Search

Extensions to the canonical LCDM model which incorporate elastic scattering between dark matter and Standard Model radiation predict the suppression of matter perturbations on small scales and modifications to the cosmic microwave background anisotropies. Studying these scenarios not only allows us to constrain the particle physics properties of dark matter, it also reveals if they can alleviate remaining tensions in cosmological data sets. In this context, the present thesis considers several aspects of dark matter-photon and dark matter-neutrino interactions. One central aspect of our work is the accurate description of additional scattering terms in the numerical solutions. In the context of dark matter-photon interactions, we demonstrate the robustness of earlier studies with respect to inconsistencies in the tight coupling approximation and to the negligence of the dark matter sound speed. Accounting for the latter, however, potentially tightens limits from large-scale structure observations for light dark matter candidates. Our updated constraints, derived from the Planck 2015 data release, are about 20% tighter than previous CMB limits in the most conservative case. We further extend dark matter-photon interactions to a mixed dark matter scenario in which two components, one collisional and one collisionless, contribute to the relic abundance. In particular a small fraction of interacting dark matter impacts the matter power spectrum in a fashion very similar to massive neutrinos. In this case, CMB data only imposes weak constraints on the interaction strength, and our Fisher forecast for the DESI survey predicts notably larger error margins on the neutrino mass. Dark matter-neutrino interactions alter the initial conditions and cause inconsistencies in the ultra-relativistic fluid approximation, which impact the matter power spectrum on small scales. Still, we can reinforce the validity of previous studies that neglected those effects. In addition to the canonical collisional damping, dark matter-neutrino interactions imply the suppression of structure by mixed damping. Indeed, our exploration of the parameter space reveals the relevance of mixed damping for observational constraints. To provide insight into the underlying physical processes, we derive an analytical approximation to the evolution of dark matter perturbations in the mixed damping regime, which accomplishes to capture all qualitative features of our full numerical results.

The standard model of cosmology has been enormously successful both at reproducing many observed properties of the Universe, and at predicting others. Despite this success one of its key components, a dark matter particle, has not been observed in targeted searches or indirect detection experiments. In addition, a number of discrepancies have arisen between observational proxies of DM structure and the small-scale predictions of this leading cosmological model, challenging its status as the standard paradigm. In this thesis we focus on two distinct but related lines of enquiry. In the first, we address the gap in observational capability concerning the satellite galaxies of the Milky Way. These objects are sensitive probes of the underlying distribution of dark matter, which is determined by the properties of the dark matter particle itself. Using partial observations of the Galactic satellite population, we introduce and use a Bayesian approach to infer the total luminosity function of these objects. We predict that there are $124^{+40}_{-27}$ (68 per cent confidence level) satellite galaxies brighter than ${\rm M}_V=0$ within $300\, {\rm kpc}$ of the Milky Way, and that half of this population should, in principle, be detectable by the forthcoming Large Synoptic Survey Telescope. In the second strand we use these estimates to test the predictions of alternative models to the standard paradigm and place robust lower limits on their allowed properties. We focus on two models of warm dark matter: thermal relics, and sterile neutrinos in the Neutrino Minimal Standard Model ($\nu$MSM). For the former we obtain a robust lower limit on the mass of the dark matter particle, ruling out with 95 per cent confidence models with particle mass, $m_{\rm th}{\leq}1.95\, {\rm keV}$, which is competitive with existing constraints. In the latter case the model depends on the size of the primordial lepton asymmetry, which we parametrize as $L_6$. Assuming a particle mass of $m_{\rm s}=7\, {\rm keV}$-motivated by observations of an unexplained $3.55\, {\rm keV}$ line in X-ray spectra of galaxy clusters-we exclude values of $L_6\geq50$, in agreement with other work.

Dark matter (DM) and neutrinos provide the two most compelling pieces of evidence for new physics beyond the Standard Model (SM) but they are often treated as two different sectors. A tantalising avenue of investigation is the possibility that a stronger connection between these two particles exists. In this thesis, we explore the phenomenological implications of a neutrino-DM coupling and show that the complementarity between cosmological observables and indirect detection searches can be used to exclude large regions of the parameter space for different DM models. After conducting a complete study of all the possible renormalizable scenarios with such a coupling, we discuss two gauge-invariant realisations of models where the DM phenomenology is dominated by its interactions with neutrinos. While in these models, neutrinos set the strongest constraints, they can also be an obstacle in our quest to understand DM. Indeed, they will soon become a source of an important background for direct detection experiments. Here, we also compute the changes in this background in the presence of new physics within the neutrino sector. We find that it can increase significantly for light DM masses. This means that future discovery claims by direct detection experiments must be carefully examined if a signal is found well above the expected SM neutrino background.

High optical nonlinearities can be achieved at the single photon level by coupling the photon states to strongly interacting Rydberg excitations under the conditions of electromagnetically induced transparency. The nonlinear response in Rydberg quantum optics comes as a direct result of the long range dipole-dipole interactions via Rydberg blockade and interaction induced dephasing. In this thesis, Rydberg quantum optics experiments are performed with two spatially separated mediums with a nonlinear response at the single photon level. Two ultracold rubidium atomic clouds are tightly confined by a pair of in-vacuo aspheric lenses such that only a few Rydberg excitations can exist in each one of the clouds simultaneously. The long range character of the dipole-dipole interactions leads to the generation of quantum states of light. The effective photon-photon interactions are directly observed as an anti-correlation in the simultaneous photon retrieval as a result of the non resonant van der Waals interactions. The collective Rydberg excitations, stored in the non-overlapping mediums, experience an additional spatial non-uniform phase. In addition, we experimentally characterize the cooperative optical response of a cold atomic medium at a single photon level. This aims to exploit the enhancement of the spontaneous emissions decay rate (superradiance) as a dependence of the number of atoms involved in the ensemble. The observed atomic response in two different regimes gives a further understanding of the system dynamics and unlocks a promising potential route towards the implementation in scalable, multichannel quantum optical devices.

The standard ΛCDM is currently the most accepted theory of the Universe. The model is characterised by a nearly-scale invariant primordial curvature power spectrum from inflation, cold and non-interacting dark matter particles and a cosmological constant Λ. Despite the success of ΛCDM, the nature of the dark sectors is still unknown. Moreover, the model faces some small-scale observational controversies that are still not resolved. In this thesis, we investigate structure formation in alternative scenarios to ΛCDM using high-resolution cosmological simulations. In the first part, we focus on non-cold dark matter (e.g. warm dark matter) and non-standard inflation (e.g. thermal inflation) models, that display damped matter density fluctuations on small scales. We investigate first the effects of adding thermal velocities to the gravitationally-induced velocities in simulations of warm dark matter. Considering different non-standard linear power spectra, we then assess which features survive the non-linear evolution and leave interesting signatures in non-linear power spectra and halo statistics. Exploiting these results, we present a new smooth-k space filter, to use in the Press-Schechter approach, to model the dark matter halo mass function, which overcomes shortcomings of other filters used in the literature. In the second part, we quantify the effects of modified gravity on neutral hydrogen abundances and 21-cm power spectra, finding that the HI clustering is a powerful test of gravity at redshifts z~3.

Suspensions are systems composed of particles dispersed in a fluid. This is an industrially important set of materials, whose members are capable of exhibiting a diverse range of phenomena. The behaviour of dense suspensions, where the volume fraction of particles is close to the volume fraction of jamming at which the suspension is unable to flow in the limit of vanishing imposed stress, is particularly challenging to model and explain. In this thesis, we report theoretical research on three dense athermal suspensions, supported in each case by particle simulations. By studying systems in which particles do not undergo Brownian motion, we are able to identify behaviour generic to both thermal and athermal suspensions, and provide insight into the underlying cause. The particle simulations are found to be of great importance in challenging the assumptions of models, testing model predictions and providing direct microstructural insight into the mechanisms by which dense suspensions evolve. We first extend a model of discontinuous shear thickening in steady-state homogeneous dense suspensions under simple shear to a dynamical one-dimensional model of the suspension, spatially resolved along the vorticity axis with periodic boundary conditions. This model encapsulates a theory of shear thickening in which a suspension of frictional particles transitions from frictionless to frictional rheology as repulsive interparticle forces that prevent contact between particles at low stress are overcome at high stress. We show that in our model, large homogeneous systems are linearly unstable to perturbations along this axis at high volume fractions within a range of imposed stresses. We characterise two long-time inhomogeneous states, both of which are unsteady but periodic. We then test our predictions with particle simulations of a suspension of frictional particles with short-ranged repulsions, finding both states at least transiently, according to parameter regime. The second suspension we consider corresponds to the first in the limit of vanishing interparticle repulsion, after a reversal of the direction of shear from steady state. This system is strongly dimensionally and symmetrically constrained: shear-rate provides the only timescale, while the system is invariant under inversion of the vorticity axis (or, equivalently, simultaneous inversion of the flow and flow gradient axes). We leverage these constraints to develop a systematic approach to modelling the evolution of the ``fabric tensor'', a traceless and symmetric rank-2 tensor related to the second-order spherical harmonic expansion of the distribution of (near-)contact pairs of particles commonly used in the literature to encode the suspension microstructure, as a function of itself and the imposed velocity gradient tensor. By fitting the models to data from particle simulations of the appropriate system, we show that such models are unsuccessful at linear order in the fabric tensor components, and are unlikely to contain any physical insight at higher orders. We then test the suitability of the fabric tensor as a description of the microstructure directly using the particle simulations, and conclude that a second-order spherical harmonic description captures the pair distribution poorly shortly after reversal. We find that a fourth-order description captures the pair distribution well. Finally, we study a suspension of soft elastic particles close to and above jamming using particle simulations with periodic boundary conditions. We prepare the suspension at a non-zero temperature and then allow it to relax athermally to the global system's local minimum. We observe non-trivial dynamics, such as slow power-law decay of the root mean squared velocity of particles, and coarsening with a power-law growth of velocity correlation lengths. This suggests that generic athermal physics may in fact underlie non-trivial dynamics commonly associated to thermal effects.

This thesis provides some theoretical and phenomenological work on two of the outstanding problems in modern physics, namely, the extreme smallness of neutrino masses and the relative abundance of matter over antimatter. A theorem is developed relating the smallness of the light neutrino masses to the degree of lepton number violation in some seesaw extensions of the Standard Model of particle physics. It is shown that for exactly massless light neutrinos there must be an exact lepton number symmetry. Then the viability of thermal leptogenesis as a resolution to the baryon asymmetry problem at different scales is assessed using more sophisticated numerical tools than have previously been applied. It is shown that, if fine-tuned solutions are allowed, the scale may be lowered to ~10⁶ GeV. Using these results, it is shown that, if CP violation comes purely from the phases of the PMNS matrix, thermal leptogenesis may still be viable over a range of scales covering ~10⁶ - 10¹³ GeV. It is also shown that thermal leptogenesis is viable in the Neutrino Option, in which the Higgs potential has its dimensionful parameter provided by loop corrections from the heavy Majorana neutrinos in the type I seesaw at a mass scale ~10⁶ GeV.

Diabetic ketoacidosis (DKA) is a life threatening complication in children with type 1 diabetes. In DKA, the body breaks down fatty acids as an alternative energy source producing high concentrations of acidic blood ketones. Normally, DKA is treated by a ``guess'' dosage of intravenous insulin infusion following blood analysis. There is an unmet need for alternative, non-invasive methods to the inaccurate, untimely blood tests to monitor each patient's response to the treatment in real time. This will help in determining the optimum insulin dosage and adjusting the treatment protocol. Breath-acetone measurement is a promising non-invasive alternative as it is proportional to blood ketone concentrations. This thesis describes building a device based on the cavity-enhanced laser-induced fluorescence (CELIF) technique for real-time, online, non-invasive breath acetone measurements. CELIF combines the sensitivity of laser-induced fluorescence (LIF) and the absolute measurement capabilities of cavity ring-down spectroscopy (CRDS) into one cross-correlated technique. The device is capable of making an acetone CELIF measurement in 100 ms, with a concentration dynamic range of 1.6--2000 ppm, covering the range of breath acetone concentration a DKA patient might have. The response time of the device is fast enough to follow a real breath pattern, with a $10 \text{--}90\%$ rise time of the CELIF signal of 370 $\pm$ 15~ms, and a 90--10\% fall time of 850 $\pm$ 21~ms, which is enough for the signal to rise and find the maximum acetone concentration, then decay back to the background level before the next breath arrives. The performance of the acetone CELIF device was validated by using a selected-ion flow-tube mass spectrometer (SIFT-MS). The results show that this device is useful for reliable online breath acetone analysis. Subsequently, the validated CELIF device was tested for breath acetone measurements in fasted healthy human subjects, using a home-built, online, buffered end-tidal breath sampler.

In this thesis we consider the Next-to-Next-to-Leading Order (NNLO) corrections to single charged electroweak boson production with associated QCD radiation in hadron-hadron collisions, calculated using the antenna subtraction method to regulate infrared (IR) divergences. Results are presented alongside the neutral current case for the inclusive transverse momentum spectrum and subsequent ratios both with and without the addition of state-of-the-art resummation results. In the former case a comparison to CMS data is also provided. We also discuss the phenomenological implications of the results when one or more jets are reconstructed from the QCD radiation. Particular attention is given to the impact on valence quark content of Parton Distribution Functions (PDFs) through a comparison with experimental results from the ATLAS, CMS and LHCb collaborations. We then discuss the use of fixed-order QCD predictions for inclusive Drell-Yan production in the context of an effective Weinberg angle extraction using triple-differential data taken by the ATLAS collaboration at √s=8 TeV, using the kinematics to extend the predictions to Next-to-Next-to-Next-to-Leading Order (N³LO) for certain parts of the measurement. Finally, using the antenna subtraction method we derive the NNLO QCD corrections to di-jet production in charged-current Deep Inelastic Scattering (DIS), allowing the first comparison to ZEUS data at this order. These results are then combined with inclusive structure functions using the method of projection-to-Born (p2B) in order to derive the first exclusive fiducial predictions for single jet inclusive production in charged-current DIS to N³LO. A comparison to data is performed, where we observe reasonable agreement with the experimental results from ZEUS.

The Standard Model of particle physics has been remarkably successful at explaining the behaviour of nature at very high energies. It has been thoroughly tested by experiments at the Large Hadron Collider and almost all of its predictions have agreed closely with observations. Despite this, there are many phenomena that it cannot explain, such as the origins of neutrino masses. Therefore, the Standard Model alone cannot provide a complete explanation of reality, and so there must exist physics beyond it. However, finding it is particularly difficult due to the aforementioned success of the Standard Model. In this thesis, we continue the search for this new physics by carrying out five separate studies that could contribute towards the overall goal. We calculate new limits on specific extensions of the Standard Model by using lepton-flavour-violating decays of τ leptons. We also use electroweak precision measurements for the first time to constrain the trilinear self-coupling of the Higgs boson, and perform the most precise calculation to date of the production of a heavy neutrino via gluon fusion. Considering machine-learning techniques, we improve the robustness of an autoencoder used for unsupervised searches for new physics, and we develop a new approach to using neural networks for solving differential equations, which we apply to the calculation of cosmological phase transitions. A general theme across all of these investigations is the importance of a high precision, both in terms of the theoretical calculations, and the experiments through which they are tested.

A proposed solution to the increased computational demands of Extremely Large Telescope (ELT) scale adaptive optics (AO) real-time control (RTC) using many-core CPU technologies is presented. Due to the nearly 4x increase in primary aperture diameter the next generation of 30-40m class ELTs will require much greater computational power than the current 10m class of telescopes. The computational demands of AO RTC scale to the fourth power of telescope diameter to maintain the spatial sampling required for adequate atmospheric correction. The Intel Xeon Phi is a standard socketed CPU processor which combines many (< 64) low power cores with fast (>450GB/s) on-chip high bandwidth memory, properties which are perfectly suited to the highly parallelisable and memory bandwidth intensive workloads of ELT-scale AO RTC. Performance of CPU-based RTC software is analysed and compared for the single conjugate, multi conjugate and laser tomographic types of AO operating on the Xeon Phi and other many-core CPU solutions. This report concludes with an investigation into the potential performance of the CPU-based AO RTC software for the proposed instruments of the next generation Extremely Large Telescope (ELT) and the Thirty Meter Telescope (TMT) and also for some high order AO systems at current observatories.

The organisation and kinetics of charges at solid and soft interfaces play a central role in biological signalling processes and are vital for energy storage technologies as well as our understanding of heterogeneous catalysis. At the molecular-scale, such interfacial behaviour remains stubbornly difficult to characterise, due to the short-ranged interactions between ions, their aqueous solvent and surface groups. Thus, continuum-scale models quickly break down, especially close to the interface and with high charge densities. This thesis addresses the question of ionic organisation using atomic force microscopy (AFM), which uniquely combines sub-nanometre spatial resolution and the ability to probe relatively long timescales. The use of small oscillation amplitudes allows the topography of the ionic layer to be mapped while simultaneously extracting physical properties from the sample or the interface itself, with time resolution spanning from tens of milliseconds to minutes. The structure of ions at hydrophilic interfaces is shown to be delicately sensitive to the charges' molecular structure (in the case of larger buffering agents) and their charge density (for simple alkali cations). Specifically, the cations' interactions with a model lipid membrane and the waters around it lead to an attractive correlation energy which generates nanoscale networks that evolve over the course of many seconds. These ionic structures directly reduce the effective stiffness of the lipids, providing a mechanism for the spontaneous control of membranes' mechanical properties. These ionic networks are significant in the case of confined fluids and provide an efficient means of lubrication even under high pressures in sub-nanometre gaps. When sheared, such fluid films are revealed to be non-Newtonian, with dynamics that depend on the velocity and lengthscale of the motion. The results highlight the greatly damped kinetics of ions and water molecules at interfaces, and shed light on the mechanisms behind their transport through and along biomolecules.

The effective management of radiological emergencies where members of the public not carrying conventional dosimeter have been exposed to doses of ionising radiation requires individual dose estimates to support medical triage. Biological and physical methods have been developed to address this issue. New materials and techniques have been sought to reinforce preparedness for such emergencies. Alternative materials, such as clothing, shoes, paper, plastic items, nail polish or banknotes were investigated using thermoluminescence (TL) and optically stimulated luminescence (OSL). Most of the materials and fabric tested exhibited either no detectable response to dose using luminescence technique, or a weak response yielding detection limits above 2 Gy, with the exceptions of a blue polyester fabric responding to infra-red stimulated luminescence (IRSL) and some types of polymer-based fabric that were found to have luminescence favourable characteristics for short - term dosimetry and particularly those containing mineral fillers. The most promising were fabrics containing calcium carbonate fillers, where the TL response to β radiations was measured with a detection limit as low as 4 mGy, and a relatively low native signal in the region of interest (≤ 200 °C). The fading was found to be slower for samples stored at -15 °C compared with samples stored at ambient temperature. A blind test was carried out and confirmed the potential of bags containing calcium carbonate fillers to provide reliable dose estimate for radiological triage. Furthermore, the TL signal of calcium carbonate fillers contained in the fabric of bags offers several advantages for accident dosimetry compared with other methods, such as a rapid dose assessment, the low cost value of the material and availability, and the possibility to map radiological doses is the fabric covers sufficient surface.

Super massive black holes (SMBH) are known to reside at the centre of massive galaxies, and are visible during their growth phases as active galactic nuclei (AGN). Current theoretical models of galaxy evolution require AGN feedback processes to reproduce many of the fundamental properties of galaxies and the intergalactic medium. In an effort to constrain the effect of AGN feedback on star formation in AGN host galaxies, this thesis uses observations to test predictions from the cosmological simulations. I present ALMA and integral field unit (IFU) observations of AGN host galaxies to trace obscured and unobscured star formation as well as ionised gas kinematics. Using deep ALMA continuum observations and multi-wavelength photometry I estimate specific star formation rate distributions of 81 X-ray AGN at z=1.5--3.2 with AGN luminosities of $10^{43}$--$10^{45}$ ergs s$^{-1}$. Comparison of the observations with predictions from the EAGLE cosmological simulations shows that AGN feedback is responsible for broadening the sSFR distribution of both active and inactive galaxies by suppressing their star formation. In the second scientific experiment, I present IFU and ALMA observations of eight X-ray AGN at z=1.4--2.6 with AGN luminosities of $10^{42}$--$10^{45}$ ergs s$^{-1}$ to investigate the connection between AGN driven ionised outflows and star formation. Using these observations, I conclude that star formation in AGN host galaxies is not instantly suppressed by AGN driven outflows, consistent with the global conclusions from my earlier study. I reach this conclusion whenever I use obscured or unobscured star formation tracer. Furthermore, I conclude that it is necessary to use H$\alpha$ emission with caution when using it to trace star formation in AGN host galaxies. In the last scientific experiment, I investigate star formation in three quasars at z$\sim$2.5 that were previously presented in the literature as having evidence for suppressed star-formation at the location of ionised outflows. Using new ALMA band 7 continuum observations and re-analysing the existing archival H$\alpha$ observations, I do not observe any suppression of star formation in these quasars. Based on the evidence from all of my studies, I conclude that AGN feedback does not instantly suppress star formation on a global scale, but rather the feedback may have an impact seen on smaller spatial scales (< 4 kpc), or on longer timescales than a single AGN episode.

The interpolation from supersymmetric to non-supersymmetric heterotic theories is studied, via the Scherk-Schwarz compactification of supersymmetric $6D$ theories to $4D$. A general modular-invariant Scherk-Schwarz deformation is deduced from the properties of the $6D$ theories at the endpoints, which significantly extends previously known examples. This wider class of non-supersymmetric $4D$ theories opens up new possibilities for model building. The full one-loop cosmological constant of such theories is studied as a function of compactification radius for a number of cases, and the following interpolating configurations are found: two supersymmetric $6D$ theories related by a $T$-duality transformation, with intermediate $4D$ maximum or minimum at the string scale; a non-supersymmetric $6D$ theory interpolating to a supersymmetric $6D$ theory, with the $4D$ theory possibly having an AdS minimum; a ``metastable'' non-supersymmetric $6D$ theory interpolating via a $4D$ theory to a supersymmetric $6D$ theory.

For the next generation of extremely large telescopes, Focal Anisoplanatism (FA) renders single Laser Guide Star AO useless. Here we analyze a novel LGS alternative configuration with corresponding wavefront sensing and reconstruction method, termed Projected Pupil Plane Pattern, to solve the problem of Focal Anisoplanatism. With PPPP, turbulence is sensed during uplink by a laser beam projected as a collimated beam from the whole telescope primary mirror. Phase changes due to the turbulence introduce intensity variations that then increase in amplitude with propagation distance. By observing the distribution of intensity at two distant planes, the Transport-of- Intensity equation can be used to determine the phase aberration encountered during the uplink path. A simple imaging camera can then be used to measure the wavefront by imaging the backscattered light patterns. We have successfully demonstrated PPPP works both by simulation and laboratory experiment, where we find that PPPP can achieve equivalent performance to a SH WFS associated with a NGS. However it is shown that the main problem of PPPP is its low Signal-to-Noise Ratio if a 20W laser is used. To reduce the requirement for high laser power, an alternative reconstructor based upon nonlinear Artificial Neural Networks has been developed, and provides a wavefront with measurement error around 160nm RMS with a single 200W laser on a 4-m diameter telescope. PPPP is therefore ready for a practical on sky test, which we are currently undertaking at Electro Optical Systems (EOS) Debris Laser Ranging (DLR) system, Australia.

Metal-free organic molecules exhibiting thermally activated delayed fluorescence (TADF) and/or room temperature phosphorescence (RTP) have attracted great attention due to their promising potential for applications in emerging optoelectronic technologies, including sensors, imaging, anti-counterfeiting and organic light-emitting diodes (OLEDs). TADF is a mechanism that can upconvert dark triplet states to emissive singlet excited states via reverse intersystem crossing (RISC). RTP emitters instead show a substantial high triplet formation yield along with a relatively fast triplet radiative decay rate and suppressed non-radiative deactivation pathways. Both TADF and RTP are promising and efficient approaches to harvesting the non-emissive triplet states, which can overcome the limitation of 25% internal quantum efficiency imposed by spin-statistics in OLEDs. Although the development of metal-free organic TADF and RTP emitters has greatly progressed in recent years, various challenges still exist concerning the full understanding of the mechanisms. The photophysical properties of metal-free organic donor-acceptor-donor (D-A-D or D-A) molecules were investigated in this thesis. The aim is to investigate the roles of isomerization and molecular conformation, and the effects of energy alignment of relevant electronic excited states on the TADF and RTP mechanisms. The work starts with the study of a series of D-A-D molecules and their structural isomers. Dual emission with contributions from both TADF and RTP is observed in some of these isomers, while others are purely fluorescent. This work highlights the similarity of these two mechanisms and the influence of D-A linkage on the intersystem crossing rates. This is followed by an investigation of the influence of molecular conformation on the TADF and RTP mechanisms, using the steric effect induced by the presence of alkyl substituents to manipulate the conformation of D-A-D emitters. The exciting topic of conformational control of RTP and TADF mechanisms in molecular crystals is then explored. Subsequent chapters address the effects of the substituents with different electron-donating, and electron-withdrawing strength on TADF emitters as a facile way to tune the electronic interactions and the energy of TADF luminescence. A clear understanding of how these effects influence the mechanisms will give clear guidelines for the design of novel and more efficient TADF and RTP emitters.

Flexible electronic is a field of technology for assembling electronic circuits and devices on flexible substrates; which are characterized by their low cost, conformable and light weight properties compared to conventional silicon-based electronic devices. The addition of ferromagnetic materials to flexible substrates will allow the introduction of additional functionalities including memory and sensors. Here, I present a detailed study of the development of perpendicular magnetic anisotropy (PMA) in Pt/Co/Pt multi-layered systems and demonstrate the conditions for controlling the anisotropy. Two typical kinds of substrates have been introduced for preparing trilayer thin-film structure by deposition of magnetic films (Pt/Co/Pt) on them. Magnetron sputtering was used to deposit a series of Pt/Co/Pt trilayers where magnetic anisotropy was studied using Atomic Force Microscope, x-ray diffraction, x-ray reflectivity, Magneto-Optical Kerr effect magnetometry and Hall-effect measurements. The effect of Co thickness on the anisotropy, coercivity and switching behaviour were examined and compared between Hall-effect measurements and silicon substrates. The substrate was found to have a significant effect. A clear PMA was observed for Co thicknesses that range from 0.3 to 0.7 nm. Additional increments in the Co thickness makes the magnetization return to the in-plane direction, with a rapid drop in both the coercivity and the remanence. Further improvements in the PMA were obtained through the thickness of the Pt buffer layer, with higher coercive fields for both flexible plastic and silicon dioxide substrates. However, the thickness of the Pt capping layer did not have any systematic effect on the anisotropy. The results of a study of the interplay between microstructure and the magnetic properties of ultrathin Ru/Co/Ru and Pt/Co/Pt trilayer thin films with PMA is also presented. The maximum values of PMA are observed for the Co thickness by using Pt in comparison with Ru. The effective magnetic anisotropy and coercive field are very sensitive to Ru buffer layer thickness. The values of coercive field increase approximately from 10 of using Ru in comparison with about 1000 Oe for the Pt case. This is associated to the larger grains growth of Pt while Ru has a smaller grains growth. The surface roughness have an impact on the Hall resistivity on the ferromagnetic/non-magnetic thin films. Observation of the magnetic domain structure by means of polar kerr microscopy reveals that out-of-plane magnetization reversal occurs through then nucleation of bubbles on silicon dioxide.

In this thesis we investigate the diffraction of cold atomic gases by optical lattices. Of particular interest to atom interferometry is the implementation of a high momentum-transfer ``beam-splitter,'' which may be achieved by inducing quantum resonance in such an atomic gas. We use Monte Carlo simulations to investigate these quantum resonances in the regime where the gas receives laser pulses of finite duration, and demonstrate that an ε-pseudoclassical model for the dynamics of the gas atoms reproduces quantum resonant behavior for both zero-temperature and finite-temperature non-interacting gases. We show that this model agrees well with the fully quantum treatment of the system over a timescale set by the choice of experimental parameters. In similar setups, the depth of a laser lattice may be measured by exposing an atomic gas to a series of off-resonant laser-standing-wave pulses, and fitting theoretical predictions for the population found in each of the allowed momentum states. We present an analytic model for the time evolution of the atomic populations of the lowest momentum-states, which is sufficient for a weak lattice, as well as numerical simulations incorporating higher momentum states for both relatively strong and weak lattices at zero and finite temperature. We propose a new approach to characterizing the depths of optical lattices, in which an atomic gas is given a finite initial momentum, leading to high amplitude oscillations in the zeroth diffraction order which are robust to finite-temperature effects. We present a zero-temperature analytic formula describing such oscillations, extend it to include atoms with initial momenta detuned from our chosen initial value, and analyze the full finite-temperature response.

Cu2ZnSnS4 (CZTS) quaternary semiconductor compound has potential properties for low cost thin film solar cells. It is composed of abundant elements and non-toxic material, with desirable properties for thin film photovoltaic (PV) applications such as high absorption coefficient close to 10-4 cm-1 and a band gap close to 1.5 eV. CZTS has been successfully fabricated by non-vacuum hot injection methods with a pure sulphur source. High concentrations of CZTS nanoparticle inks were deposited onto clean glass by spin coating techniques to study the CZTS nanoparticle ink quality. This thesis reports the study of the influence of fabrication conditions for CZTS including temperature and time on structure and optical properties. This reveals that these temperature conditions 185, 205, 225, 245 and 265 oC with reaction times of 0.5, 1.0, 1.5 and 2.0 hours have strong effects on CZTS properties. The nanoparticles were deposited onto glass by the spin coating method. We report measurements made by UV-VIS spectrophotometer, Scanning electron microscopy, TEM, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results confirm energy band gaps decreases with increasing reaction temperatures. For increasing reaction times from 0.5 to 2.0 h the energy band gap increases with increasing reaction time. X-ray diffraction and Raman spectroscopy were used to study the structure of CZTS nanoparticles and show the crystal structure improves with increasing reaction temperature and times. It is also observed from SEM and TEM measurements that the size of particles increases with increasing reaction conditions. CZTS nanoparticles of differing composition (stoichiometric, Cu-poor/rich and Zn-poor/rich) are prepared to investigate the impact of composition on the properties of the thin films. Films are characterised using X-ray diffraction, electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. Cu-rich sample has Cu2S secondary phases identified by a Raman peak at 475 cm-1. The result confirmed that the band gaps depend on the copper and zinc content as well as the particle size. The implications for PV devices are discussed. The results confirm that the optimum conditions for synthesising high quality nanoparticles are 225 oC for 1.0 h. this results in nanoparticles of dimension 35 nm with band gap of 1.5 eV, which are Cu-poor and Zn-rich. The effect of thin film annealing parameters including temperature, time, ramping rate and atmosphere on the structure and optical properties are studied. XRD, Raman Spectroscopy, SEM and EDX are used to analyse films and demonstrate that the crystallinity of CZTS and homogeneity of elements improves with annealing conditions. This is an important factor for CZTS thin-film solar cells. The crystallinity, structure and chemical composition of CZTS thin film increased and improved under H2S+N2 atmosphere. It is concluded that annealing at 500 oC for 1 h with 10 oC/min under H2S (20%) +N2 (80%) atmosphere is a suitable condition for CZTS thin films used in solar cell devices. CZTS solar cell devices which consist of substrate, back contact layer, followed by the main layer in this project; absorber layer, buffer layer, window layer, transparent oxide layer and grid layer were deposited as Mo foil/CZTS/ CdS/ i-ZnO/ ITO/ Al. Different thicknesses of CZTS layers were to study optical and electrical properties using Photoluminescence (PL) and I-V measurements. For PL measurements, the main peaks may correspond to CZTS in the range 1.35-1.55 eV. This study focused on the first four peaks, P1, P2, P3, and P4 at energies 1.38, 1.45, 1.55 and 1.63 eV for all samples. The most intense PL peak found at 1.55 eV is thought to be due to recombination of CZTS. The results confirm that all peaks have k-values less than 1. The values of k are in the range 0.75-0.92 ±0.02 which are related with radiative transition involving defect states. The PL peak energy has a blue shift to higher energy with increasing excitation power. However, a small peak shifts in the range between 1.0 and 4.0 meV/decade is seen in all samples. PL results confirm radiative transitions from defect states with electronic levels which are influenced by fluctuating potentials. This limits solar device performance. SEM images of devices show some unexpected features at the interface between CZTS and other layers: CdS and ZnO. This is shown to impact I-V characteristics. Damage in the device layers leads to loss of current collection and more investigations identified the source of the damage. Cross section images show uniform CZTS layers in the majority of devices. EBSD cross sections are used to identify the quality of crystal and phase orientation in the layer.

Small scale tests of the nature of dark matter require simulations which incorporate baryonic physics. In this thesis we study how the inclusion of baryonic physics affects the abundance and properties of dark matter halos and their substructures. We introduce a new high-resolution hydrodynamical zoom simulation of a 10^13 Msun galaxy group, which we use to study the properties of halos and subhalos relevant to strong lensing tests of the cold dark matter model. We also compare two hydrodynamical simulations of Milky Way-mass halos, Apostle and Auriga. We find that the number of subhalos, as well as the slope of the subhalo mass function and the subhalo velocity distribution, is altered significantly depending on the implementation of baryonic physics.

Numerous experimental observations place Dark Matter (DM) as a central character in our cosmological history. Many extensions to the Standard Model of particles physics provide candidates for DM, often predicting interactions additional to gravity. This gives us the opportunity to experimentally probe these extensions and determine the nature of DM. In this thesis, we explore how direct DM detection could be used most effectively to achieve this goal. With this in mind, we have developed a tool for performing multidimensional parameter scans. This tool allows us to evaluate the capabilities of current and future detectors for detecting and understanding DM interactions. We show that by extending the energy region analysed, detection sensitivities and parameter reconstruction can be improved substantially. These insights play an important role in more global analyses, where hints of DM could come from other experiments, but verification depends on direct detection.

In the last twenty years sub-millimetre surveys have uncovered numerous sub-millimetre bright galaxies (SMGs), sites of some of the most intense star-formation in history with rates peaking over a thousand solar masses per year. For the majority of the twenty years since their discovery, the identification and the constraints on the properties of individual SMGs has been limited by the relatively coarse resolution of single-dish sub-millimetre telescopes (typically FWHM ~ 15"). The more recent development of millimetre interferometers, such as the Atacama Large Millimetre/Sub-millimetre Array (ALMA), provides us with an order of magnitude increase in resolution over the single-dish surveys allowing us to precisely identify the individual SMGs. In this thesis I present an analysis of two surveys of single-dish sub-millimetre sources which have been followed up with ALMA to identify the individual SMGs. The first survey is of the 716 SCUBA-2 sources detected in the UKIDSS UDS field where our ALMA follow-up detects 708 SMGs. From the extensive multi-wavelength coverage in this field we derive photometric redshifts for each SMG, finding a median Zphot=2.61±0.09 with a high-redshift tail containing 33⁺³₋₂% of Zphot > 3 SMGs. We find the blending of multiple galaxies into a single-dish source in 11±1% of our ALMA maps, and this rate of multiplicity increasing for brighter single-dish flux with a rate of 28±2% for S^deb_850≥5mJy. The photometric redshifts of these multiples suggest ≳30% are physically associated, which potentially points to merger activity playing a role in the intense star-formation rates for a significant percentage of SMGs. I also present the results of an ALMA survey of four SCUBA-2 sources in the centre of the Z=1.46 cluster XCS J2215, where the high-resolution imaging shows an elevated density of SMGs with 14 detections. Detections of ¹²CO in six of these cluster galaxies suggest they are recent accretors with larger fractions of warmer gas than comparable field galaxies suggesting ram pressure stripping is preferentially removing the cold gas content.

Triplet state harvesting is an important issue in the area of organic electronics, including organic light emitting diode (OLED) technology that has already entered the global market. In the aim to achieve efficient light-emitting diodes the photophysical properties of OLED emitters need to be understood in great detail. This work is devoted to triplet state harvesting in OLEDs. In this work a set of triplet-harvesting systems comprising exciton, excimer or exciplex emitters are characterized and used to fabricate prototype devices. The first system is based on metal-free emitters, using acridone or phenothiazine, which show thermally activated delayed fluorescence (TADF) or room-temperature phosphorescence (RTP). The competition between the rate of deactivation pathways affecting the triplet state and the reverse intersystem crossing (RISC) rate determine whether these molecules emit through TADF or RTP. The second system explores the effects of different substitution patterns on the properties of excitonic tetradentate ONNO Pt(II) complexes, and their performance in OLEDs, revealing a complicated host-to-guest energy transfer mechanism in doped films. The third work in this thesis explores the properties of newly synthesized Pt(II) metal complexes that have been found to efficiently form photoluminescent excimers and have strong potential to be used in solution-processed OLED devices. The photophysical characterisation of these complexes doped in film has revealed co-existence of excimer and aggregate emissions. Finally, the last two works in this thesis are focused on small molecule and polymer-based exciplex blends that exhibit efficient TADF emissions and can be used to fabricate solution-processed or vacuum-deposited OLEDs. The photophysics of these exciplex systems is characterised in-depth and undoubtedly demonstrates that local triplet states are not involved in the RISC process in this blends, which is in clear contrast with most exciton small molecule TADF systems. Furthermore, in this work a clear rationale for the observation of emission decaying in power law regimes is obtained for the first time.

All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets, in applications such as fusion, means we must understand and control the high sensitivity of critical current density Jc to strain ε. This thesis presents detailed Jc(B,T,ε,θ) transport measurements as a function of field B, temperature T, strain ε and angle θ with respect to the applied magnetic field, on high field superconductors that include the very widely observed inverted parabolic dependence for Jc(ε). It is usually assumed that a coincidence occurs between the Fermi energy and a peak in the density of states in the unstrained state which leads to a peak in the superconducting properties in the unstrained state. The long-standing interpretation of Jc(ε) data attributes the inverted parabolic strain behaviour to the averaged response of the underlying material. Features of the data in this work are identified in both HTS REBCO tape and LTS A15 wires which show that both of these assumptions are incorrect. A new analysis is presented which shows the inverted parabolic nature of Jc(ε) is the result of competition between domains with opposing strain dependencies and successfully accounts for the features of the data incompatible with standard assumptions. It is concluded that the origin of the competing domains in REBCO tape lies in twinned domains and in A15 wires is caused by percolative current flow across grain boundaries orientated in different directions which respond to an applied strain in different manners due to the Poisson effect. This work provides fresh insight into how improvements might be made to high field superconductors under strain.

Biological membranes rarely exist as free-floating structures but are often confined and supported by various cellular assemblies such as the cytoskeleton and the extracellular matrix. It has already been shown that biological and polymeric substrates can modulate the morphology and response to various stimuli of supported lipid bilayers significantly. The interaction between such structures and the membrane are obviously important yet remain poorly understood even in minimal or synthetic systems. The work of this thesis utilises a variety of fluorescence microscopy and atomic force microscopy (AFM) techniques to investigate the behaviour and structure of supported lipid bilayers, in particular how interfacial features of their support substrate influence and modulate their morphology and biophysical properties. First, surface modification of polydimethylsiloxane is systematically explored, in particular how the interfacial properties of such a polymer substrate can be modified to create fully and partially plasma-treated interfaces that stably support lipid bilayers. Lipid patch formation on such substrates is then investigated, revealing that the membrane undergoes significant morphological reorganisation after vesicle fusion has completed forming a lipid patch. The underlying mechanisms can be altered by substrate interactions following different pathways for fully and partially plasma-treated PDMS substrates. Furthermore, partially plasma-treated substrates are demonstrated to be capable of specifically depleting cholesterol from supported lipid membranes, while stably supporting the other remaining phospholipid species. Studies of cholesterol depletion of lipid patches possessing liquid-ordered and disordered domains reveal a disruption in domains structure, with the partitioning of fluorescent dyes into regions from which they were previously excluded. This structure perturbation was found to be reversible upon the reinsertion of cholesterol into the bilayer. Many of the discussed mechanisms are only observed in the presence of a substrate, emphasising the importance of substrate interactions in both functional biomembranes and the development of supported membrane technologies.

The evolution of Active Galactic Nuclei (AGNs) is crucial to galaxy evolution, given that AGNs affect their host galaxies via AGN feedback. In this thesis, I present predictions for the evolution of supermassive black holes (SMBHs) and AGNs from the semi-analytic model of galaxy formation galform, over a range of redshift (0 < z < 15) and wavelength (from radio to X-ray). First, I compare SMBH masses and AGN optical to X-ray luminosities from the model for z < 6 to observations, and explore the evolution of typical SMBHs within the model. I find that the median SMBH spin evolves very little over this redshift range. Secondly, I present predictions for z ≥ 7 for future surveys by JWST, EUCLID, ATHENA, and Lynx. I find that Lynx will detect the smallest SMBHs in the smallest host galaxies and host haloes, and that the predictions are generally insensitive to the SMBH seed mass. Thirdly, I predict the evolution of jet powers and (core-dominated) radio luminosities from the model for z < 6, and compare the evolution of these to observations. I predict the jet powers, halo masses, and fuelling mechanisms that dominate the model predictions. Finally, I present predictions of radio luminosities, lobe sizes and Fanaroff-Riley types of radio sources by combining a radio lobe evolution model appropriate for extended sources with the galaxy formation model. I find that this model generally is in good agreement with observed radio properties at z = 0, except for the fractions of Fanaroff-Riley sources, the number of low luminosity radio sources in high stellar mass galaxies, and the number of large sources. I explore the effect of varying different free parameters of this radio model, and suggest potential improvements.

This thesis presents the very first experimental realisation of splitting and recombination of robust bright-solitary-matter waves on a narrow repulsive potential barrier. This system has intrinsic interest for fundamental studies of soliton phase and for the realisation of a soliton interferometer. An upgraded imaging system is presented, which is capable of imaging the bright-solitary-matter waves in-situ and across a wide range of magnetic fields. The system uses a combination of an offset-locked imaging laser, a high-intensity probe beam and a microwave-transfer adiabatic rapid passage of the atoms to an auxiliary imaging state which has favourable transition properties. BECs of ⁸⁵Rb are created by direct evaporative cooling, using an upgraded crossed optical dipole trap. Condensates of up to 7000 atoms are created, with greater than 80% purity. We develop a wavefunction engineering protocol, which allows us to transfer the condensate into a quasi-1D potential and systematically demonstrate regions of interatomic interactions where bright-solitary-matter waves can be formed, as well as regions where condensate collapse or breathing-mode phenomena are observed. The bright-solitary-matter waves are very robust, propagating without measurable dispersion for over 20s. The splitting of a soliton into two daughter solitons on a narrow blue-detuned optical potential is presented. We demonstrate full control over the transmission coefficient by varying the barrier height. Velocity selection between the outgoing daughter solitons is observed and quantified, whereby the transmitted daughter soliton always has a higher centre of mass kinetic energy than the reflected daughter soliton. Velocity-selection-mediated recombination on a wide barrier is demonstrated, as well as interference-mediated recombination on a narrow barrier. We observe large shot-to-shot fluctuations for the narrow barrier which are fully consistent with independently-determined uncertainties in the barrier position. For the first time we explore this experimentally and theoretically, both with Gross-Pitaevskii simulations and analytical approximations, putting new limits on the required parameters for future phase-sensitive interferometric measurements.

Research in thermally-activated delayed fluorescence (TADF) emitters is gathering momentum and rapidly progressing towards commercial application in display industry. Successful TADF combines design strategies that result in thermal up-conversion of nonemissive triplets into emissive singlet excitons, increasing the maximum internal efficiency from 25 to 100 % in purely organic systems. Its performance can therefore compete with current leading emitters used in industry, however it is not without its hurdles and a full understanding of how to produce efficient TADF systems and stable organic light-emitting diodes (OLEDs) is still elusive. This thesis aims at illuminating strategies to achieve highly efficient and stable TADF. By examining the photophysical aspects of different donor-acceptor systems (D-A, D2-A, D3- A, D4-A and D-A-D) and, more importantly, establishing comparisons between different subsets of molecules, subtle but important aspects of the performance of these emitters are isolated to allow understanding of future design rules for better combinations. These comparisons are then correlated with the emitters' performance in devices. In a multi-donor platform, the inherent TADF mechanism and in host are both considered by comparing the effect of number and position of donors as well as rigidity and polarity of the host environment to the D-A angles. A separate comparative study elucidates the real heavy atom effect in an emitter with dual emission from two different conformations. Furthermore, the application of a well-established spectroscopy technique novel to TADF tests its physical mechanism by probing character and mixing of the excited states involved. Finally, in a more application-driven approach, a combination of three different TADF molecules for the production of white light is studied in simple device structures. In this sense, guidelines of how to produce optimised TADF systems emerge, moving the technology ever close to its industrial application.

Quantum error correction protocols are an essential element in the design of any circuit-model quantum computer. In this thesis, I introduce the coherent parity check (CPC) framework for quantum error correction. CPC codes have a fundamental structure in which quantum parity check measurements are stored coherently and compared over time. The specific advantage of the CPC code structure is that it provides a way of creating new stabilizer codes from the starting point of any sequence of parity checks. I show that this freedom in the choice of parity checks can be used to derive methods for the construction of distance-three quantum codes based on almost any distance-three classical code. The CPC framework has further applications in machine search routines for code discovery, as well as in the design of bespoke codes tailored for the demands of a given device. Another feature of CPC codes is that they can be represented as factor graphs of the type commonly seen in classical error correction and machine learning. I outline a procedure for this mapping, and demonstrate how a quantum code can be derived by manipulating its factor graph representation. The aim of the factor graph mapping for CPC codes is to make it easier to adapt well-developed techniques from classical information theory for use with quantum codes. This will make the CPC framework a useful tool for the theoretical and practical study of quantum error correction codes as large-scale quantum computers move closer to becoming a reality.

The unexplained origin of neutrino masses suggests that these neutral and weakly interacting particles might provide a portal to physics beyond the Standard Model. In view of the growing prospects in experimental neutrino physics, we explore new theoretical models and experimental searches that can shed light on the existence of low-scale particles with very small couplings to ordinary matter. Our efforts highlight a vast landscape of models where neutrino physics offers our best chance of discovering such hidden sectors. Along the way, we revisit the Standard Model physics of neutrino trident production with a modern calculation and explore its phenomenology at neutrino facilities. As shown here, this type of rare neutrino scattering process can probe unexplored anomaly-free extensions of the Standard Model with a complementary, and often more powerful, search strategy to to the well-known searches in neutrino-electron scattering. As to new models, we propose a novel neutrino mass model resembling the inverse seesaw, where neutrino mixing stands as the most prominent portal to dark sectors and dark matter. In our dark neutrino model, neutrino masses are generated radiatively, with the vector, scalar, and neutrino phenomenology displaying an unique interplay. Later, we devise new methods to search for these dark neutrinos using neutrino-electron scattering data, aiming to discriminate among new physics explanations of the MiniBooNE anomalous results. Finally, we discuss light and heavy conventional sterile neutrinos in the context of νSTORM, an entry-level neutrino factory for precision neutrino physics.

Giant impacts dominate many planets' late accretion and evolution, but the detailed consequences of these violent events are still poorly understood. In this thesis I use 3D smoothed particle hydrodynamics simulations of giant impacts to study three primary topics: the impact origin of Uranus' obliquity; numerical convergence with increasing resolution; and atmospheric erosion by giant impacts. To these ends, the SWIFT code is developed to model planetary impacts with 1000 times more simulation particles than the current standard, and an efficient method for creating relaxed initial conditions is presented. A suite of simulations of giant impacts onto the young Uranus confirms that the planet's high obliquity can be explained by a wide range of impact scenarios. For some grazing collisions, most of the impactor's ice and energy is deposited in a thin shell in the target's outer ice layer, which might help explain Uranus' observed lack of heat flow from the interior. Follow-up simulations with just over $10^8$ simulation particles reveal that standard simulations with fewer than $10^7$ particles fail to converge on even bulk properties like the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these large-scale results reliably, but even $10^8$ particles may not be sufficient to study the detailed composition of scattered debris. This improvement in resolution then enables the first full, 3D simulations of atmospheric erosion on terrestrial planets by giant impacts. For head-on collisions, there is a rapid change with increasing impact speed from very little erosion to total loss. However, for grazing impacts there is a more gradual change with speed and a non-monotonic dependence on the impact angle. These projects highlight the necessity of high-resolution simulations in three dimensions to capture the complexity of giant impacts. In the final chapter, a model of the Moon's argon exosphere is used to test competing explanations for the strange features observed by the recent LADEE mission. The persistent overdensity of argon over the maria can only be reproduced by a localised endogenic source. This offers a novel probe of the lunar interior late after its origin in a giant impact.

Quantum interference constitutes one of the sharpest divides between our classical intuition and the weirdness of the quantum world. Whenever there are multiple paths leading to the same outcome, it is the interplay of associated probability amplitudes that determines measurement results. In the double slit experiment, wave-like interference governs the distribution of particles on a detection screen. Extending to multiple particles allows behaviour that cannot be described by a classical wave, such as the bunching of photon pairs in Hong-Ou-Mandel interference. In both these examples, the ability to distinguish the interfering paths leads to degradation of interference and a return to behaviour described by probabilities. Understanding how this transition evolves for larger multiparticle quantum systems is of fundamental interest and also vital for developing photonic quantum technologies that rely on many-photon interference. In this thesis we investigate the role of distinguishability in the interference of three and four independent photons by using polarisation and time delays. For three photons, a new type of collective distinguishing phase appears that goes beyond a pairwise description of the similarity of quantum states. We use nonlinear photon sources and a fibre interferometer to experimentally demonstrate the ability to tune and isolate this phase in interference statistics. Extending to four photons we show that, contrary to intuition from two-photon experiments, the interference of photons prepared in distinguishable states is possible due to a four-particle phase. This is confirmed experimentally using a bulk four-mode interferometer. We then extend our study to include the effects of state impurity in the interference of three photons. New properties of this interference enable diagnosis of realistic experimental imperfections and characterisation tasks impossible using only two photons. Finally, we link discussions of state distinguishability to the group representations underpinning the exchange symmetry of multiparticle wavefunctions.

The last few years have witnessed a remarkable explosion in advances in the field of photovoltaics (PV) based on organic semiconductors as the light absorbing material, both in terms of device efficiency and in terms of the physical understanding of the underlying processes. This thesis aims to contribute to this field by examining two different types of solar cells based on light-absorbing organic semiconductors. It consists of two main parts connected through their focus on the role of charge transfer (CT) states on device performance, and more specifically on a focus on effect of CT states on open-circuit voltage losses. The first two results chapters focus on the fabrication and analysis of novel hybrid heterojunction solar cells based on the molecular inorganic semiconductor copper thiocyanate (CuSCN) in conjunction with light absorbing organic semiconductors, which show surprisingly high charge generation efficiencies. In the first results chapter I show that the formation of a hybrid CT-state formed at the interface between CuSCN and organic semiconductors is key to the charge generation process in these cells. In in the second results chapter I present the discovery of a remarkable nanostructured interface formed between CuSCN and PC70BM via solution processing with which it is possible to fabricate semi-transparent devices with power conversion efficiencies (PCEs) exceeding 5%, which is amongst the highest reported efficiencies for hybrid heterojunction solar cells. The final two results chapters focus on the role of CT-states in affecting the voltage losses of bulk-heterojunction (BHJ) solar cells based on non-fullerene acceptors. In the third results chapter I present the device optimization and characterization of a solar cell based on a novel polymer:non-fullerene acceptor combination, and show that it is possible to fabricate solar cells with PCEs exceeding 13%, amongst the highest reported efficiencies at the time. I extend the analysis of the role of the CT-state in achieving this high efficiency in the last results chapter, where I present a new model that helps to explain the observation of very low non-radiative voltage losses in blends with a low driving energy for charge separation through the processes of hybridization of CT and local excited states.

Plasmon-induced hot carriers processes in metallic nanostructured materials are becoming an important field of research due to their potential technological applications. For example, hot carriers have been found to have favorable properties for photocatalysis, photodetection and solar energy harvesting. Although there have been significant experimental efforts to understand hot carrier properties in nanoplasmonic systems, there exists a lack of theoretical models to underpin experimental findings and guide experimental progress towards promising systems. In this work we carry out theoretical calculations of hot carrier phenomena in mono-metallic and bi-metallic nanoparticles and also in thin metallic films. In particular, we propose a simple theoretical model to describe hot-carrier generation in mono- and bi-metallic nanoparticles (NPs). For this, we combine a classical description of the plasmons in nanostructures with quantum-mechanical calculations of the electronic structure. The decay of the plasmon into hot carriers is described using Fermi's golden and the optical and electronic properties of the NPs are derived within the quasistatic approximation and solving the Schroedinger equation for a spherical well, respectively. For the mono-metallic nanoparticles, we present results for a set of 6 plasmonic metals (Na, K, Al, Cu, Ag, Au). For the bi-metallic core-shell nanoparticles, we screen 100 different material combinations and predict their efficiency for photocatalytic applications. For the plasmonic thin films, we combine ab initio density-functional theory with the two-temperature model to describe the ultrafast thermalisation dynamics of hot electrons in titanium nitride. We investigate the lifetime of hot carriers due to the scattering with phonons and also include the effect of defects.

The study of quantum field theories and supersymmetric quantum field theories has thrived in the recent decades. An aspect of this study has a particularly geometrical nature and as such has also started to spearhead progress in related fields of mathematics. This aspect is the study of vacua. Traditional computations in quantum field theory understand particle states as perturbations around a vacuum state. Many theories have a plethora of candidates to be this vacuum state. Different choices of vacuum imply different properties of the particles in the theory. These physical differences can be understood by creating a moduli space: an affine variety in which each point of the variety corresponds to a different vacuum of the theory. The physical properties of the vacua are then realized as geometrical properties of the different points in the variety. In this thesis we analyse these properties and develop new techniques to further their study: the Kraft-Procesi transition and the magnetic quivers. In order to do so we employ a particular fruitful approach to this type of problems: embedding the quantum field theories at hand in a string theoretical background. By this procedure, the low energy dynamics of branes in the string theory set up correspond to motions along the moduli space of vacua of the theory. The new results that we present here aim to be a set of tools that can readily be applied to the analysis of many different supersymmetric quantum field theories. The goal is that the tools are based on manipulations of simple diagrams, which should make them easy to remember and easy to implement.

Since Bell, Kochen and Specker formulated the first notions of contextuality, a number of frameworks have been created to help make philosophical and mathematical sense of the concept. Two of the most common are the Equivalence-based approach due to Spekkens, and the Sheaf-theoretic framework of Abramsky and Brandenburger. Each of these can support a hierarchy of strengths of contextuality, which can include possibilistic concepts. This thesis will explore the differences between the two approaches, and explore what can be demonstrated by using weaker notions of noncontextuality. Notions of noncontextuality are important for proving no-go theorems which highlight that many properties that are true of classical mechanics fail to be compatible with quantum predictions. Often, the notions of noncontextuality used are very strong assumptions, and even weaker flavours of noncontextuality can be used to draw the same, or similar conclusions. An important class of such notions are, possibilistic assumptions that only care about whether or not events are possible or impossible, rather than the exact Born rule probabilities that are applied to events. Even these weaker notions are often impossible to resolve with the logical structure and predictions of quantum theory. Contextuality has been demonstrated to be a resource for many quantum computational operations, and the contextual fraction has been shown to be an important resource. We introduce a metric that can differentiate between different quantum mechanical scenarios with contextual fraction 1, and demonstrate analogues of CHSH and Tsirelson bounds for this quantity. We also investigate the computational complexity of calculating the values of some metrics for nonlocality and contextuality, completing the classification of possibilistic nonlocality scenarios, as well as other metrics of nonlocality analogous to our metric of maximal contextuality.

This document will explain the process of developing systems and equipment to observe adiabatic shear bands as they form during experiments. This will involve explaining the scope of the problem which relies on the speed required to cause a system to effectively become adiabatic. The adiabatic nature of the deformation under shear loading at high rate is what allows the thermal softening to overwhelm strain and strain rate hardening. This requires a suitable test bed, and this thesis will explain the development of a suitable arrangement of a miniaturised Klosky Bar. This thesis will also explain a method of observing the deformation as it occurs, using a speckle pattern applied to a well-polished sample surface and digital image correlation to observe changes over the course of the experiment. This system needs a specific set of requirements be met for the sample so that shear bands can be preferentially formed in view of the imaging systems. The development of those samples is also explained. Additional methods of analysis are also preliminarily investigated, including x-ray phase contrast imaging and thermoreflectance temperature measurements. This thesis presents some of the results gathered during this investigation, conducted to attempt to find the ASB characteristics of Ti-6Al-4V. This project found the critical strain for as received Titanium-6- Aluminium-4-Vanadium is 0.25, and critical strain rate may be as low as 1x10^3, and showed that deformation through the shear region is not laminar. The directions this project's work could then be taken is explained in the final section.

In this thesis we will explore the development of a realist quantum theory based on the decoherence functional using the co-event interpretation of Quantum Measure Theory. The Sum-Over-Histories theory of quantum mechanics will provide the bedding for a Hilbert-space-free stochastic-like theory that can accommodate spacetime-like objects, and can therefore be applied to quantum gravity and cosmology, as well as give an alternative perspective on quantum phenomena. The primitive objects of the theory are histories, which give different accounts of a system's evolution, and the decoherence functional, which sums the quantum interference between these histories. Quantum Measure Theory and Generalised Quantum Mechanics (a theory close to Decoherent Histories) then give alternative interpretations of the decoherence functional's relation to reality. In these theories, the decoherence functional is mathematically constrained in analogue to probability measures. However, one of the conditions, called weak positivity, can be lost under composition of isolated systems. We will extend this composition argument to take the case for a stronger condition of strong positivity for decoherence functionals. The bulk of the report will then focus on the co-event interpretation of Quantum Measure Theory, where co-events give full accounts of which events do or do not occur for a given system. The quantum nature of reality is expressed through the breaking of classical logic within these co-event descriptions. We will focus on evolving co-event schemes, which dynamically construct co-events to describe the reality of an evolving system in tandem with its progression. The evolving co-event schemes will be shown to reproduce classical logic when they are applied to classical systems. Moreover, similar to classical stochastic theory, these schemes will be shown to be invariant under the inclusion or exclusion of non-interfering histories. We will also explore a number of outstanding problems for these schemes, and will propose some potential modifications.

Photonic quantum information processing is a key element for scalable quantum technologies, and has applications in secure long-distance quantum communication and connecting nodes of a quantum computation network. However, logical photon-photon gates and state-of-the-art single photon sources rely on probabilistic processes. Quantum memories are devices that enable storage and on-demand recall of quantum states of light, and have been highlighted as a vital component in photonic networks to overcome the scaling problem by synchronising probabilistic processes. The Raman memory has a large storage bandwidth and high synchronising capacity, and is an ideal candidate for local synchronisation. However, previous demonstrations of the Raman memory suffer from four-wave mixing noise, which prohibits quantum level operation. In this thesis I investigate methods to increase the signal to noise ratio in the Raman memory. I investigate increasing the light-matter coupling strength to boost the memory efficiency, and then explore two different methods to suppress four-wave mixing noise. I demonstrate that operating the Raman memory in a cavity is successful in reducing four-wave mixing, but it is technically challenging to maintain a high memory efficiency. I investigate a new method of noise suppression by introducing an absorption feature at the frequency of the unwanted noise field. This technically simple method is successful in reducing the noise by an order of magnitude, and will be applicable to many quantum memory protocols. In the final section of this thesis I explore the temporal mode properties of the Raman memory. I demonstrate that the Raman memory is single mode and can be used to separate and manipulate temporal modes of light. This positions the Raman memory as a key device for enabling high-dimensional photonic quantum information processing, and enhancing light-matter interactions. These results pave the way towards an efficient, low-noise, mode-selective quantum memory.

In this thesis a comprehensive study of the highly lattice mismatched ternary alloy GaAsBi is presented and its suitability for use in future high efficiency photovoltaic cells assessed. A large non-linear bowing of the band gap in GaAsBi allows the alloy to realise a 1.0 eV band gap with as little as 6% Bi and 0.7% compressive strain, which is highly desirable for next generation quad- junction photovoltaic cells. However, despite the promising electronic properties GaAsBi remains in the early stages of development and further understanding of the affects of Bi incorporation on GaAs is required. Minimisation of extrinsic front surface reflection losses can lead to appreciable performance enhancements in both triple and quad junction solar cells. A small enhancement of 0.7 percentage points in power conversion efficiency is observed when applying an optimised triple layer anti- reflection coating to a triple junction solar cell, however a more appreciable improvement of 1.5 percentage points is demonstrated when the triple layer coating is applied to a potential quad junction design. This demonstrates the importance of minimising extrinsic losses when wishing to realise high efficiency photovoltaic cells in excess of 50%. Samples containing bulk GaAsBi up to 3.7% are examined using time integrated and time resolved photoluminescence spectroscopy. Clear evidence of alloy disorder is observed, with a red-blue-red shift, or 's-shape', evident in the temperature dependence of the peak luminescence energy, and a strong spectral dependence of the effective carrier lifetime at low temperature. Selective excitation of the sample is employed to investigate an anomalous low energy emission peak centred at 0.978 eV, which is attributed to defect related recombination in the N+ GaAs sub- strate. This initial study highlights the presence of alloy disorder in GaAsBi and the caution that should be employed when evaluating the optical characteristics of novel semiconductor alloys on doped substrates. The nature of disorder induced localised states resulting from fluctuations in the band edge potential is assessed. The magnitude of the s-shape, is demonstrated to decrease with increasing excitation power, indicative of the filling of localised energy states. Examining the luminescence spectrum lineshape under varying excitation power indicates that a Gaussian distribution with standard deviation σGauss = 100 meV and a localisation energy up to 81 meV best describes the profile of the localised distribution of states extending from the valence band edge. Detailed analysis of the thermal quenching of the luminescence under varying excitation power indicates that the distribution of localised energy states is made up of two Gaussian components with standard deviations σ1 = 62 meV and σ2 = 20 meV. Good agreement between the spectrum lineshape and thermal quenching analysis highlights consistency in the Gaussian profile of the localised density of states, which is in contrast with the classical exponential interpretation and other reports in the literature. The affect of inherent alloy disorder on the voltage performance of prospective GaAsBi based solar cells is investigated. It is shown that the open-circuit voltage may degrade by up to 202 meV with Bi fractions up to 5.5%, indicating that GaAsBi may not be as suitable as initially anticipated as a potential 1.0 eV sub-cell material. In light of this analysis GaAsBi is considered as a potential replacement for InGaAs in future upright metamorphic solar cells. Both abrupt absorption and material limited absorption calculations are presented and demonstrate that GaAsBi with as little as 2.8% Bi, incurring only 0.25% compressive strain on the Ge substrate, can offer improved device performance over the existing InGaAs based architecture under single sun AM0 illumination. This demonstrates the potential of GaAsBi for producing the next generation of highly efficient photovoltaic devices and highlights a new pathway to achieving the next milestone goal of 50% efficient solar cells.

In 2012 the CMS and ATLAS experiments announced the discovery of a boson with mass around 125 GeV and properties consistent with that of the Higgs boson predicted by the standard model (SM). Despite this success, many observations about the universe appear to be inconsistent with the SM phenomenology, implying the need for beyond the standard model (BSM) physics. Many BSM scenarios require extended Higgs sectors, which usually include additional Higgs bosons and predict different properties of the 125 GeV state. Two approaches may be used to test such scenarios: search for the additional bosons, or measure the properties of the 125 GeV state to test the comparability with the SM. This thesis explores both of these options. A search for additional Higgs bosons in the mass region 90 GeV-3.2 TeV decaying to pairs of tau leptons is presented. This analysis uses the dataset collected by CMS in 2016, which corresponds to an integrated luminosity of 35.9 fb −1 . No statistically significant excesses are observed. Exclusion limits are set on the cross sections times branching fractions. The results are also interpreted in the context of the minimal supersymmetric standard model (MSSM). A measurement of the CP-properties of the Higgs Yukawa coupling to top quarks is also performed using Higgs boson production via gluon fusion in association with two jets. The analysis of the azimuthal angular correlations between the jets is sensitive to the CP mixing angle α_hgg. This analysis uses the datasets collected by CMS in 2016 and 2017 which correspond to an integrated luminosity of 71.8 fb −1 . The best-fit value of α_hgg is measured to be −59 ◦^{+47◦}_ {−50◦} . This constitutes the first direct measurement of the CP-properties of a fermionic Higgs coupling.

The solar wind is a sparse and hot plasma, flowing from the surface of the Sun to the edge of the heliosphere. Our best knowledge of the solar wind close to the Sun comes from the two Helios spacecraft launched in the 1970s, but this will soon be joined by data from Parker Solar Probe and Solar Orbiter. This thesis re-visits in situ measurements of the solar wind taken by Helios in preparation for these new missions. Measurements of solar wind protons and alpha particles taken by Helios are re-analysed to extract the thermal anisotropy of the particle distribution functions. The new dataset is used to investigate three areas of solar wind formation and evolution. The properties of number density structures inside 1 AU located away from the heliospheric current sheet are investigated and shown to account for < 1% of the solar wind. Their use as tracers of the ambient solar wind speed is validated, and 90% of the structures detected are shown quantitively to be too small to be detected in existing remote sensing images. A new solar wind source identification scheme is constructed for in situ measurements made inside 1 AU, with distinct solar wind sources classified into three categories based on proton temperature anisotropy and Alfvénicity. This categorisation scheme is used to demonstrate that the traditional categories of slow and fast solar wind do not map well to distinct solar sources. The radial evolution of alpha particle temperatures inside 1 AU is presented. Alpha particles are found to undergo heating in the magnetic field parallel direction, and cooling in the magnetic field parallel direction during propagation. This is shown to be at least due to the influence of temperature anisotropy instabilities. These results also imply that alpha particles are heated in the perpendicular direction below 0.3 AU.

String theory possesses duality symmetries that relate different string backgrounds. One symmetry is known as T-duality symmetry. In general, when n dimensions are toroidally compactified, the T-duality group is O(n, n, Z). String theory has another duality symmetry known as S-duality, which does not commute with T-duality. The full S-duality group is SL(2,Z). The last duality symmetry is U-duality symmetry, which is En(Z) for type II string theory on an n-torus. Duality symmetries tell us that strings experience geometry differently from particles. In order to understand string theory, a new way to understand string geometry is required. In this thesis, first we introduce some basic ideas on duality symmetries in string theory, namely, T-duality, S-duality, and U-duality. Next, we review string field theory. We, then, provide the basic constructions of DFT and EFT. Next, we consider the finite gauge transformations of DFT and EFT. The expressions for finite gauge transformations in double field theory with duality group O(n,n) are generalized to give expressions for finite gauge transformations for extended field theories with duality groups SL(5,R), SO(5,5) and E_6. Another topic is the T-duality chain of special holonomy domain wall solutions. This example can arise in string theory in solutions in which these backgrounds appear as fibres over a line. The cases with 3-torus with H-flux over a line were obtained from identifications of suitable NS5-brane solutions, and are dual to D8-brane solutions. This T-duality chain implies that K3 should have a limit in which it degenerates to a long neck capped off by suitable smooth geometries. A similar result applies for the higher dimensional analogues of the nilfold. In each case, the space admits a multi-domain wall type metric that has special holonomy, so that taking the product of the domain wall solution with Minkowski space gives a supersymmetric solution.

The Tokai to Kamioka (T2K) long baseline neutrino oscillation experiment was designed to make precise measurements of neutrino oscillations. It uses a muon (anti-)neutrino dominated beam produced at the Japan Proton Accelerator Research Complex (J-PARC) on the east coast of Japan, aiming towards the Super-Kamiokande (SK) detector 295 km west. The neutrino beam is sampled by the two near detectors ND280 and INGRID, 280 m downstream of production. These measure the neutrino flux and interaction cross-sections to reduce the impact of systematics for oscillation analyses. The work presented herein details the process of using ND280, INGRID, and external data to best constrain the predicted event rates at SK. The analysis proceeds by using a Markov Chain Monte Carlo method which simultaneously fits ND280 and SK data without assumptions on the underlying posterior probability density function. The two analyses detailed herein reduce event rate uncertainties at SK from 12-14\% to 2-4\%, enabling world-leading oscillation parameters to be extracted from T2K. Numerous run-by-run and detector-by-detector studies were performed and alternate models investigated, all of which were deemed compatible within uncertainty. This body of work has been included in the official T2K results presented in 2017 and 2018, and its use continues beyond that.

When structures with a strong confinement of the electromagnetic fields are considered, the near-field dynamics becomes integral to the description of a semiconductor. Not only it provides feedback from the environment, as is the case in a laser system, but it further mediates the interaction between different, otherwise independent, positions. In such a context, a full-field spatio-temporal description is essential to faithfully describe the dynamics of either an extended semiconductor system or a set of spatially separated emitters. This thesis highlights the importance of combining a complex (many-body and band-resolved) model of semiconductor carrier dynamics with a full-field description of the electromagnetic fields by presenting some applications. With the recent rise in popularity of atomically thin materials, semiconductors can be embedded in increasingly smaller optical environments, whose properties can only be studied by self-consistently combining carrier and field dynamics. The ability to calculate the linear and non-linear response of a system under arbitrary excitation conditions is shown. This is performed without any prior knowledge of the electromagnetic environment and can thus be extended to complex geometries. By embedding active materials in a tailored environment, the complex interaction of the two can be exploited to engineer the optical response of the system by using a self-consistent modelling technique. The complex dynamical interaction between field and gain in a semiconductor laser is another example of a system in which a self-consistent model is required. Here, a set of one-dimensional simulations is reported showing how the output of a semiconductor laser is highly sensitive to perturbation arising from sub-wavelength dynamics of the gain medium. By introducing a random patterning of the laser cavity, a novel approach to the suppression of dynamical instabilities in a laser output is demonstrated. This scheme, based on complex wave interference, is introduced by spatially perturbing the optical environment.

Several pieces of astrophysical evidence, from galactic to cosmological scales, indicate that most of the mass in the universe is composed of an invisible and essentially collisionless substance known as dark matter. A leading particle candidate that could provide the role of dark matter is the Weakly Interacting Massive Particle (WIMP), which can be searched for directly on Earth via its scattering off atomic nuclei. The LUX-ZEPLIN (LZ) experiment, currently under construction, employs a multi-tonne dual-phase xenon time projection chamber to search for WIMPs in the low background environment of the Davis Campus at the Sanford Underground Research Facility (South Dakota, USA). LZ will probe WIMP interactions with unprecedented sensitivity, starting to explore regions of the WIMP parameter space where new backgrounds are expected to arise from the elastic scattering of neutrinos off xenon nuclei. In this work the theoretical and computational framework underlying the calculation of the sensitivity of the LZ experiment to WIMP-nucleus scattering interactions is presented. After its planned 1000 live days of exposure, LZ will be able to achieve a $3\sigma$ discovery for spin independent cross sections above 3.0e-48 cm^2 at 40 GeV/c^2 WIMP mass or exclude at 90\%~CL a cross section of 1.3e-48 cm^2 in the absence of signal. The sensitivity of LZ to spin-dependent WIMP-neutron and WIMP-proton interactions is also presented. All the sensitivity projections are calculated using the LZStats software package, which is discussed in detail in this thesis. In addition, this work classifies key systematic uncertainties by their impact on the WIMP sensitivity and motivates the inclusion of the highest-ranked into the analysis likelihood function. The effect of some of these systematics on the reconstruction of the WIMP cross section is also studied and it is found to be sub-dominant.

This thesis describes the development of technology and experimental techniques for scalable trapped-ion quantum computing. An architecture for scalable quantum computing is presented in which 171Yb+ ions are trapped in linear R.F. Paul traps and their hyper-fine states are used to form qubits. Quantum logical operations can be carried out by applying R.F. and microwave fields. This thesis presents significant improvements in the experimental setup used for quantum computing, as well as new theoretical developments and experimental demonstrations of new high- fidelity quantum control methods. A new procedure for carrying out high- delity two qubit gates using static magnetic eld gradients and resonant microwave elds is presented. It is shown that this has the potential to enable signi cantly improved delities compared to current methods. Improved methods for measurement and statistical analysis in trapped-ion experiments are also shown. These should enable more accurate measurement of two-qubit gate delities, which becomes increasingly important as delities increase. A new system for generating arbitrary R.F. and microwave waveforms is presented, including a bespoke software control system allowing the user to de ne custom pulse sequences, which represents a signi cant improvement in the scalability of the quantum control system. Experimental developments towards a system for carrying out on-chip trapped-ion quantum logic in a static magnetic eld gradient are also presented. A new technique for generating multi-level control methods is introduced, based on reducing a multi-level system to an e ective two-level system. High- delity quantum control methods generated using this technique, which form a will form a key part of the scalable quantum computing architecture, are implemented experimentally.

The goal of achieving energy production by harnessing nuclear fusion is one of the most important scientific challenges of our time. Inside a fusion reactor structural materials are exposed to extreme doses of radiation, which over time alter their structural properties. To design efficient thermal recovery procedures for structural materials in fusion energy applications it is important to characterise quantitatively the annealing timescales of radiation-induced defect clusters. The evolution of the defect microstructure in materials at high temperature is dominated by diffusion-mediated interactions between dislocations, cavities and surfaces. This gives rise to complex non-linear couplings between interstitial and vacancy-type dislocation loops, cavities and the field of diffusing vacancies that adiabatically follows the evolution of microstructure. In this work we present a novel approach inspired by the Green's function formulation for the climb of curved dislocations, including in the same framework the evaporation and growth of cavities and the effects of free surfaces. The model makes use of boundary integral equations to solve the steady-state vacancy diffusion problem, allowing a unified treatment of multiple microstructural features in a real-space picture. After illustrating its main features, we then expand the formalism to include the treatment of a population of very small defects and dislocation loops that are below the experimental detection limit. These are taken into account through a mean field approach coupled with an explicit real-space treatment of larger-scale discrete defect clusters. We find that randomly distributed small defects screen diffusive interactions between larger discrete clusters, modifying the free diffusion Green's functions into Yukawa-type propagators. The evolution of the coupled system is modelled self-consistently, showing how the defect microstructure evolves through a non-monotonic variation of the distribution of sizes of dislocation loops and cavities, treated as discrete real-space objects.

The SHiP experiment is a proposed experiment aiming to search for new super-weakly interacting particles. The concept is based on using a very intense and high energy proton beam at the CERN Super Proton Synchrotron (SPS) which is delivered to the new Beam Dump Facility (BDF), where the experiment will search for New Physics (NP) in a zero background environment. This thesis describes several studies for the optimisation of this concept, in order to maximise its physics potential. These include studies of a benchmark signal model to understand acceptance effects, studies of the muon induced background using both simulation and a dedicated experiment at the SPS, and the optimisation of the muon shield -a crucial component of SHiP- using machine learning techniques.