Graphitic systems have an electronic structure that can be readily manipulated through electrostatic or chemical doping, resulting in a rich variety of electronic ground states. One such class of materials are graphite intercalation compounds, a signi cant number of which exhibit superconductivity at low temperatures. The optimal superconductor in this class is CaC6, which superconducts at 11.5 K. This thesis presents the rst atomic-resolution surface studies of CaC6 revealing not only the surface structure but also, via detailed spectroscopic imaging and Fourier transform techniques, a stripe phase. This stripe phase corresponds to a charge density wave with a period three times that of the Ca superlattice. The work presented in this thesis is the rst such reporting and detailed characterization of a charge density wave in a graphitic material. Of further interest is the fact that the stripe phase is found to modulate the Ca lattice but no distortion of the graphene lattice is found indicating the graphene sheets may host the ideal, purely electronic, charge density wave. The implications for the understanding of superconductivity in CaC6, graphene and the apparent ubiquity of CDWs with superconductivity in anisotropic layered materials are discussed.

No description available.

The bulk of the mass in star-forming environments is contained in photodissociated regions (PDRs) where the chemistry is controlled by the far ultraviolet radiation field coming from nearby ionizing sources. H II regions can be found throughout active star forming galaxies as they are also controlled by the presence of a nearby ionizing source. In fact, H II regions, PDRs, and molecular clouds are not distinct physical regions, but rather a series of layers that are unified by the transport of radiation from hot stars into cold gas and the flow of cold molecular gas into hot ionized regions. The relative contributions of the ultraviolet radiation field from different gas phases can be estimated from observations of several far infrared forbidden lines such as [C II]158\mum, [O I]145\mum and 63\mum. Fine structure emission line studies provide valuable information on H II and PDR regions and it is well known that they originate in the outer part of both H II and PDR regions. The only exception are the nitrogen lines that clearly come exclusively from ionized regions. With the launch of ISO the far infrared properties of galaxies were observed with greater sensitivity than ever before. This thesis deals with the study of [C II]158\mum, [O I]145\mum, 63\mum and partly CO in external galaxies. In Chapter 3 we investigate the [C II]158\mum, [O I]145\mum and 63\mum emission lines in a sample of external galaxies in order to use them as diagnostics to infer the physical conditions in the gas, such as temperatures, densities and radiation fields. This study was carried out using the photodissociation UCL PDR code, testing PDR model results against ISO LWS observations. In addition, a detailed study of the oxygen self absorption, computed by modelling the emission line profiles with the radiative transfer SMMOL code, was conducted. In Chapter 4 a detailed study of the starburst NGC 4038 has been performed by using both MOCASSIN and UCL PDR. The contribution of [C II] 158\mum, [O I] 63\mum and 145\mum lines coming from the H II region was studied using the 3D ionization MOCASSIN code. We then used the computed radiation field at the ionization front as an input for the photodissociation UCL PDR code and an examination of the oxygen self absorption and a comparison of the CO rotational emission lines with CSO observations, were also presented. Finally, to improve the interpretation of future observations such as those from ALMA and HERSCHEL, a feasibility test on the coupling of the two codes (MOCASSIN and UCL PDR) was performed and the results are presented in Chapter 5. Conclusion and ideas for future work are presented in Chapter 6.

We use numerical simulations to probe the evolutionary history of spiral galaxies such as our own Milky Way. We use the chemodynamical simulation code GCD+ to simulate several galaxies both in a cosmological and isolated environment. The simulations include gravity, hydrodynamics, radiative gas cooling, star formation, stellar evolution, metal production and feedback into the interstellar medium. We describe in detail how these physical processes are implemented in two different versions of the code adopted in this thesis. The simulations are compared with observations in order to disentangle the details of spiral galaxy formation. Several unresolved issues regarding the evolution of spiral galaxies are specifically addressed. We first analyse the properties of stars found in the bulge component of our simulated spiral galaxies, finding that stars formed during mergers at different epochs show different elemental abundance ratio [α/Fe]. Stars formed during one of the merger events retain a systematically prograde rotation at the present time demonstrating that ancient orbital information may still be preserved in the present day kinematics of bulge stars. Next, we analysed the radial abundance gradients along the disc, comparing with recent observations from the Milky Way. We found that the disc contains a greater fraction of young stars in the outer regions, with older stars in the inner regions. This could explain the positive [α/Fe] and negative [N/O] gradients with radius. These radial trends are a natural outcome of an inside-out formation of the disc and could thus explain the recently observed positive [α/Fe] gradients seen in Milky Way disc open clusters. Finally, several isolated galaxy evolution simulations were carried out using the new and improved version of our N-body/smoothed particle hydrodynamics code. We show that our models with higher energy feedback from supernovae and stellar winds more closely resemble the observations of spiral galaxies.

The inclusive measurement of charm production in deep inelastic scattering with the ZEUS detector at HERA is presented in this thesis. In a first step, the existing algorithm from the inclusive secondary vertexing analysis was generalised using the signed impact parameter of jet-associated tracks to extract the heavy quark content of the data sample. In a second step, the algorithm was extended by using the decay vertex in events containing a well-reconstructed secondary vertex, the impact parameter in events not containing a secondary vertex, and a combination of vertexing and tracking information in all other events. The combined algorithm has reduced the relative statistical and systematic uncertainties on the measured double differential cross sections in Q2 and x. The relative statistical error has been reduced by 40 % on average, whilst the systematic uncertainty decreased due to an improved tracking efficiency. In order to extend the kinematic region and further reduce the uncertainty on the extrapolation of the charm contribution, F2cc, to the structure function of the proton the cut on the transverse jet energy has been relaxed from ET > 4.2 GeV to ET > 2.5 GeV. An extrapolation of the visible cross sections to the full kinematic phase space was performed allowing an extraction of F2cc to be made for a well-defined set of points in the x and Q2 plane. A comparison with previous results shows that lowering the cut on the transverse energy has particularly reduced the uncertainty on the extrapolation in the low Q2 and low x region. The present analysis is well described by next-to-leading order QCD predictions generated with the HVQDIS program. At present it is the most precise measurement of charm production cross sections with the ZEUS detector, and will have a significant impact on future combinations of ZEUS and H1 data.

Fluctuations in the cosmic microwave background (CMB), the radiation left over from the Big Bang, contain information which has been pivotal in establishing the current cosmological model. CMB data can also be used to test theoretically well-motivated additions to the model, including pre-inflationary relics (signatures of bubble collisions arising in eternal inflation) and topological defects that form after inflation (cosmic strings and textures). These relics typically leave sub-dominant, spatially localised signals, hidden in the “noise” of the primary CMB, the instrumental noise, foreground residuals and other systematics. Standard approaches for searching for such signals involve focusing on statistical anomalies, which carry the danger of extreme a posteriori biases. The self-consistent approach to this problem is Bayesian model comparison; however, the full implementation of this approach is computationally intractable with current CMB datasets, and will only become more difficult with data from the next generation of CMB experiments. I will describe a powerful modular algorithm, capable of coping with the volume of data, which combines a candidate-detection stage (using wavelets or optimal filters) with a full Bayesian parameter-estimation and model-selection stage performed in pixel space within the candidate regions. The algorithm is designed to fully account for the “look-elsewhere” effect, and its use of blind analysis techniques further enhances its robustness to unknown systematics. Finally, I will present the results of applying the algorithm to hunt for the signatures of bubble collisions and cosmic textures in the seven-year data from the Wilkinson Microwave Anisotropy Probe.

Mechanical experiments on single polymers have provided force-displacement data for a number of species, including multi-stranded polymers such as DNA and collagen. The interpretation of this information at the mesoscale requires the use of statistical mechanics to calculate the free energies of various structures. In this thesis, we look initially at the behaviour of a single stranded polymer in extension and go on to consider two particular multi-stranded polymers, DNA and collagen, both of which have di erent but similar tertiary structures. In simplifying the multistranded structures evaluate the partition function by a transfer matrix approach in one dimension. The numerical solution of the equations provide the restoring force as a function of the extension of a single strand, as well as a description of the molecular distortion that is induced. In DNA we investigate molecular breakage schemes for base-pair potentials that simulate the severing of bonds when a single strand is pulled, and the tearing of one strand from another in triple-stranded collagen.

Density functional theory calculations were employed to explore the interaction between water and the {1014} surface of calcite. In addition a defective {1014} surface and stepped surfaces in contact with water were investigated. A series of percentage water coverages and water configurations were explored, including dissociated water states. Static relaxations found associated water to be favourable on the {1014} surface, although a metastable dissociated state 1.77eV higher in energy was found. Molecular dynamics (MD) simulations of low water coverage reveal fluctuations in the H-O water bond when the H atom is directed towards a surface CO3 ion. Desorption of an H2O molecule was observed in simulations above 900K. Water was found to be strongly bound to the perfect {1014} surface, with an adsorption energy of -0.91eV. MD simulations of a defective {1014} surface found water to favour dissociation at CO3 vacancies. However, water at Ca vacancies diused across the surface to form a bond with the nearest surface Ca ion. Water was also found to favour an associated state at both acute and obtuse steps. On all these imperfect surfaces water was found to adsorb strongly to the surface, with adsorption energies ranging from -0.99eV to -1.60eV.

Zinc Oxide (ZnO) exhibits a plethora of physical properties potentially advantageous in many roles and is why it one of the most studied semiconductor compounds. When doped or in its intrinsic state ZnO demonstrates a multitude of electronic, optical and magnetic properties in a large variety of manufacturable morphologies. Thus it is inherently important to understand why these properties arise and the impact potentially invasive sample preparation methods have for both the function and durability of the material and its devices. Coherent X-ray Diffraction Imaging (CXDI) is a recently established non-destructive technique which can probe the whole three dimensional structure of small crystalline materials and has the potential for sub angstrom strain resolution. The iterative methods employed to overcome the `phase problem' are described fully. CXDI studies of wurtzite ZnO crystals in the rod morphology with high aspect ratio are presented. ZnO rods synthesised via Chemical Vapour Transport Deposition were studied in post growth state and during in-situ modification via metal evaporation processing and annealing. Small variations in post growth state were observed, the physical origin of which remains unidentified. The doping of a ZnO crystal with Iron, Nickel and Cobalt by thermal evaporation and subsequent annealing was studied. The evolution of diffusing ions into the crystal lattice from was not observed, decomposition was found to be the dominant process. Improvements in experimental technique allowed multiple Bragg reflections from a single ZnO crystal to be measured for the first time. Large aspect ratio ZnO rods were used to probe the coherence properties of the incident beam. The longitudinal coherence function of the illuminating radiation was mapped using the visibility of the interference pattern at each bragg reflection and an accurate estimate of the longitudinal coherence length obtained, \xi(L) = 0.66\pm 0.02 \mu m. The consequences for data analysis are discussed. The combination of multiple Bragg reflections to realise three dimensional displacement fields was also approached.

The study of the interaction of positrons with atoms and molecules has become increasingly popular, because more and more experimental activities have become feasible. Although exchange effects are absent, the polarisation effects, caused by the attractive nature between the positron and the target electrons, make the positron-molecule collisions more diffcult to handle than the corresponding electron collisions. This thesis gives the calculations of positron collisions with polar molecule H2O and non-polar molecules, H2 and C2H2 at energies below the positronium formation threshold. All calculations were carried out using the modied version of the UK molecular R-matrix code. Due to the large permanent dipolar nature of water molecule, the three models tested give very similar results. However, for positron collision with non-polar molecules, the polarisation effects can be very important in the calculation. The molecular R-matrix with pseudostates (MRMPS) method has been employed to analyse the positron collision with non-polar molecules, and found to lead to an excellent representation of target polarisation. C2H2 is the simplest molecule that has very enhanced annihilation parameter Zeff, which can be determined by the total scattering wavefunction. So a new sub-code was developed for calculating Zeff based on the UK R-matrix polyatomic code and employed to treat positron collisions with H atom, H2 and C2H2 molecules. It has been found that Zeff values are also sensitive to the degree of polarisation included in calculations and are greatly improved by use of the MRMPS method.