The negatively charged nitrogen vacancy centre is a promising candidate for use as a single photon source for linear optical quantum information, and as a solid state spin for solid state quantum information and room temperature magnetometry. However low photon collection efficiency is a problem for each of these applications. We demonstrate how photon losses due to refraction can be eliminated through the use of Solid Immersion Lenses (SILs) nano-fabricated on the surface of diamond. Coherent electron spin manipulation and readout is demonstrated on NV- centres under SILs. We show initial investigations into the effects of FIB fabrication on the NV- centre's coherence time, and demonstrate unitary quantum process discrimination on between two non orthogonal processes. In order to improve collection efficiency further it is necessary to couple NV- centres to optical micro cavities. This requires a higher degree of precision in the measurement of the NV- centres position than is possible using conventional confocal microscopy. We investigate spectral self interferometric microscopy as a method for precision measurement of the depth of an NV- centre. Finally we show coherent manipulation of photons emitted from a near infra-red colour centre in diamond using a single integrated waveguide chip. This is used to verify wave particle duality of the photons.

Neutron scattering from the free surface of liquid He has been used to investigate the distribution of 3He atoms in extremely dilute 3He-4He solutions at temperatures down to 0.08 K In particular evidence has been sought and found for the existence of Andreev trapping states for 3He just below the surface. The experiments were conducted in ISIS at the Rutherford Appleton Laboratory using the CRISP instrument (a general purpose reflectometer). The neutron scattering from the surface of 4 He (industrial and high purity) also was investigated and compared with dilute 3He-4He solutions. The 3He surface layer was observed for the first time using a small-angle neutron scattering (SANS) technique. The thickness of the layer is strongly dependent on temperature and 3He concentration. Even in the case of industrial 4He and even at 2.3 K (higher than superfiuid transition) a substantial amount of 3He atoms was found close to the surface. In the case of extremely pure 4He the 3He atoms on the surface were not observed. At low temperature (0.4 K) a very thin layer of 3He atoms was observed and the calculated thickness (based on our experimental results) is approximately ~10 A of pure 3He on the top of the mixture. It is followed by a diffusive interface area (also observed experimentally) of 3He-4He mixture of thick layer approximately ~200 A, just below that almost pure 3He thin layer on the top. With increase of the concentration the thickness of the top layer increases until it reaches a limit above which the 3He starts dissolving in the bulk superfiuid liquid 4He. Neutron critical opalescence from 3He gas/vapour was observed for the first time and the information for 3He density was obtained in a broad range of temperatures 1.4 - 50 K. The data was compared with the results gathered by using different techniques. Far from the critical temperature all methods yield similar results. However, near the critical temperature of Tc = 3.32 K, the densities obtained from neutron transmission results are lower than other published results.

Torque magnetometry techniques have been employed to study the quantum Hall effect in several AIGaAs/GaAs-heterostructure two-dimensional electron gas samples at filling factors between 1 and 4. Two magnetometers were employed to acquire measurements for three samples an existing instrument was used to acquire data for two samples and a novel instrument has been developed in which the output signal sensitivity is increased by 700% during experiments on a third sample. The third sample was also illuminated in situ. The samples exhibit breakdown-like behaviour in two forms. The first is the saturation of the magnetic moment peak size with respect to increasing sweep rate. A simple charge-up model, which treats the charge density of the sample in terms of a capacitance around the edge of the sample, was used to analyse experimental data. Temperature dependence of the longitudinal conductivity is analysed with respect to a published model of Polyakov and Shklovskii. The charge-up time constant, rc (10--105) seconds, decreased with increasing temperature and was found to follow a straight line when plotted on a logarithmic scale against temperature. Characteristic temperatures extracted from the data lie in the range T0 * (0.2--2.0) Kelvin. Decay time measurements were performed to acquire the decay time constant rd. Two regimes of decay were observed, the first exhibiting a time constant of the order of several seconds followed by a second phase with a much larger time constant many minutes or hours. The second form of breakdown was demonstrated as a type of noisy breakdown clearly observed at filling factor 2 in two samples. This noise, of the form of sudden jumps followed by more gradual growth, was interpreted using an edge charge-up model and is thought to be consistent with the sand-pile model of the theory of self-organised criticality. As a result the frequency of occurrence of noisy jumps as a function of their particular size is seen to follow a power law. Time constants of individual noise jumps were found to lie mainly in the range (1--10) seconds.

This dissertation consists of a theoretical investigation into the transport and coherence properties of indirect excitons in coupled quantum wells (QWs) at helium temperatures. The motion of excitons along the quantum well plane is described through a quantum diffusion equation and the possibility of excitonic cloud formation is studied both due to the natural potential fluctuations and externally applied confining potentials. The photoluminescence (PL) of decaying excitons is used as a probe for their properties such as concentration, effective temperature and optical lifetime. The exciton thermalisation from an initial high energy to the lattice temperature is achieved within their lifetime due to a very effective coupling between the exciton states and a continuum of phonon states, a direct consequence of the relaxation of momentum conservation along the growth direction of a QW. Moreover, the natural spatial separation between electrons and holes prevents their recombination, resulting in long lifetimes. The dynamics of the system of excitons in optically-induced traps is also studied and the numerical solution of the quantum diffusion equation provides an insight into the extremely fast loading times of the trap with a highly degenerate exciton gas. The hierarchy of timescales in such a trap allows for the creation of a cold and dense gas confined within the trap, opening a new route towards the long sought Bose-Einstein Condensation (BEC) in solid state. Finally the issue of exciton spatial coherence is studied and an analytic expression for the coherence function, i.e., the measure of the coherence in a system, is derived. A direct comparison with large coherence lengths recently observed in systems of quantum well excitons and microcavity polaritons is attempted and interesting conclusions are drawn regarding the build up of spontaneous coherence in these systems.

Recent advancements in the semiconductor industry have been driven by the extreme downscaling of device dimensions enabled by innovative photolithography methods. However, such nano-scale patterning technologies are impractical for large-area electronics primarily due to extremely high cost and incompatibility with large-area processing. Therefore, alternative techniques that are simpler, more scalable and compatible with large-area manufacturing are required. This thesis explores the technological potential of two recently developed patterning techniques namely interlayer lithography (IL) and adhesion-lithography (a-Lith) for application in the field of large-area nano/electronics. The IL method relies on the use of a pre-patterned metal electrode that acts as the mask during back illumination of a photoresist layer followed by a conventional lift-off process step. On the other hand in the a-Lith approach, the surface energy of a patterned metal electrode is modified through the use of surface energy modifiers such as organic self-assembling monolayer (SAM). Following, a second metal is evaporated on the entire substrate. However, because of the present of the SAM, regions of metal-2 overlapping with metal-1 can easily be peeled off with the aid of an adhesive layer (e.g. sticky tape) leaving behind the two metal electrodes in close proximity to each other. Analysis of the resulting structures reveals that inter-electrode distances < 20 nm can easily be achieved. The method was then used to develop innovative process protocols for the fabrication of functional self-aligned gate (SAG) transistor architectures. Best performing devices exhibited charge carrier mobility in the range of 0.5-1 cm2/Vs, high current on-off ratio (~104), negligible operating hysteresis and excellent switching speed. Using the same a-Lith process protocol, low-voltage organic ferroelectric tunnel junction memory devices were also developed by combining the metal-1/metal-2 nanogap electrodes with a ferroelectric copolymer deposited in-between them. Controllable ferroelectric tunnelling was observed enabling the devices’ conductivity to be programmed using low biases and hence been used as a non-volatile memory cell. The alternative and highly scalable patterning methods described in this thesis may one day play a significant role on how largearea electronics of the future would be manufactured.

No description available.

Superconductors are commonly used in many magnet systems available to day. One of the most popular is Nb3Sn, due to its high upper critical field and uncomplicated structure. It is however, not particularly well under stood at the microscopic scale with respect to grain boundaries and strain. Grain boundaries appear when forming the material and are used as an effective way to increase the upper critical field by increasing the normal state resistivity, but there is a trade-off, as the critical current density drops as the material becomes more disordered. In addition, when the material is strained by magnetic fields, the superconducting properties will vary. Much experimental work has been performed to study these experimental effects, but a first-principles study gives a unique insight into the intrinsic properties of the material itself. This thesis gives a record of investigations into the strain and grain boundary dependence of Nb3Sn as well as determining whether we can use density functional theory to determine superconducting properties from first principles. This work includes an efficient implementation of a method to calculate the electron-phonon coupling matrix elements from first principles via density functional perturbation theory. This method is tested on some simple metallic elements and shown to provide coupling strengths close in agreement to experimental work.

With the aid of Lang-Firsov transformation, a single particle Green's function describing the propagation of small Fröhlich polaron has been derived. One- and two-dimensional spectral functions are studied by expanding Green's function perturbatively. SIS tunnelling cuprates can also be described by some closely-related type of this Green's function. The theory allows us to determine the characteristic phonon frequencies, gap parameter, inverse-lifetime parameter and the electron-phonon coupling from the tunnelling data.

This thesis reports the synthesis and characterisation of several layered pnictides, chalcogenides, and oxychalcogenides, with an emphasis on materials that exhibit high temperature superconductivity. High and low temperature techniques have been used to synthesise new materials and modify their properties. The majority of this work has been focused on the synthesis of superconducting materials with the general formula Ax(NH3)y(NH2)zFeSe, where A is an alkali metal. These materials are formed from a co-intercalation of alkali metals, ammonia, and alkali metal amides into the interlamellar space of pre-formed tetragonal FeSe. There is a remarkable increase in Tc associated with this intercalation, from 8 K in FeSe to a maximum of 46 K in the products. A range of characterisation techniques including neutron and X-ray diffraction, EXAFS, and SQUID magnetometry have been used to identify a variety of crystal structures, compositions, and properties adopted by these materials. The synthesis procedure of these materials where, A = Na and K, has been studied in-situ via powder X-ray diffraction at world-class central facilities, revealing new phases, intermediates, and activation energies. The Kx(NH3)y(NH2)zFeSe phases are found to undergo a topotactic decomposition step to become Kx'Fe2-y'Se2 phases on annealing, which has also been studied by in-situ powder X-ray diffraction. Additional studies on Na1-xFe2-yAs2 and CaFeSeO have been performed. Na1-xFe2-yAs2 is the product of a room temperature deintercalation of sodium and iron from NaFeAs, which changes the superconducting properties of the material. XAFS measurements have been used to characterise the local structure of the materials, which supports the conclusion that iron is deintercalated from the parent material and gives new insight into the effect of the iron vacancies on the local structure. CaFeSeO is a newly discovered material that adopts a never-before-seen crystal structure, which has been solved from powder X-ray diffraction data. Intricate vacancy ordering exists in the material, which contributes to a peculiar mixture of magnetic behaviours including signatures of a spin glass, ordered antiferromagnet, and an ordered ferromagnetic component. All of these behaviours however, can be rationalised by the nuclear and magnetic structure of the material that have been refined using powder neutron diffraction.

Nanoelectromechanical systems (NEMS) can measure very small forces and mass as has been showcased in the last decade by the demonstration of measurements ranging from single spin detection and mass spectroscopy to the read-out of the quantum ground state of a mesoscopic resonator. Mass spectroscopy with NEMS is particularly appealing because the vibrational frequency of NEMS is a sensitive function of its total mass; thus minute changes in mass due to added or removed adsorbate will change the resonance frequency of a nanomechanical resonator. Indeed, single molecule detection has recently been demonstrated using NEMS as a sensitive mass detector. To maximize mass as well as force sensitivity, resonators with low mass and high quality factors are required. Extreme stiffness, low mass, a high Young's modulus and good conductivity makes one atom-thick graphene a most suitable candidate for NEMS. However, achieving quality factors higher than 103 at room temperature has been a bottleneck for graphene NEMS. Extensive studies have been carried out on graphene NEMS by employing both optical, and electrostatic transduction techniques. Optical transduction requires large and complicated experimental setups. This restricts the use of this technique to low temperatures and high magnetic fields. Electrostatic sensing, another commonly used technique requires more complex circuitry and can damp the motion due to electrostatic force. In this thesis the use of piezoresistive transduction to transduce motion of graphene resonator is explored. Major advantages of piezoresistive sensing over other sensing methods are its fairly linear response, robustness, simple measuring circuitry and implementation. It has been demonstrated in the present work that piezoresistive sensing is not only a simple but also an extremely effective electrical readout method for graphene based nanoelectromechanical systems. The first part of the thesis starts with an introduction to Nanoelectromechanical systems (NEMS), explaining how it originates from simple electromechanical systems and then later evolved from Microelectromechanical systems (MEMS). Finite element method (FEM) analysis confirms that the stresses are concentrated at the legs of H-shaped mechanical resonator which we have used to maximize the piezoresistive effect of graphene. Modal analysis is performed employing Comsol Multiphysics to carry out the simulations in order to predetermine the frequency range, which is as the same order of the experimentally measured resonance frequencies of the devices. Thermoelastic damping (TED) simulations are carried out to show comparison between different structures of the resonators. Detailed fabrication processes using standard e-beam lithography to fabricate fully suspended H shape graphene resonator have been developed. Graphene resonators are electronically characterized using piezoresistive sensing. Detailed measurements such as piezomechanical and thermomechanical, frequency and time domain measurements are carried out. One order higher Q-factors (103) than the previous reported values for double side clamped beams in ambient temperatures has been measured. The minimum detectable mass and force resolution of such resonators are estimated using the experimental results to be an astounding 0.95-1.54 zeptograms (10-21 g) and 11.7-21.6 aN/Hz1/2 at room temperature respectively. Various nonlinearities in graphene resonators such as nonlinearity in spring constant as well as higher order nonlinear damping are carefully considered. Simulations as well as experimental results showing various nonlinear effects such as saddle node bifurcation, super-harmonics, Pol-Duffing and unstable states are discussed. In the last part of the thesis, some additional data of the higher order resonance modes with symmetric and asymmetric shape of the devices have been demonstrated. Interesting behaviors such as peak splitting, resonance and anti-resonance peak, have been experimentally observed. This is further confirmed with the experimental results from the commercial cantilever using AFM.