrical conductance of the imidazole and benzimidazole series with increasing the length. This study establishes that hydrogen-bonding plays an important role in determining the conductance of these molecular-scale junctions. The calculations, therefore, focus on the effect of H-bonding formed from N and H atoms on both the electrical conductance. The second topic of this thesis is presented in chapter 5. In that chapter, I have computed the transmission coefficient for series of polycyclic aromatic hydrocarbons including naphthalene, anthracene and tetracene. The conductance has been obtained with either with thiol or pyridyl anchor groups linked to the aromatic cores, either at para-para positions or meta-meta positions. The conductance of these molecules is predicted using density functional theory calculations. The results demonstrate consistently higher conductance in the meta series, after adding a first 5-membered ring, compared to the para series. The conductances of these new polycyclic aromatic cores are not much affected by adding a second 5-membered ring for meta connectivity. It is observed that the width of the gap between the HOMO and LUMO resonances can be controlled by the stacking geometry of these structures and their conductances are determined by the difference in Fermi energies relative to the frontier orbitals of the molecules due to the two different anchor groups.

Optical and optomechanical systems are exploited for quantum technologies to perform highly precise measurements for fundamental physics as well as biological and engineering tasks. Theoretical studies using tools from statistics and quantum information can greatly aid studies of the sensing capabilities of quantum systems and experimental designs. These allow us to quantify the amount of information about a parameter encoded in a quantum state itself and examine how that can be extracted through measurements. In this thesis we show how mechanical squeezing and measurements beyond position can be utilised to improve the precision of wavepacket expansion measurements which can test collapse models of quantum mechanics. Particularly, that squeezing can compensate for the free-fall time which is often the most significant limiting factor in such experiments, and measurements of other quadratures can offer increased precision. We demonstrate that the use of additional optical fields to measure the displacement of a free mass in a radiation-pressure limited interferometer cannot surpass the ultimate precision of the single-mode interferometer. This work applies to the likes of laser-interferometric gravitational wave detectors. Finally, we venture into multi-parameter estimation: we calculate bounds for multi-mode Gaussian states used to estimate many phase shifts and optimise over the input states. In contrast to the—far less experimentally feasible—multi-mode generalisations of N00N states we see no significant improvement when using multi-mode Gaussian states for the task. Given their comparable performance in single-phase estimation this is a new limitation of Gaussian states appearing only at the multi-parameter level.

Cold atom systems in optical lattices provide a promising platform for a wide variety of applications, ranging from quantum simulation to quantum metrology, due to their extremely high tunability and the ability to derive microscopic models under well-controlled approximations that allows us to model them. The proper characterization of those systems requires, in many scenarios, taking into account that they are subject to some dissipation sources, as dissipation can drastically modify the behaviour of the known phases of matter or even generate new ones. In this thesis, we investigate several important examples of dissipative many-body dynamics. The first one relates to the use of engineered coupling to the environment,both coherent and dissipative, to robustly create spin-symmetric fermionic states. This scheme, which combines a Raman transfer between Bloch bands and sympathetic cooling with a reservoir gas, prepares entangled states that exhibit quantum enhanced precision for metrology. In the second topic we explore, we focus on the study of one-dimensional spinless fermions and hard-core bosons. We observe how dissipation induces differences in local observables that are identical in the closed system. The third topic that we include in this thesis focuses on characterizing the role of dissipation, specifically particle loss and dephasing, in the long-time behaviour of many-body localized systems. We analyze under which conditions dissipation leads to thermalization in the localized phase. In all these projects, we make use of tensor network techniques to tackle the open system dynamics combining matrix product states and matrix product operator approaches, in both cases, exploiting symmetries in the system to optimize the numerical performance. All in all, the application of open system ideas to the study of quantum many-body problems provides not only an improved description of the realistic scenario but also can give access novel tools to engineer cold atomic systems in regimes that are not accessible for closed systems.

This thesis describes the implementation of an optically pumped caesium magnetometer containing a 1:5mm thick microfabricated vapour cell with nitrogen buffer gas, operating in a free-induction-decay configuration. This allows us to monitor the free Larmor precession of the spin coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarise the atomic sample;however, synchronous modulation of the light field actively drives the precession and maximises the induced spin coherence. Both amplitude- and frequency-modulation have been adopted producing noise floors of 3.4 pT / √Hz and 15.6 pT/√Hz, respectively,within a Nyquist limited bandwidth of 500 Hz in a bias field comparable to the Earth's (~50 μT). We investigate the magnetometers capability in reproducing time-varying magnetic signals under these conditions, including the reconstruction of a 100 pT perturbation using signal averaging. Additionally, we discuss a novel detection mode based on free-induction-decay that observes the spin precession dynamics in-the-dark using Ramsey-like pulses. This aids in suppressing the systematic effects originating from the light-atom interaction during readout, thus vastly improving the accuracy of the magnetometer whilst maintaining a sensitivity that is competitive with previous implementations. This detection technique was implemented further to measure the spin relaxation properties intrinsic to the sensor head, useful in determining the optimal buffer pressure that extends the spin lifetime and improves the sensor's sensitivity performance.

This thesis aims to provide a fuller understanding of a highly important but still controversial aspect of Isaac Newton's natural philosophy: the role of occult, or at least non-mechanical, principles in his natural philosophy. The most obvious of these was his belief that gravity was an attractive force which operated across empty space, and so was an occult actio in distans. But there are other aspects of Newton's work which would have been regarded by Cartesian contemporaries as occult; such as his belief that light can be an active component within bodies, that light and other matter can be converted into one another, and that bodies are not inert and passive but manifest various principles of activity. R.S. Westfall, suggested in the 1970s that Newton's unprecedented success as a natural philosopher was due to the fact that he combined two seemingly antithetical traditions of natural knowledge, the mechanical tradition, and what he called the Hermetic tradition. This thesis replaces Westfall's outdated notion of a "Hermetic" tradition with broader occult or natural magical traditions and shows how they formed the context within which Newton's own work was formed. The thesis is not primarily a study of Newton's work but a study of the work of earlier English thinkers who can be seen to have established the occult traditions which were subsequently taken up by Newton. Each chapter, therefore, focuses on a different aspect of occult ways of thinking in natural philosophy during the early modern period, and finally shows, in the conclusion to each chapter, how these ideas appeared in Newton's work, and, as Westfall suggested, contributed to his unprecedented success. Over six chapters the thesis considers theories that the world system is a network of radiating forces analogous to light rays, that gravity is an attractive force analogous to magnetism and operates at a distance, that matter has the power to attract and repel other matter, or has the power to incessantly emit active material effluvia, or the power to vibrate. It also shows how beliefs about the mathematical principles of natural philosophy, and the usefulness of the experimental method made possible, and supported, these theories about occult principles. The focus is on English thinkers and developments in English natural philosophy. This is not just an arbitrary choice but reflects sympathetic attitudes to occult ways of thinking in English thought which are shown to derive from the first natural philosophers in England to acquire international reputations since the Middle Ages, John Dee, Francis Bacon, and William Gilbert. Writing before the mechanical philosophy was conceived, these three thinkers all embraced occult ideas and left them as a legacy for subsequent English thinkers, up to and including Newton. The thesis shows that the combination of occult and mechanical traditions discerned in Newton's work by Westfall was in fact highly typical of English thinkers who combined occult ideas deriving from Dee, Bacon, and Gilbert, with the emerging mechanical philosophy. This marked trend in English natural philosophy reached its culminating point in the work of Newton, but Newton's achievement was only possible because of what had gone before. The thesis shows, therefore, that Newton's achievement crucially depended upon this English background.

Microresonators have the ability of strongly enhancing the propagating optical field, enabling nonlinear phenomena, such as bi-stability, self-pulsing and chaotic regimes, at very low powers. It is fundamental to comprehend the mechanisms that generate such dynamics, which are crucial for micro-cavities-based applications in communications, sensing and metrology. The aim of this work is to develop a scheme for the control of nonlinear regimes in microresonators, assuming the interplay between the ultra-fast Kerr effect and a slow intensity-dependent nonlinearity, such as thermo-optical effect. The framework of the coupled-mode theory is applied to model the system, while the bifurcation theory is used to investigate a configuration in which the power and frequency of a weak signal can control the behaviour of a strong pump. In this regards, this study demonstrates that the effect of a parametric interaction, specifically the four-wave mixing, plays a fundamental role in influencing the nature of the stationary states observed in a micro-cavity. The results show possible new strategies for enhanced, low-power, all-optical control of sensors, oscillators and chaos-controlled devices. Moreover, the outcomes provide new understanding of the effect of coherent wave mixing in the thermal stability regions of optical micro-cavities, including optical micro-combs.

The Super-K experiment determined that neutrinos are massive particles by observing the oscillation of atmospheric neutrinos. The SNO experiment confirmed this measurement by observing neutrinos from the Sun. The SNO+ experiment is intended to study the nature of neutrino masses by replacing the heavy water used in SNO with scintillator. The main goal of the experiment is to search for neutrinoless double-beta decay within 130Te. The SNO+ detector is much more sensitive to radioactive contamination than the SNO experiment. For this reason an external LED calibration system has been developed so the detector can be calibrated without risking contamination of the scintillator volume. This thesis describes the commissioning of this calibration system and its performance during the water phase of SNO+. The scintillator volume is separated from the surrounding detector via an acrylic vessel. As scintillator is less dense than water the position of the vessel is expected to shift throughout the lifetime of SNO+. A method to determine the position of the vessel using the external LED system is detailed as well as its performance. A measurement of the scattering length of the water surrounding the acrylic vessel using the same LED system is also presented. Calibration of the detector will also be performed using sources deployed within the vessel, and a study on the angular distribution of light from these sources and the effect of hardware upgrades is also presented. During the lifetime of the SNO+ experiment, the detector will be sensitive to neutrinos emitted from a supernovae within the Milky Way. A software framework was developed to accurately simulate the main interaction channels for supernova neutrinos within SNO+. The software was used to determine the burst trigger effciency during the water phase as well as a procedure to follow in the case of the burst. One outstanding problem in the field of neutrino physics is the neutrino mass hierarchy; whether there are two heavy and one light or two light and one heavy neutrino mass eigenstates. Neutrinos from a supernova burst may be able to solve this problem. A simple study is performed to determine the sensitivity of various detectors. The same analysis is performed using the aforementioned software to explore any systematic effects which may alter the sensitivity.

This thesis describes experimental work on implementing single and two qubit gates in ¹⁷¹Yb+ ions using methods suitable for a large scale quantum computer. By combining a magnetic field gradient with microwave and radiofrequency radiation, the spin and motional states of the ions are coupled which allows multi-qubit operations to be performed, as well as providing individual addressing of ions in frequency space. A dressed state qubit is used which exhibits an increase of over two orders of magnitude in the coherence time for a qubit that it is sensitive to the magnetic field gradient. Using this system, single qubit gates are characterised using the technique of randomised benchmarking, resulting in a measured average error per gate of 9(3)x10ˉ⁴. A new type of two qubit gate is experimentally demonstrated, which in comparison to a standard two qubit gate shows significantly increased resilience to two major sources of gate infidelity: heating of the motional mode of the ions during gate operations, and incorrectly set gate field frequencies. These types of errors are expected to become increasingly important with the move towards quantum processors with large numbers of qubits. Using this same technique, a two qubit gate is also demonstrated at a higher initial temperature with a significantly improved fidelity compared to standard methods. These gate techniques are used to demonstrate work towards implementing positiondependent quantum logic, a method which could remove the correlation between the number of ions and the number of gate fields required in a large scale quantum computing architecture. A method to move the dressed state qubit through a magnetic field gradient while preserving quantum information is demonstrated, as well as a method to optimise the phase of a two qubit gate of unknown phase in order to implement a CNOT logic gate. This provides a path forwards to demonstrating a CNOT gate using position-dependent quantum logic.

In this thesis, a software framework called Data Quality Monitoring for High Energy Physics (DQM4hep) is presented, intended as a generic and adaptable online monitoring and data quality monitoring framework for high-energy physics experiments and testbeams. The framework and its development and deployment is discussed, using a number of testbeams as examples. The first group of these testbeams took place within the AIDA-2020 and CALICE collaborations, using the framework on the CALICE-AHCAL prototype. Following this, the framework was also used in the IDEA combined testbeam at the CERN Super Proton Synchrotron. The result of these testbeams was proof that the framework is capable of being adapted easily to a wide variety of detector types and experiments, demonstrating that it has fulfilled the requirements of the AIDA-2020 collaboration. Following this, it was also shown that DQM4hep can be used for online analysis of the IDEA testbeam, performing a similar role to more traditional o ine analysis using ROOT. Also presented is a physics analysis as part of the detector and physics for the Compact Linear Collider collaboration (CLIDdp). The analysis was performed using the hadronic decay channel of the e+e- → tth process at a centre-of-mass energy of 1.4 TeV. The goal of this analysis was to obtain an updated sensitivity on the measurement of the top-Higgs Yukawa coupling at the planned Compact Linear Collider. The analysis used Monte Carlo generated physics samples and several stages of modern processing, including Pandora Particle Flow Algorithms. Combined with a similar study of the semi-leptonic decay channel, the uncertainty of the coupling measurement was found to be 3.86%.

This thesis describes an ion trap magnetometer, in which experimental hardware and theory for trapped ion magnetometry has been developed and demonstrated. Quantum magnetometry has been a fast expanding field in recent years due to the plethora of military, medical and security applications. The current quantum magnetometer literature has shown great improvements on the standard magnetometer success metrics such as sensitivity, spacial resolution, frequency tuneability, noise shielding and vector field resolution. Ion trap magnetometry could potentially boast several advantages over some of the better established methods such as, nano-meter spatial resolution, broadband tuneability over radio-frequency (RF) and microwave frequencies, and magnetic field noise shielding and sensitivity improvements through dynamic decoupling methods; all in room temperature environments. The experimental setup discussed uses a micro-fabricated Paul trap for ion confinement due to the scalability of this technology. The method for sensing that was demonstrated utilises long-wavelength radiation dressed states to extend the coherence time of a single 171Yb+ ion beyond the bare states and to allow for a high level of frequency tuneability. The coherence times measured were T₂ = 0:645 s for RF field sensing at 15.616 MHz and T₂ = 1.153 s for microwave field sensing at 12.658466 GHz. Sensitivities of SB = 125.857 ± 26.155 pT/ √Hz for RF magnetic fields and SB = 102.054 ± 11.046 pT/ √Hz for microwave magnetic fields were achieved experimentally. A novel micro-fabricated Paul trap for trapping multiple ion clusters was developed to improve sensitivity values by increasing the total number of trapped ions. The design confines ions over a two-dimensional surface spanning 0.65 x 1.00 mm which would also allow for magnetic field gradient measurements. Efforts have been made to miniaturise certain experimental hardware to further the development of portable trapped ion experiments. For this, a miniaturised RF resonator that could be used for radial confinement of ions within Paul traps has been developed. At the current stage of development it is capable of applying a peak RF voltage of 114.9 V with a Q value of 70 at a resonant frequency of Ω/2π = 12.65 MHz.

Despite there being a significant development in polymer composites for multiple applications over the past few decades, there are still many difficulties relating to the effective distribution of nanoparticles such as Graphene and other 2D materials in preparing structure and minimising aggregation. To overcome these obstacles a simple method for modulating the properties of the composites by controlling the organization of the fillers, using an ordered lattice of polymer particles as a template, is described. This technique can facilitate the self-assembly of nanomaterials while preserving their useful properties and allows well-defined interface between the polymer and the nanomaterial resulting in a honeycomb-like arrangement. The ideal case would be to obtain a segregated percolating network whereby low loadings of nanoparticles are added to obtain high performance systems and thus reduce the cost for possible industrial applications. The enhancement of the mechanical and electrical properties of the composites suggests the use of these materials for different applications such as transport applications, where the combination of high strength and lightweight is required. Moreover, to reinforce systems that are very delicate such as membranes or fibres or for possible applications in sensor technology, molecular electronics, supercapacitors, electrochromic devices and pressure sensitive adhesives materials. The physical properties of these composites can be tailored using different matrices such as natural rubber, or a bimodal particle system, to create materials with high density and low void fractions, and hence very low percolation threshold. Furthermore, the combination of excellent monolayer fraction and homogenous dispersion of the GO within the polymer dispersion leads to highly uniform films, with good distribution of the filler throughout, in a segregated network. A reduction in situ proceeds to improve the innate properties of the GO, but also to modify the polymer in close proximity through and exothermic process, thus providing a much improved interface between filler and matrix, improving both the electrical and mechanical properties. The advantage of this method to organize the graphene nanoparticles in a segregated pathway does not require the use of expensive equipment or materials, and it is a promising way to open up pathways to tunable electronic composite materials on a large scale.

This thesis presents the theoretical foundation and design of a planar magnetic field source for the Geonium Chip Penning trap. Conventional Penning traps are limited in their scalability by the size of their magnetic field sources, which often take the shape of large and expensive superconducting solenoids. For many practical applications, such as portable mass spectrometry and quantum radar, these conventional Penning trap systems would be unsuitable, due to their large size and need for very high currents (of the order of a few hundred amperes). The Geonium Chip Penning trap aims to overcome this problem by proposing a novel planar magnetic field source comprising several superconducting coils confined to a plane. The electrode surfaces of the trap sit less than a millimetre above the magnetic field source such that the trapping magnetic field is oriented parallel to the electrode surfaces. In this thesis, I explore many of the fundamental design issues concerned with developing such a planar magnetic field source. In particular, I devise an entirely new scheme of remotely magnetising a plurality of closed superconducting loops to carry currents in persistent mode, and show how persistent supercurrents of hundreds of amperes can be remotely induced with input currents of the order of a few (one to two) amperes. I introduce, and experimentally verify, a theoretical framework that allows for a high degree of control over the magnetic field distribution. From this framework, I present an optimised design of a planar persistent current-mode magnetic field source theoretically capable of achieving a 0.1 T field homogeneous up to fourth order in the Taylor expansion about the trapping position. This magnetic field source is due to be constructed and tested in the coming months.

The Standard Model (SM) of particle physics provides no explanation of the lightness of the Higgs particle found at the Large Hardron Collider (LHC) in 2012 compared to the natural scale of its mass, the Planck scale. This problem leads to the study of the class of physics models known as composite Higgs models, where the Higgs boson is considered to be a bound state of a new strongly interacting gauge theory. In this type of models, elementary particles have to couple with this composite state in order to gain masses. The dependence on the composite partners of the top quark, known as the top-partners, of Higgs production through gluon fusion has been studied. There it was found that, due to a subtle cancellation between the contribution of the top and that of the top partners, it is not possible to infer the top-partner mass from that process. However, there has been a study on the Higgs plus jet production from gluon fusion in a model with a top and an additional top partner. In that case, the transverse momentum distribution of the Higgs boson showed a depenence on the top-partner mass and coupling. In this thesis we extend that study by considering Higgs production with a jet in explicit composite Higgs models, which has never been considered before in the literature. In particular, we consider composite Higgs models where the right handed top quark tR was considered to be a composite state of a strong interacting sector containing either one or two top-partner multiplets. We then study Higgs production in association with a jet in these models, and in particular we examine thoroughly the impact of increasing number of the top partner multiplets. The models studied in our work were categorised according to the representation of the top partners and the way the standard model left-handed doublet is embedded in the representations of the symmetry of the strong sector. In the case where there is only one top partner multiplet in the models, we derived the explicit forms of the Yukawa couplings of the top quark and the top partners, and the CP-odd couplings that are present as a result of having a bound state tR. In the case where there are two top partners multiplets, we discussed the behaviour of the Yukawas and the masses of the top partners as a function of the input parameters of the models. Numerical values of the masses, Yukawa couplings, and CP-odd couplings were calculated for both cases, and these values were input in a numerical programme to calculate the transverse momentum distribution of the Higgs. Various deviations from the Standard Model behaviour appear. They are typically model dependent, and have been studied on a case-by-case basis. In particular, we have discussed the difference between models with one and two top-partners.

Liquid phase exfoliation (LPE) has been demonstrated to be a powerful and versatile technique for scalable production of two-dimensional (2D) nanomaterials, such as graphene and molybdenum disulfide (MoS2) which allows for their processing into a wide range of structures. LPE can be understood in terms of the chemical physics of the interactions of the liquid with the nanosheets. Here, it is shown that the prototypical solvent for LPE of 2D materials, N-methyl- 2-pyrrolidone, undergoes chemical modification during exfoliation which gives rise to increased absorption and photoluminescence, making it particularly unsuitable for dispersion of photoluminescent nanomaterials such as MoS2. A subsequent study identifies the influence of solvent properties on the exfoliation process and presents a model which allows for consistent size selection of few-layer nanosheets from any chosen solvent. Using this understanding, applications-driven solvent selection can be used to identify alternative solvents which facilitate processing of liquid-exfoliated nanosheets into composite and thin film structures. This approach allows for exfoliation into water-immiscible solvents to enable assembly of liquid-exfoliated 2D materials can be assembled at the interface between two immiscible liquids as solid-stabilised emulsions where the nanosheets act as both stabiliser and functional material. An understanding of the chemical physics of these emulsions is developed in terms of surface energies which allows for both measurement of the surface properties of the stabilising nanosheets and design of emulsions for potential applications as inks, composites, sensors and energy storage devices. In addition, Langmuir deposition can be used to assemble densely-packed ultrathin films at the air/water interface. This method is used to prepare few-layer MoS2 nanosheet networks, which exhibit interesting spectroscopic properties. Furthermore, these films exhibit high conductivity which is attributed to doping by nanosheet edges. The combination of nanoscale film thickness and increased conductivity highlights their potential for optoelectronic devices. As such, this study demonstrates that, through understanding of exfoliation and size selection, interfacial assembly represents a promising approach for realisation of functional composites and thin films, enabled by ultra-thin interfacial films of 2D nanosheets.

Quantum sensors are a new upcoming technology that offers to push the limits of how we can detect incredibly small signals in the DC and low RF regimes using magnetometers such as super-conducting quantum interference device (SQUID); N-V centres in Diamond, atomic vapour cells and atomic traps. All these technologies are currently being heavily invested in to push the boundaries of magnetometry for commercial applications. The quantum sensor I have been working to develop is set apart in several different aspects: unlike other quantum sensors it is very sensitive at the high RF and even 12.6 GHz microwave radiation; it has the ability to be rapidly tuned to different frequencies; due to the microwave decoupling technique we employ, the sensor is not sensitive to fluctuations in the DC field and therefore does not require bulky shielding or a laboratory environment to function. To this end I have built a demonstrator system capable of sensing RF and microwave fields and have measured the sensitivity of this device in both of these regimes. There has also been work on developing a portable version of the demonstrator and this work is ongoing within the group.

This thesis details work conducted as part of the experimental collaboration responsible for the neutron electric dipole moment (nEDM) experiment based at the Paul Scherrer Institute (PSI). The nEDM is a sensitive probe of a broad range of new CP violating physics beyond the standard model, however it remains elusive: while historic experiments since 1951 have increased in sensitivity by over six orders of magnitude, a nonzero nEDM is yet to be detected. Many theories of physics beyond the standard model predict neutron EDMs of a size that would be detectable by current and next generation experiments, and it has been said that measurements of the neutron EDM have ruled out more theories than any other experiment. One explanation of the smallness of the neutron EDM, the Peccei-Quinn theory, invokes a novel particle, the axion, which is also a credible dark matter candidate. The axion is yet to be detected. The work covers three main areas. First, contributions to the data analysis technique used to analyse the main dataset to produce a new world-leading limit on the neutron EDM. Second, an auxiliary measurement campaign to map the magnetic field within the experiment's magnetic shields is described, and the analysis of these datasets to provide corrections for several critical systematic effects is presented. Finally, a novel analysis of the data taken at a previous-generation nEDM experiment is used to derive the first experimental limits on the coupling of axion-like dark matter particles to gluons is described. These exclusions are up to 1000 times stronger than previous results for cosmologically interesting 10-22 eV axions.

In recent years carbon nanomaterials have shown great potential in various applications; among them gas sensing and supercapacitor electrodes. Devices reported in the literature, which are built to exploit the properties of the nanomaterials are often based on expensive multistep processes. As nanomaterials start to be commercially available, the remaining challenge is to find scalable fabrication methods to take advantage of the nanomaterial characteristics. In this thesis, manufacturing techniques and materials are presented which enable technically and commercially interesting gas sensors based on nanomaterials with interdigitated electrodes. Graphene flake dispersions are readily available on the market in large quantities. Currently most available materials are actually multilayer graphene rather than single pristine sheets of carbon atoms. Flake sizes are typically below 1 µm preventing the readout of individual sheets. Nevertheless these small graphene sheets can be made to form a percolating film when dispensed and compressed in a L-S trough. Such films can be transferred to readout electrodes which have been pre-patterned by laser ablation. This simple three step process creates a sensor which is read-out by resistance measurement. In this work, sensors have been manufactured, with this device architecture, showing performance compatible with personal safety applications. Their detection limit of 1 ppm for NH3 with a response time of 90 seconds competes with current commercial systems. Investigation into the sensing mechanism of these percolating films reinforces the scalability of the fabrication method. Using reducing (NH3) and oxidising (acetone) gases we determined the origin of the sensor's response. KPFM and Raman analysis show basal plane doping of the graphene sheets in opposite direction when exposed to the reducing and the oxidising agent. The resistance of the film increases in both cases when exposed to the gases. This leads to the conclusion that the dominating sensing mechanism in the percolating graphene film takes place at the contacts of the flakes in the film. This sensing mechanism allows the use of even relatively low quality graphene flakes to make effective devices. Defects in the basal plane play a negligible role within the films as the edge properties are responsible for the adsorption. Another, less well-investigated nanomaterial with gas sensing properties is carbon nanofoam (CNF). CNF is formed in a diffusion limited aggregation process where individual carbon clusters form elongated networks with a "web"-like appearance. Synthesis of CNF and deposition onto readout electrodes in a single step is made possible by using a pulsed laser. The CNF is formed during the interaction between the laser and a precursor material, positioned close to the electrode onto which the CNF is to be deposited. This direct deposition method makes a film with good adhesion and uniformity. Metal functionalization using physical vapor deposition can be applied to the CNF in order to modify its sensing properties. Preliminary gas sensing studies, exposing both CNF and metallised CNFs to a changing humidity environment show responses of +70 % for CNF and -30 % for metallized CNF when the humidity is decreased by 17%. This is among the highest reported response of carbon nanomaterials. The reaction of the CNF to dry air changing environment only takes 13 seconds to reach steady state while the functionalised material takes 32 seconds. The metallization of the CNF follows the percolation law as the foam acts as a scaffold for the sputtered metal particles. The sign of the response when exposed to a dry environment inverts when the percolation threshold is crossed. At the threshold the sensitivity to a change in humidity is suppressed. As water is a major interferent for carbon nanomaterials in gas sensing devices it is important to find ways to suppress the response of a device towards water. CNF has also been used to make supercapacitors and their behaviour has been characterised. Characterising the CNF in supercapacitor electrodes reveals a large influence of the gravimetric capacitance on the mechanical properties of the foam. Supercapacitors were made using directly-deposited CNF (as above) and also by transfer through a water sub-phase. The CNF lifts from the substrate when immersed into water and can then be picked up with another substrate. This transfer not only induces a change in morphology but also introduces a compressional stress. These effects more than double the gravimetric capacitance from 17 Fg−1 to 42 Fg−1. The fragility of the CNF that this reveals indicates the necessity of a one-step deposition process as the properties of the CNF are easily changed if mechanical force is applied. Nevertheless the transfer enhances the specific capacitance greatly. In summary, laser-deposition technology and a combination of laser ablation and L-S deposition allows scalable fabrication of gas sensor devices based on carbon nanomaterials. Similar devices show interesting supercapacitive properties but they do not, so far, approach the state of the art as defined by gravimetric capacitance.

The intra-cluster medium (ICM) plays a key role in galaxy formation. The cooling of hot gas and its recycling due to feedback are key parameters in understanding the regulation of star formation. Semi-analytical models (SAMs) are quick simulations that allow us to test our understanding of galaxy formation processes. Most of them produce results agreeing fairly well with various galaxy observations, however they fail in reproducing the ICM properties. In this work we focus on the active galactic nuclei (AGN) feedback and cooling mechanisms affecting the ICM, by developing new physically motivated methods that give a more accurate ICM description in the L-Galaxies SAM. We start by correcting the baryon content of each halos in the simulation. Due to halo mass fluctuations, halos ended up with an over density of baryons. This problem was resolved by introducing an extra phase for the baryons to keep track of the gas that would be considered leaving the halo (during contractions) or inflating (halo growth). Although this solution solved the baryon problems, it did not answer the question of the excess of hot gas stored inside the virial radius. We investigated different feedback mechanisms, from SNR to black holes and found that ejecting the gas with powerful AGN jets is compulsory to reduce the hot gas content of the halos. In order to reduce the gas content, a new model of AGN feedback was implemented and tuned to reproduce the observational gas fractions available in the literature. The AGN can not only now reheat cold and cooling gas, but also eject it via powerful jets. In addition to this feedback, four new black hole accretion models were compared, based on different gas reservoir. The most accurate agreement with observations for most AGN and galaxy properties came from a model where the AGN was fed by accretion of cold clouds from the ISM of the host galaxy. Finally we investigate the effect of our improved feedback on the ICM itself. In addition to the new AGN feedback, we developed a more physical cooling mechanism based on Bremsstrahlung radiation and a Beta profile of the gas. This change enabled us to give X-ray predictions for our model to compare with observational data, including the most recent XMM results. We found that the change in cooling only slightly affects the results (gas fractions, metallicities), as expected. However the X-ray luminosities of our groups and clusters of galaxies are now in agreement with observations, mainly due to the gas content reduction done by the new feedback mechanism. This new version of L-Galaxies manages to reproduce both galaxies and ICM properties in fairly good agreement with observational data.

General Relativity (GR) is a highly successful theory whose predictions are still being confirmed a hundred years later. However, despite its significant success, there still remain questions beyond the realm of its validity. The reconciliation of Standard Model (SM) and GR or Quantum Mechanics (QM) and GR point towards the need for a potential modification of GR or a consistent theory of quantum gravity (QG). The purpose of this thesis is to explore classical and quantum gravity in order to improve our understanding of different aspects of gravity, such as black holes (BHs), exotic compact objects (ECOs) like Boson Stars (BS) and gravitational waves (GW). We follow recent advancements in the field of Effective Quantum Gravity (EQG) by noticing that gravity naturally lends itself to an effective framework. The cut off of this effective theory is set to be the Planck mass since this is where UV effects are expected to take over. We focus on finding low energy quantum corrections to General Relativity by using the effective 1PI action and the modified gravity propagator. These include predictions of two new gravitational wave modes in addition to the usual classical GW mode predicted by GR. We investigate and make comments on whether these modes could have been produced by the events observed by LIGO and the energy scales in which these could be possibly produced. In the next project, we investigated whether there exists a correction to the quadrupole moment formula in GR to calculate the energy carried away by gravitational radiation. We apply the corrected formula to calculate the gravitational radiation produced in a binary black hole system in the effective quantum gravity formalism. We make comments regarding its regime of validity. While working in the field of gravitational waves, an interesting aside was modelling of Exotic Compact Objects such as Boson Stars which could also potentially act as black hole mimickers. We calculated analytically the gravitational radiation background produced by binary BS systems. We also commented on and put constraints on their possible detectability by LISA. Last but not the least, an important area in QG is the study of black hole thermodynamics. Corrections to the Bekenstein-Hawking area theorem have been calculated in various quantum gravity approaches and have been found to have a logarithmic form. In the last paper of this thesis, combining insights from Effective Quantum Gravity and Black Hole Thermodynamics, we motivate a generalised Area-Entropy law for black holes building upon the idea of an adiabatic invariant. This allows us to find interesting constraints on the number of fields in a consistent theory of quantum gravity. This work is particularly interesting because of its potential consequences in finding minimal extensions to the standard model and combining the standard model with a consistent theory of gravity.

This thesis aims to understand the processes driving galaxy evolution across a range of environments, focusing on submillimeter-radio interferometric observations to characterise the interstellar medium (ISM) of galaxies. I investigate the formation of the first massive, passive galaxies in clusters, as a key step towards establishing the prominent environmental trends seen at low redshift. Cl J1449+0856 is an excellent case to study this - a galaxy cluster at z=2 with an already virialised atmosphere. Thanks to the significant over-density of galaxies in Cl J1449+0856, we have uncovered a diverse range of cluster members at z=2. I use multi-wavelength observations to study how dust-obscured star-formation, ISM content and Active Galactic Nuclei (AGN) are linked to environment during this crucial phase of cluster evolution. I find that the dense cluster environment significantly increases the star-formation efficiency and gas excitation of the massive galaxies, and conclude that these effects are driven by the high number of mergers, interactions and AGN in the cluster core. I then examine the sub-M* population in the cluster, probing this important and so-far poorly characterised ISM regime. I place these low-enrichment galaxies on ISM scaling relations, and find evidence for increased gas-to-dust ratios in this regime at z=2, compared with the local Universe. I quantify the effect of low metallicity on high-J CO transitions, finding that both the diffuse and denser gas phases are significantly photo-dissociated at z=2. Finally, looking towards future surveys of obscured star-formation across all environments, I start preparations for the Square Kilometer Array (SKA). I construct high-resolution mock images of an entire survey field as observed by the SKA, containing galaxies with a variety of morphologies and star-formation rates at 0 < z < 3. This will both inform future observational strategies, and enable us to efficiently analyse the large amounts of SKA data that will soon become available.

Hyperbolic materials are anisotropic media which exhibit metallic or dielectric behaviour depending on polarisation. Natural hyperbolic materials as well as hyperbolic metamaterials, composites with engineered optical properties that opened up new avenues for light manipulation, have an unprecedented ability to concentrate light on deeply subwavelength scales which promises a wide variety of new applications in nanophotonic technologies. This work presents three new material platforms for the realisation and control of hyperbolic dispersion and describes their optical properties. These include a metamaterial based on an array of plasmonic nanocones, heterostructured metamaterial based on Au-ZnO-Au metaatoms and a natural hyperbolic material CuS. A combined experimental and theoretical study of the optical properties of CuS colloidal nanocrystals show that they exhibit anisotropic plasmonic behaviour in the infrared and support optical modes with hyperbolic dispersion in the visible spectral range. Heterostructured, layered Au-ZnO-Au nanorod metamaterials supporting guided modes were developed with the introduction of a nanoscale dielectric gap in the the meta-atoms. The role of the shape of meta-atoms forming the array has been studied on the example of transformation of nanorods forming the metamaterial into nanocones. The plasmonic mode structure of the individual nanocones and pronounced coupling effects between them provide multiple degrees of freedom to engineer both the field enhancement and the optical properties of the metamaterials. These metamaterials are the first so-called gradient refractive index metamaterials that behave as a medium with elliptic optical dispersion in the region of the nanocone apexes and hyperbolic optical dispersion in the region of the bases. A scalable manufacturing process for these metamaterials allowing mass-production at the centimeter scales has been proposed and developed. The introduced natural and engineered plasmonic hyperbolic materials bring about new opportunities for future exploration and applications of these unusual systems in nanophotonics for linear and nonlinear light manipulation, fluorescence control, surface enhanced Raman spectroscopy as well as hot-carrier plasmonics and photocatalysis.

In this thesis, I investigate how to construct a self-consistent model of deformed general relativity using canonical methods and metric variables. The specific deformation of general covariance is prediction by some studies into loop quantum cosmology. I firstly find the minimally-deformed model for a scalar-tensor theory, thereby establishing a classical reference point, and investigate the cosmological effects of a non-minimal coupled scalar field. By treating the deformation perturbatively, I derive the deformed gravitational action which includes the nearest order of curvature corrections. Then working more generally, I derive the deformed scalar-tensor constraint to all orders and I find that the momenta and spatial derivatives from gravity and matter must combine in a very specific form. It suggests that the deformation should be equally affected by matter field derivatives as it is by gravitational curvature. Finally, I derive the deformed gravitational action to all orders, and find how intrinsic and extrinsic curvatures differently affect the deformation. The deformation seems to be required to satisfy a non-linear equation usually found in fluid mechanics.

Relativistic effects, such as spin-orbit coupling, are important in heavier elements and trigger some interesting phenomena such as magnetic anisotropy, orbital magnetization, etc. For a careful treatment of these effects, we need to solve the Dirac equation. However, it is not a trivial problem and requires the use of some special techniques due to the complicated nature of the equation. We describe the implementation of this task and its effect on the results of electronic structure calculations, including magnetic susceptibilities and exchange interactions, computed relativistically. The coherent potential approximation (CPA) is a convenient tool for treating alloys. It is applied here to (Ni1-xFex)1-yZy (Z=Cu, Cr, Mn, and Rh) alloys to calculate their magnetization, Curie temperatures, and other properties. Interest in these materials is due to the growing need for weak magnets with low-saturation magnetization and reduced Curie temperatures for some applications in microelectronics.

This thesis outlines the development and implementation of an exact tech-nique for the analysis of a particular class of open quantum systems. Start-ing from a generalised Caldeira-Leggett model, a set of coupled stochastic di˙erential equations are derived as an evolution equation for the reduced density matrix of an arbitrary open system interacting (in a generalised manner) with a bath of harmonic oscillators. These equations are appli-cable even in the case of external driving and strong environment cou-pling. They also permit a more general class of initial states, where the combined system and environment are in full thermal equilibrium. Col-lectively these equations are known as the Extended Stochastic Liouville Equation (ESLE). The ESLE is derived by casting the system+environment density matrix as a path integral in both real and imaginary time. In this form, it is possible to obtain the reduced system density matrix using influence functional techniques. Applying the two-time Hubbard Stratonovich transformation to this path integral, one obtains the ESLE. This consists of two evolution equations, accounting for a propagation in imaginary time followed by real time. Both equations contain stochastic terms which are non-trivially correlated and when averaged over realisations, give the exact reduced density matrix of the system. A first application of the ESLE to a spin-boson model is also discussed. This is used as a proof of principle that the noises required by the ESLE can be generated numerically, and amenable to practical calculation. The impact of the ESLE's generalisations in the description of a two-level sys-tem being driven from equilibrium is also discussed. An equivalent classical analysis is performed using Koopman-von Neu-mann (KvN) mechanics (an operational Hilbert space formalism which puts the quantum and classical descriptions on the same footing). In this setting, the ESLE derivation reproduces the Langevin equation directly from classical mechanics. Finally, the KvN formalism is used to explore some adjacent topics. In particular, a theory of classical self-adjoint ex-tensions as a measure of local entropy conservation is developed.

The routine use of nanomaterials in the clinic is growing ever closer with the continued development of biocompatible particles for a range of biological uses. Non-invasive imaging and therapeutic modalities that utilise light are an area of particular interest for the incorporation of semiconductor nanocrystals, which exhibit unique optical properties. Nanomaterials with near infrared optical properties, whether it be absorption or photoluminescence, are critical for use in biological systems due to the minimal absorption and scattering of light in this region. As well as developing appropriate optical properties, an understanding of the behaviour of nanomaterials in vitro is vital for the eventual use of nanomaterials in medicine. This thesis explores the preparation of such biocompatible nanomaterials, focusing on copper chalcogenides for near infrared fluorescence imaging or photothermal therapy applications. A large portion of the work focused on CuInS2 quantum dots for near infrared fluorescence imaging, looking both at organic- and aqueous-phase syntheses. Materials were optimised for near infrared photoluminescence and colloidal stability in water, which involved investigation of phase transfer processes for the organic-phase particles. Biocompatibility testing of the resulting nanomaterials indicated that encapsulated organic-phase quantum dots were highly cytotoxic, whilst the aqueous synthesis method produced quantum dots suitable for in vitro imaging. The latter were subsequently used for the optical imaging of tumour-bearing mice and demonstrated potential as near infrared fluorophores. Alongside the preparation of these quantum dots, a near infrared imaging system for the laboratory was developed, to aid rapid characterisation of materials. The simple, cheap near infrared camera allowed for the identification of fluorescent nanomaterials immediately after synthesis, for rapid verification of synthesis success. Also, the preparation of quantum dots in bubble wrap is described as a glassware-free alternative to traditional methods. The ability to synthesise nanomaterials in a cheap and accessible way could open up this chemistry to schools and resource-limited environments. Plasmonic copper sulfide nanocrystals were also prepared by a novel, self-capping route using a dithiocarbamate single-source precursor. The resulting nanocrystals were characterised, and attempts made to elucidate the ligands present on the surface of the particles after synthesis. Preliminary biocompatibility assessment in HeLa cells proved promising and the particles were successfully used to heat water via photothermal process.

This thesis is devoted to the study of dynamical symmetry enhancement of black hole horizons in string theory. In particular, we consider supersymmetric horizons in the low energy limit of string theory known as supergravity and we prove the horizon conjecture for a number of supergravity theories. We rst give important examples of symmetry enhancement in D = 4 and the mathematical preliminaries required for the analysis. Type IIA supergravity is the low energy limit of D = 10 IIA string theory, but also the dimensional reduction of D = 11 supergravity which itself the low energy limit of M-theory. We prove that Killing horizons in IIA supergravity with compact spatial sections preserve an even number of supersymmetries. By analyzing the global properties of the Killing spinors, we prove that the near-horizon geometries undergo a supersymmetry enhancement. This follows from a set of generalized Lichnerowicz-type theorems we establish, together with an index theory argument. We also show that the symmetry algebra of horizons with non-trivial uxes includes an sl(2;R) subalgebra. As an intermediate step in the proof, we also demonstrate new Lichnerowicz type theorems for spin bundle connections whose holonomy is contained in a general linear group. We prove the same result for Roman's Massive IIA supergravity. We also consider the near-horizon geometry of supersymmetric extremal black holes in un-gauged and gauged 5-dimensional supergravity, coupled to abelian vector multiplets. We consider important examples in D = 5 such as the BMPV and supersymmetric black ring solution, and investigate the near-horizon geometry to show the enhancement of the symmetry algebra of the Killing vectors. We repeat a similar analysis as above to prove the horizon conjecture. We also investigate the conditions on the geometry of the spatial horizon section S.

Nanoparticles are characterized by their small and finite-size. Indeed they are made of tens to thousands of atoms, corresponding to a size of 1 to tens nanometers. Nanoparticles' finite-size entails three significant implications: internal transitional symmetry breaking occurs, electronic confinement effects are relevant, and nanoparticle's surface to volume ratio is not neglegible. In turn, at the nanoscale, we witness: the occurrence of many peculiar structures, the arising of unique optical, magnetic, and catalytic properties (different from the one of the single atom and of its bulk counterpart) and characterized by peculiar structureproperty relationships, the failure of bulk thermodynamics modeling in the prediction of the nanoparticle structural stability, also for the case of relatively large systems. Indeed size- and shape-effects are known to non-trivially affect the (meta)stability of a nanoparticle against melting and pre-melting. Borrowing the words spelt by Richard Feynman during his famous talk 'There is plenty of Room at the bottom': "When we get to the very, very small world ... we have a lot of new things that would happen that represent completely new opportunities for design". Tailoring the nanoparticles architecture - i.e. size, shape, chemical composition and ordering - at will, one could harness their full potential in technological devices which will trigger a revolution in many fields, ranging from biomedicine to catalysis and optics. The identification of optimal nanoparticle architectures for target purposes is, from here on, mentioned under the name of rational design. The complexity inherent to this practice represents a longstanding high-reward challenge as it hinges on the understanding of how each structural feature contributes towards the nanoparticle global chemophysical property. Moreover, the rational design of nanoparticles for target application should encompass the in depth study of the (meta)stability of the chosen bespoke nanoarchitecture. Indeed, coming back again to Feynman's notorious contribution to nanoscience, he mentioned that "... [he was] not afraid to consider the final question as to whether, ultimately - in the great future - we can arrange the atoms the way we want; the very atoms, all the way down! ... (within reason, of course; you can't put them so that they are chemically unstable, for example)". A degree of atomistic control of the nanoparticle components is currently at reach during both colloidal and physical growth methods. Thus, it is of fundamental importance to assess whether the so synthesized nanoparticles present a satisfactory structural stability, or if their inherent (meta)stability manifests and determines too dramatic changes in the nanoparticles chemophysical properties, and thus in the performance of the device which exploits them. This thesis will contain a discussion of the application of state-of-the-art sampling techniques to probe the complexity of the conformational and energetic landscape of noble and quasi-noble metal nanoparticles of 100-1000 atoms. Conversely, a thorough characterization of the mechanisms driving melting and pre-melting will be sought and rationalized in terms of size, shape, and composition effects. As a final case study, the structural properties of Pt nanoparticles will be carefully assessed to predict their performance for Oxygen reduction reaction, identifying design criteria towards the synthesis of nanoarchitectures with enhanced catalytic properties. Chapter I will introduce the state-of-the-art of metallic nanoparticles characterization, synthesis, theoretical modelling, and application as nanocatalyst. Chapter II will present the numerical techniques employed to study the melting and pre-melting of metallic (group X and XI) nanoparticles and the prediction of their catalytic properties. Chapter III and IV will follow with a thorough investigation of solid-solid transitions in mono- and bimetallic nanoparticles. Structural rearrangements will be discussed with a focus on size and composition effects determining whether they are concerted or diffusion driven. Chapter V will tackle a discussion on how to disentangle univocally kinetic and thermodynamic contributions determining phase changes in metallic nanoparticles as well as to discriminate faithfully solid and liquid structures. Finally, Chapter VI will detail the application of a method based upon a geometrical descriptor to determine design criteria towards the synthesis of Pt nanoparticles highly active towards oxygen reduction reaction.

Much of our knowledge regarding cellular structure and function has derived from our ability to visualise distinct processes through fluorescence microscopy. The unparalleled combination of specificity and compatibility with live samples renders the fluorescent microscope an invaluable imaging modality across the life sciences. Owing to the diffraction of light, the resolution of an optical system is inherently limited. For centuries, a fundamental, immutable resolution was thus imposed on the fluorescence microscope and the study of cellular features existing beyond this limit was unfathomable. Circumvention of the diffraction barrier however was achieved experimentally in 2006, and the advent of super resolution microscopy, for whose discovery was awarded the 2014 Nobel Prize in Chemistry, represented a paradigm shift in our apperception of the resolution issue in microscopy. A repertoire of super resolution microscopy methods have since emerged, allowing unprecedented access to the cellular architecture on the nanoscale. The focus of this Thesis is the implementation of single molecule localisation microscopy (SMLM) for the study of sub-cellular fibrous organisation, which routinely achieves ~ 30 nm spatial resolution. Concurrent with the remarkable technological advances in the field of SMLM is the need for complimentary data analysis methods, to help tune SMLM into a quantitative imaging tool. Despite variations in the working principles employed to achieve SMLM images, all modalities share a common data output: a spatial point pattern (SPP). One must therefore embrace new strategies for the study of SMLM images, compared to conventional fluorescence microscopy methods; a technical challenge. This challenge is exacerbated when studying filamentous structures, whose topologies and cytoarchitectures are typically complex. This Thesis aims to deliver analysis methods for the study of fibrous SPPs generated by SMLM, to compliment the varied tools currently available for clustered SPPs. The presented analysis methods will be validated by use of simulated data and applied to the study of the actin cytoskeleton at the T cell synapse. The actin cytoskeleton is responsible for maintaining a wide range of cellular behaviours, and its malfunction has been implicated in various human diseases. Filamentous actin is subject to extensive remodelling to execute diverse cellular tasks, mediated by a catalogue of accessory proteins. Besides its structural role in maintaining cell morphology, the functional role of actin in regulating cell signals is a current topic of great interest. This Thesis primarily investigates the nanoarchitecture of the actin cytoskeleton at the T cell immunological synapse via SMLM. The effect of the actin crosslinking protein α-actinin on the nanoscale organisation of actin at the mature immunological synapse, and its role in the clustering of the linker for activation of T cells (LAT), a prominent signalling molecule involved in T cell activation, will be investigated.

In the centre of every major galaxy is a supermassive black hole. Some of these power an active galactic nucleus (AGN) in which the black hole is growing by accreting the luminous disc of material around it. As well as consuming matter, AGN can eject it in the form of powerful jets travelling at relativistic velocities. I present detailed studies of two narrow-line Seyfert 1 galaxies which exhibit powerful jets and high-energy gamma-ray emission. I explore the relationship between the disc and jet in these sources and suggest that their jets are relatively underpowered. Although AGN are too small and distant to be spatially resolved, temporal studies can reveal information about the processes occurring within the central engine. I conduct a temporal spectroscopic study of a hypervariable AGN which dimmed and rebrightened by a factor of three over four years. I demonstrate that the event is due to an intrinsic change within the accretion disc, and is not due to obscuration by an external body. Such dramatic variability poses a challenge to our current models of accretion discs. I draw attention to some new models which confront this problem. Finally, I place my findings in the context of the current literature and discuss some of its limitations and open questions. I highlight how future, planned observatories will help us to address these issues and deepen our understanding of AGN.

A fundamental question in astronomy is how galaxies form and evolve. How does gas flow into and out of galaxies? What physical processes drive the evolution of the star formation rate history? What is the role of dusty star formation at high redshift? To answer these questions we must understand the complex interplay between galaxies and the surrounding circum-galactic medium and we have to study the evolution of molecular gas and dust in galaxies. We present the ALMACAL survey utilizing ALMA calibration observations for science. We use this unique dataset to study the evolution of molecular gas and dust in galaxies with cosmic time. Using this survey, we select a sample of CO emission line detections in gas-rich galaxies first identified as intervening absorbers. From this parent sample we select the three galaxies detected in multiple CO emission lines for a further analysis and follow up observations. Ultimately we are aiming for a better understanding of the population of gas-rich galaxies. As a pilot study, we use VLT/MUSE to follow up one absorption-selected system at z ~ 0.5 detected in multiple CO transitions. We find in total four galaxies at the absorber redshift, one of which was detected in CO. This provides further evidence that the connection between absorber and host galaxy is more complex than a simple one-to-one relation. We find that most probably the absorbing gas is tracing intra-group medium. Next we focus on the multiple CO transitions and study for the first time the CO spectral line energy distribution of absorption-selected galaxies. We find evidence for more excited ISM conditions compared to the Milky Way. This indicates that previous studies of absorption-selected systems might overestimate the molecular gas mass in some galaxies. Furthermore, we suggest that absorption-selected galaxies may preferentially trace group environments. In addition to the local baryon cycle in single objects, we study the global baryon cycle over cosmic time. To understand the processes that drive the evolution of the star formation rate history, we trace the evolution of the molecular gas mass density over cosmic time using intervening molecular absorption. In the currently largest available dataset of quasar spectra in the submillimetre regime, ALMACAL, we do not detect intervening CO absorption. We place constraints on the evolution of the molecular gas mass density. This suggests, combined with complementary measurements from the literature, a strong evolution following that of the star formation rate history. Finally, half of the star formation activity in the Universe is expected to take place in dusty star-forming galaxies. We use our ALMACAL dataset to search for dusty star-forming galaxies observed at 680μm. We determine the first high-frequency number counts at 680 μm free of source blending and cosmic variance effects. At this wavelength we find that we resolve the majority of the extragalactic background light.