Observations of four rich clusters of galaxies made using the Position Sensitive Proportional Counter (PSPC) instrument onboard the German Rontgensatellit (ROSAT) observatory were analysed. These data were complemented by ROSAT Wide Field Camera (WFC), Ginga and optical Automated Plate Measuring Machine (APM) information. Cluster properties, including the temperature profile, gas mass and total gravitational mass were investigated. Estimates for the mass deposition rates of central cooling flows, where appropriate, were made. Substructure was investigated using isophotal analysis and the employment of hardness ratios. The presence of central, intrinsic absorption, discovered previously in some similar clusters was confirmed. This is probably due to cold gas clouds. In the case of A2199, the WFC/PSPC data provided useful constraints to the partial covering fraction of these putative gas clouds. Abell 2142, which is classified in the optical as a binary cluster was shown to have a complex substructure. It is highly elliptical out to ~ 2 Mpc which can be explained in terms of a merger between two subclusters of differing sizes. The temperature and luminosities measured for A2218 and A1061, both of which have high velocity dispersions, indicate that these dispersion measurements are overestimated. Substructure is the most plausible explanation; spatial analysis supports this view. The galaxy density profiles obtained from the APM data were found to be consistent with the King approximation, and their luminosity functions were in reasonable agreement with the 'universal' Schechter model with a characteristic absolute magnitude of Bj = -21.6. Finally, the x-ray data showed that the gas mass fraction continues to rise with radius at the limit of detectability of ~ 2 Mpc. Including the optical data, the baryonic fraction at this radius is = 0.3. This result is difficult to reconcile with the universal fraction determined from standard cosmological nucleosynthesis models.

The UK ROSAT Wide Field Camera was used to carry out the first all-sky survey at extreme- ultraviolet wavelengths (EUV). In this thesis, an optimised method for the extraction of survey light curves is developed and applied to an all-sky sample of eighty-one cataclysmic variables. Twenty-two sources were detected with high confidence. The sample of detected systems is dominated by AM Hers, which are found to be exceptionally strong EUV sources. Six of these are new systems, identified through optical follow up of survey sources, and these EUV-selected systems are found to have the most extreme EUV/optical flux ratios. Orbital periods are detected in most AM Hers, and the implications of the new orbital-period distribution of AM Hers are discussed. Phase-folded light curves of a flux limited sample are presented and spectral constraints are applied. Three AM Hers show evidence of spectral modulation with phase. Three intermediate polars are detected, though a distinct EUV emission component is required only in the case of RE 07514+14. Upper limits to other systems show that RE 0751 is unusually bright within the bandpass of the WFC. Limits are placed on temperatures and absorption columns of similar components in other systems, but the WFC does not provide useful limits on the luminosities of such components. The energy balance of intermediate polars remains an open question. Six non-magnetic cataclysmic variables were detected, four of which are dwarf novae caught in their outburst state. These include VW Hyi and SS Cyg, which are discussed in the context of extensive multi-wavelength coverage; incorporating optical, ultraviolet and X-ray observations. SS Cyg was detected as a strong EUV source during decline, which was found to be faster than the optical decline. VW Hyi was observed throughout an optical outburst, with but no enhancement of EUV flux. This suggests the EUV flux detected by EXOSAT must lie softwards of ~ 70 eV. WFC limits show that the boundary layer of VW Hyi during outburst must be less luminous than the disk. In VW Hyi and SS Cyg the hard X-ray flux is suppressed during outburst, and remains low until the very end of optical decline. This is taken as evidence that the hard X-ray flux is regulated by the boundary layer.

The structural and magnetic properties of rare earth metal overlayers have been investigated using techniques utilising synchrotron radiation. The magnetic ordering in ultrathin Tb layers was studied using Spin Polarized Photoelectron Diffraction. Magnetic information is obtained from the variation with temperature of the relative intensity of the peaks in the Tb 4s multiplet splitting. Short range magnetic order is shown to exist in the ultrathin Tb film up to temperatures significantly higher than the bulk transition temperature. The growth mode of Gd films on Mo(110) was investigated using Surface X-ray Diffraction. The formation of the Gd adlayer was found to change with substrate temperature. At room temperature the growth mode was Stranski-Krastanov, but at higher temperatures, the growth proceeds by step-flow and the reflected X-ray intensity decays. The growth curve is parabolic at the start of deposition up to 1monolayer before becoming a constant value for the duration of the deposition. The epitaxial growth was found to improve if the substrate was first exposed to a controlled dosage of oxygen gas. Oscillations in the reflected X-ray intensity were observed up to a coverage of 7 monolayer. It is proposed that oxide clusters form on the surface and act as nucleation sites which facilitate epitaxial growth. The magnetic properties of ultrathin Gd films have been studied using Circular Magnetic X-ray Dichroism. To induce magnetisation in the Gd film, a buffer layer of Fe was deposited on to the Mo(110) substrate prior to deposition of the Gd film. Sum rules were used to calculate the expectation value of the orbital angular momentum operator, (Lz), which was found to increase as the thickness of the Fe buffer layer was increased.

The latest technological developments have resulted in a push for faster, cheaper and simpler photodetector technologies across a wide range of temperatures, wavelengths and sensitivities for use in industrial and research applications. Graphene, a 2D allotrope of carbon, is seen as an interesting route for future photodetectors. Recent research into graphene has focussed on fundamental physics, fabrication processes and future commercial applications. Fundamental research has demonstrated many interesting properties, including the potential for high carrier mobility, high conductivity, approximately constant photon absorption and extreme tensile strength. These properties led to promising developments for graphene-based photodetectors, such as the demonstration of ultrafast photodetection on a femtosecond timescale for pulsed lasers. This document discusses novel graphene-based photodetector technologies from concept to theory, design, fabrication and experimental demonstration. Three detectors, from terahertz to X-ray, were fabricated in essentially the same graphene field effect transistor (GFET) structure, with photons coupling to different components of the detector to provide a measurable photosignal. A simulated cryogenic, colour sensitive, bilayer graphene single photon counting photodetector exploited the tuneable band gap of bilayer graphene to trade-off resolution against temperature to enable higher temperature operation, requiring less costly and complex cryogenics, with photons coupling directly to the bilayer graphene. The passive terahertz detector utilised photons from a broadband terahertz source coupling to antennae to generate a photoresponse via the Dyakonov-Shur effect with an NEP=0.85±0.15μWHz-0.5, with further work ongoing to demonstrate narrowband terahertz detection. The X-ray GFET was developed to investigate the energy sensitivity to X-ray photons coupling to the absorber based on work in the literature, where charge carrier modulation generates a field that changed the conductivity of the graphene channel. Using pulsed optical lasers to probe the behaviour and sensitivity of the detector gave ΔE~480keV (for E=30MeV) with a photoresponse dependent on the gate voltage. No X-ray sensitivity was observed for Fe-55 sources, but it was observed for an X-ray generator; this inconsistency possibly suggests a different mechanism, such as bolometry, to that proposed previously in the literature.

Space-based measurements of atmospheric carbon dioxide (CO2) provide global coverage with repetition times on the order of days. These measurements are used in combination with flux inversion models to track and identify sources and sinks of carbon. The ultimate goal is a better understanding of natural and anthropogenic contributions to the global carbon cycle, from which mitigation strategies and policies can be derived to deal with the effects of climate change. The algorithms responsible for inferring the atmospheric concentrations of CO2 from the high-resolution spectroscopic measurements are the so-called retrieval algorithms. This thesis focuses on two main aspects that are important for a successful retrieval strategy, and both have applications beyond CO2 retreivals. The first part of this thesis is centred around solar-induced chlorophyll fluorescence (SIF), a naturally occurring radiance signal produced by vegetation as a by-product of photosynthesis. Due to its spectral signature, it is observed by satellite measurements in the O2 A-band at 0.76 μm. Based on an established retrieval concept, the SIF retrieval was implemented and its impact on CO2 retrievals has been evaluated. The SIF retrievals themselves are of great interest to carbon cycle science, and have been used for two case studies: relating SIF to primary production, and tracking the biosphere response to the 2012 North American drought. In the second part, the focus of the thesis is on fast radiative transfer (RT) methods, which are acceleration techniques to speed up the computationally very expensive line-by-line RT calculations. A novel method based on principal component analysis has been implemented and further advanced. This allowed for the PCA-based method to be used in CO2 retrievals for measurements from the OCO-2 instrument. Finally, for the first time, a comparison of three popular fast RT schemes has been performed in a consistent way using the same retrieval algorithm.

The dwarf spheroidal galaxies (dSphs) of our Local Group pose challenges to our theories of galaxy formation and evolution in a ΛCDM Universe. Despite the wealth of information available from these systems provided by their close proximity, little is understood about their individual histories. The faintest dSphs host a single, ancient stellar population, the formation of stars in these galaxies ceased billions of years ago and they are devoid of gas. As these systems are so small, they are susceptible to gas loss, however this cannot be a simple process due to the diverse range of properties they exhibit, despite residing in haloes of the same mass. In this thesis I address two processes resulting in gas removal at early times and attempt to further our understanding of these systems by isolating each process. I use hydrodynamical simulations to explore the impact of each process to asses their significance and to identify the dominant influences on their evolution. Firstly I simulate the effect of SNe feedback on dSph progenitors undergoing an initial burst of star formation at high redshift (z~10) and the conditions under which they retain gas. In order to host an extended burst of star formation the number of supernovae must be lower than that expected if we assume star formation proceeds in a similar fashion to that in larger galaxies. The impact of an AGN outflow originating from the Milky Way on the surrounding dwarf galaxies is also simulated, which easily strips gas from these systems, leaving behind a halo with an ancient stellar population. The difficulty of retaining gas in the early Universe could be alleviated if they accreted gas during the period of extended star formation. I investigate these three process in the context of isolated dwarfs.

No description available.

It is widely believed that quantum physics is a fundamental theory describing the Universe. As such, one would expect to be able to see how classical physics that is observed in the macroscopic world emerges from quantum theory. This has so far largely eluded physicists, due to the inherent linear nature of quantum physics and the non-linear behaviour of classical physics. One of the principle differences between classical and quantum physics is the statistical, probabilistic nature of quantum theory. It is from this property that non-classical states can arise, such as entangled states. These states possess maximal correlations. However, they are not the only way in which correlations are created in quantum systems. This thesis aims to show how open quantum systems naturally contain correlations from their quantum nature. Moreover, even seemingly simple open quantum systems can behave far more complexly than expected upon the introduction of quantum feedback. Using this effect, the dynamics may become non-linear and as such behave non-trivially. Furthermore, it is shown how these effects may be exploited for a variety of tasks, including a computational application in hidden quantum Markov models and a quantum metrology scheme that does not require the use of exotic quantum states. This results in the design of systems that benefit from the use of quantum mechanics, but are not constrained by the use of experimentally difficulties such as entanglement.

The interfacial morphology and the Dzyaloshinskii-Moriya interaction (DMI) have been investigated in thin films of perpendicularly-magnetised epitaxial Pt(30 A)/Co(7 A)/Pt(10 A) and Pt(30 A)/Co(7, 9, 11 A)/Ir20Mn80(20, 50 A). In Pt/Co/Pt, altering the substrate temperature during deposition of the top Co/Pt bilayer in the range of 100 C to 300 C formed a peak in total interfacial disorder with deposition temperature of 3.3 A at around 250 C. The coercivity increased from 230 +/- 60 Oe to 400 +/- 60 Oe whilst the anisotropy and saturation magnetisation stayed constant at 15.0 +/- 0.3 kOe and 90 +/- 10 microemu, respectively, when the substrate temperature at deposition was reduced in value. In contrast, the anisotropy increased with deposition temperature if either the substrate temperature was increased, or the deposition temperature of only the top Pt layer was varied, though not both simultaneously. The bubble domain expansion technique was used to measure domain wall velocity with applied magnetic fields in various in-plane (IP) and out-of-plane (OOP) configurations. Of the three prominent creep models used, it was found that the most basic model fitted best. A moderate linear correlation was seen between DMI and the difference in disorder between the top and bottom Co interfaces, indicating that as the disorder in one interface increases, its contribution to DMI decreases, allowing the relative interface disorders to dictate the strength, and even sign, of the net DMI. The net DMI increased with increasing deposition temperature, also, showing that such a growth parameter can be used to fine-tune the DMI. Using Pt/Co/Ir20Mn80 samples, the growth temperature and working pressure were optimised for DMI measurements. Samples with 50 A of IrMn, with exchange bias strengths up to 1000 Oe, showed a high density of small, rough domains forming during magnetisation reversal. Samples with 20 A of IrMn, so without exchange bias, produced well-formed bubble domains for Co thicknesses below 9 A, with a Voronoi-like network seen in an 11 A sample. Bubble domain expansion showed severely skewed velocity against IP field graphs, with behaviour at higher fields not predicted by current models, giving DMI constant values between -0.6 and -1.0 mJ/m^2.

We present a new model of atomic decoherence by space-time perturbations. We propose that decoherence will arise as a result of two possible effects that gravitational fluctuations will have on the atom. One is that the nucleus will be displaced relative to the valence electron, which will be perceived as a sudden change in the electric potential. This will result in the wave function of the atom being partially projected into lower energy levels. The other is that the strain in space will change the local electric field as felt by the electron. This interaction will either induce a change in the angular momentum of the atom or a small shift in the transition of the energy levels, presenting two different experimental approaches for the detection of the effect. We calculate how the decoherence is related to the internal degrees of freedom of the atoms, obtaining that the effect will be more prominent for atoms initially in a highly excited state (Rydberg atoms). By applying the nuclear displacement model for the scatter- ing of neutral particles, we suggest that it could be potentially useful for the detection of weakly-interacting particles, like possible candi- dates of Dark Matter. The overall effect of gravitational waves for the strained-space model was calculated to be several orders of magnitude higher than for the nuclear displacement model, allowing for detection in different ranges of frequencies. We analyze how different quantum states are affected according to the proposed model, calculating that the information from the measurement of correlated atoms will be significantly higher. The optimal quantum state that minimizes the uncertainty of the measurement is described for an arbitrary number of atoms, giving a relation that follows closely the Heisenberg limit.