This thesis describes measurements of rare electroweak penguin decays performed with data collected by the Large Hadron Collider beauty experiment corresponding to 3 fb^{-1} of integrated luminosity. The purpose of these measurements is to search for physics beyond the theoretical framework known as the Standard Model (SM). Electroweak penguin decays are sensitive to virtual particles in extensions to the SM whose influence on the decay amplitude can be of similar strength to the SM contribution. The particular measurements that are described in this thesis are the differential branching fractions and isospin asymmetries of B -> K(*) μ^+ μ^- decays as well as the angular observables in B -> K μ^+ μ^- decays. Although results are consistent with the SM, all the branching fractions of B -> K(*) μ^+ μ^- decays tend to favour a lower value than theoretical predictions.

The ocean carries more heat poleward than the atmosphere at low latitudes, whilst the reverse occurs at high latitudes. In the Northern Hemisphere, the largest ocean-atmosphere heat fluxes occur over the Gulf Stream, suggesting that an ocean-atmosphere 'relay' is active at mid-latitudes. This thesis is concerned with the significance of the extremes in air-sea heat fluxes over the Gulf Stream. In the first research chapter, the direct interaction between the ocean and the atmosphere is examined in the ERA-Interim dataset. Based on Lagrangian trajectory calculations, the most extreme air-sea heat flux events are found to be associated entirely with air of continental origin. The subsequent heat gain in the overlying air is caused almost completely by surface heat fluxes. For average air-sea heat fluxes, the associated air is both continental and maritime in origin, with a noticeable contribution to the heat content of the air parcels from entrainment at the top of the boundary layer. The second research chapter determines the causes for variations in surface heat flux in the ERA-Interim dataset. Roughly 90% of the time, one observes a baroclinic waveguide of varying strength over the Gulf Stream, setting the intensity of the air-sea heat exchange and the mean state in precipitation and tropospheric wind divergence. A potential mechanism whereby a change in sea-surface temperature gradient could cause an alteration of these mean patterns is discussed. Finally, the link between sea-surface temperature gradients and atmospheric fronts is explored in model simulations. A smoothing in the sea-surface temperature gradient is found to broadly reduce front intensity over the Gulf Stream. Increases in front intensity are shown to be consistent with a thermal damping mechanism. A significant effect is also observed on the regional precipitation and tropospheric vertical velocity, as well as on the direction of frontal propagation.

This thesis concerns the magnetotransport properties of the iron-based superconductors, and in particular, the ferropnictides. In the low doped ferropnictides, linked structural and magnetic transitions occur which significantly alter the electronic behaviour. Simultaneous to the establishment of the magnetic ordering is the creation of small Fermi surface pockets. It has been shown that some of these Fermi surface pockets have Dirac Cone characteristics. The primary work in this thesis focuses on the existence of non-saturating quasi-linear magnetoresistance in the underdoped ferropnictides. This feature has been seen as the hallmark of Dirac cone physics due to the commonly applied quantum linear magnetoresistance model. We have explored this hypothesis by performing a series of magnetotransport experiments using the van der Pauw method on undoped BaFe$_{2}$As$_{2}$, low cobalt doped BaFe$_{1.985}$Co$_{0.015}$As$_{2}$ and superconducting BaFe$_{1.96}$Co$_{0.04}$As$_{2}$. Scattering centres have been systematically introduced using 3-MeV proton irradiation. The quantum linear magnetoresistance model predicts the quasi-linear magnetoresistance should vary with carrier scattering. We describe these experiments, and draw the conclusion that the quantum linear magnetoresistance model is incorrectly applied. Other models to explain the quasi-linear magnetoresistance are reviewed. Speculation as to the cause of magnetic hysteresis in the magnetoresistance found in some of the parent crystals studied is presented. The Hall resistivity in the parent and underdoped ferropnictides shows a clear non-linear response suggesting that the single carrier model is invalid. We find that the Hall resistivity is insensitive to the introduction of disorder. Various models are reviewed including the anomalous Hall Effect and the antiferromagnetism related anisotropic quasiparticle lifetime model. Furthermore, magnetotransport scaling techniques are considered. Only the modified Kohler's rule is satisfied and this is shown to have an intriguing Co doping dependence.

Organic upconversion of photons through triplet energy exchange between two or more molecules (OUC) has been investigated through transient absorption and fluorescence spectroscopy, kinetic rate modelling and morphological analysis of thin films. An OUC system, consisting of one sensitising molecule (PQ4Pd) and an emitting molecule (rubrene), was first studied to explore the possibility of modelling the entire OUC process with a kinetic rate model. Transient absorption spectroscopy allowed for the intermediate steps of OUC to be directly observed and fitted, producing rate constants for each step in the process. This complete model was then optimised against fluorescence measurements from a system containing PtTPBP (sensitiser) and perylene (emitter) to calculate rate constants for that system from a single fluorescence type experiment, as opposed to several orthogonal Stern-Volmer type experiments. The possibility of fabricating a thin film OUC has been investigated through microscopy, fluorescence spectroscopy and a simple Monte-Carlo model. Using a system of PtOEP (sensitiser) and DPA (emitter), it was shown that the maximal efficiency of a thin film containing these molecules suspended in a PMMA matrix is found when the matrix is between 80 and 85 weight% of the total mixture. It was shown that on short timescales (a few seconds to a few minutes), atmospheric oxygen does not adversely affect thin film upconverters of this type as local oxygen is extremely rapidly quenched (less than a ms) and fresh oxygen is not able to diffuse back into the matrix at a rate that is competitive to OUC. It was shown that the degree of intermixing of active materials is of absolute importance in this fabrication, and a novel optical technique for measuring this intermixing in air was developed, some preliminary results are included.

The plasma facing surfaces of the ITER are going to contain beryllium for the first wall and tungsten for the divertors. To test their benefits for the future operations, the JET is now run with the ITER-like walls. The ASDEX Upgrade has placed full tungsten surfaces inside it, so a large amount of tungsten dust grains are produced. The WEST divertor was recently set up in the Tore-Supra. There are some tokamaks which may not use the ITER-like materials but still metal, e.g. the FTU. Also the diagnostic tool can provide metallic dust grains in a chamber. With the high heat output of the future metallic tokamaks, much more metallic dust grains should be produced, the situation of which never occurs. We focus on studying two phenomena related to metallic dust grains in a plasma: the avalanche erosion done by high velocity impacts; and the misty plasma physics for charged droplet and bubble. For the former, we use the dust transport code, DTOKS to simulate iron dust grains re-entering the plasma, corresponding to the FTU, from the bottom. We find that only certain ranges of core plasma flow speed, launch direction and initial dust size result in acheiving a high velocity dust grain. In misty plasma, for a large droplet, we modify the electrostatic stability limit by the use of the MOML theory and the liquid pressure by the use of the conservation of the ion momentum flux. The bubble in the plasma may originate from the boiling molten layer on plasma facing surfaces or the transformation from the superheated droplets. We calculate the bubble electrostatic stability limit by the Lord Rayleigh's approach. It is surprisingly that the basic instability initiates at l = 3 rather than l = 2.

Achieving precise control over an array of ultracold molecules would provide a unique tool-set for carrying out quantum simulations and quantum computations, as a result of the molecules' rich internal structure. To realise this aim, the molecules have to be cooled and trapped. This is much more difficult for molecules than for atoms due to their complex internal structure. This thesis presents preliminary work towards realising a versatile, permanent magnet trap for buffer gas cooled molecules. Atoms are used throughout to test the feasibility of the trap, as they are easier to produce and detect. Two novel methods for trapping buffer gas cooled atoms in a permanent magnet trap are investigated. The first of these involves trapping the atoms directly from a cryogenic buffer gas cooled ablation plume. Dy atoms, with a magnetic moment of 10μB, are trapped with a lifetime of 800 ± 30 μs, thought to be limited by collisions with a high density of background buffer gas atoms remaining in the trap region. Information gained from the direct trapping experiments motivated the design of a second trapping set-up. Here, a beam of Dy atoms is first extracted from a cryogenic buffer gas source, and when this beam reaches the trapping region, a fraction of the atoms are stopped through collisions with cold helium gas present in the trapping region. This second method reduces the density of buffer gas required in the trap region. The trap lifetime achieved in this arrangement of 810 ± 40 μs is no longer than in the direct trapping experiments, but this arrangement is much more stable and repeatable. The lifetime here is also thought to be limited by collisions with background buffer gas atoms.

Despite the continued success of the Large Hadron Collider, no clear evidence for the existence of new BSM particles has been identified to date, pushing the bounds on their masses to ever higher values. As such, increasing efforts have been made to constrain all remaining regions of parameter space where light new particles could still exist. To do so reliably requires accurate Monte Carlo simulations of signal events, often in the case that hard radiation is produced together with the new particles. In this thesis, we focus on using matrix-element corrections based on the Powheg formalism to improve the simulation of hard radiation produced in new physics events. The corrections have been implemented within the Herwig++ Monte Carlo event generator, both for squark-antisquark production at the LHC and a wide range of decay modes that occur in beyond the Standard Model physics scenarios. Taking supersymmetry as a test case, we find that corrections applied to radiation generated during either the production or decays of new particles each impact on the reach of analysis strategies sensitive to high transverse momentum jets, with the most important effect occurring when the former correction is applied in scenarios featuring a compressed new particle mass spectrum. Finally, we investigate the sensitivity of the LHC to supersymmetric scenarios using monotop signatures of a single top quark produced together with missing transverse energy. We present analysis strategies sensitive to compressed regions of parameter space, and compare their expected reach at the next run of the LHC to those of more traditional search strategies.

This thesis presents progress toward the production of ultracold CsYb molecules. To this end, an apparatus capable of producing magneto optical traps of Yb and Cs was designed, built and tested. Both atoms are produced in a dual species oven and both slowed to low speeds by a single Zeeman slower. From the Zeeman slower atoms are captured in a dual-species magneto-optical trap. To cool caesium the 852 nm D₂ transition is addressed by two lasers for cooling and repump. For ytterbium the 399 nm ¹S₀ → ¹P₁ transition is addressed for the Zeeman slower and the 556 nm ¹S₀ → ³P₁ transition is addressed for the magneto-optical trap. The 399 nm light is produced by two homebuilt diode lasers in an injection-seeding setup, which can produce up to 100 mW. The 556 nm light is produced from a commercial frequency doubled fiber laser, which can produce up to 260 mW. The Zeeman slower is characterised experimentally for both Cs and Yb, and the results compared to those of a numerical simulation of the slower for Yb. The velocity distribution exiting the slower is very sensitive to the exact magnetic field profile, the laser power and detuning of the laser light. The number of atoms loaded into the magneto-optical trap was investigated as a function of the magnetic field gradient, the laser power and the laser detuning. The capture velocity of the Yb MOT is small because the linewidth of the MOT transition is narrow, and so we investigated the influence of broadening the laser linewidth by adding multiple finely-spaced sidebands to the laser light. After optimisation the caesium MOT trapped 5.5 x 10⁸ atoms at 125 ± 4 μK. The ytterbium MOT trapped 4.7 x 10⁹ atoms at 81 ± 2 μK. Lastly we demonstrate that both MOTs can be produced in the same vacuum chamber simultaneously.

Although barium titanate is one of the most widely studied ferroelectric materials, questions about the nature of its phase transition remain. There are two competing models for this transition (displacive and order-disorder) and experiments detect signs of both types of transition. To study this issue computationally requires the simulation of large, disordered systems on the atomic scale. In this PhD I have developed a new classical force field for BaTiO3 which, in essence, is an ionic model. The free parameters within this force field were fitted to data generated from density-functional theory (DFT) simulations. Properties that were not used in the fitting procedure calculated with the resulting force field were found to be in excellent agreement with those calculated via DFT. The potential energy surface of BaTiO3 has been explored in detail using DFT and the force field, enabling both sublattice and single ion displacements to be studied, and highlighting in particular the important role played by the oxygen ions. Dynamical simulations show behaviour compatible both with experiments that have been interpreted as evidence for displacive transitions and with experiments interpreted as evidence for order-disorder transitions. In particular, when the local structure of the phases was studied, the locations of the ions were found to be consistent with a displacive model, whereas the calculated distribution of polarisation densities was more characteristic of an order-disorder model. The direction of the local relative displacements of titanium and oxygen ions was also considered. The deviation of this displacement from the < 111 > directions produced excellent agreement with experimental measurements of the tetragonal phase, and a similar effect was found in the orthorhombic phase. These dynamical simulations were found to be very sensitive to a range of factors such as supercell size.

Organic electronics is an exciting new avenue for low cost electronics. The unique properties of organic semiconductors may enable a new generation of electronic devices to be fabricated into flexible, large area, and even transparent consumer products. However, for this to become a reality, many challenges must first be overcome. As the performance of these materials continues to improve, it is now necessary to look to new manufacturing methods and materials that can fully exploit the advantages of organic materials. The work presented in this thesis is focused on the development of new and high resolution fabrication methods which are compatible with organic electronic materials. The findings presented in the first half of this thesis are based on the idea that fundamentally new forms of manufacturing are required to match the unique properties of organic materials. Initially the adhesion properties of several materials are analysed with a focus on how they interact at the nano-scale. Further work then outlines how adhesion forces can be manipulated and used to produce highly aligned nano-scale electronic devices, something that until now has required high cost and specialist equipment. The second part of this thesis describes how existing fabrication methods can be modified to produce high performance organic devices. By creating self-aligned organic transistors, higher frequency device operation and enhanced performance may be possible. New materials such as graphene and low voltage nano-scale dielectrics are tested in this configuration and compared with similar devices reported in the literature.