Дисертації з теми "621.318:537.6"

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.

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.

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.

Dilute magnetic semiconductors (DMSs) are ideal candidates for spintronic devices as they exhibit both semiconducting and magnetic properties. The defining feature of a DMS material is the exchange interactions between the magnetic ions and the band electrons and holes, which leads to many of the spin behaviours observed. A fundamental property of DMSs is that a relatively small external magnetic field can cause enormous Zeeman splittings of the electronic energy levels, which allows separating of states with different spins. The giant Zeeman effect present in the DMS systems also leads to the possibility of trapping quasiparticles in an inhomogeneous magnetic field. In this thesis the effect of inhomogeneous magnetic fields on excitons in a DMS quantum well is investigated. We look at the possibility of trapping excitons in hybrid structures composed of a DMS quantum well placed a few nanometres below a nanoscale and microscale ferromagnetic disk and a ferromagnetic strip. Quasiparticles in a DMS quantum well are shown to undergo a splitting between band states for different spin components due to the giant Zeeman interaction. Due to the inhomogeneous magnetic field created by a nanoscale ferromagnetic disk in the vortex state the quasiparticles are found to be confined in a small region on the quantum well. The behaviours of excitons in the presence of both a homogeneous and inhomogeneous magnetic field is then discussed. The binding energy of a heavy hole exciton in a finite DMS quantum well in the presence of a homogeneous is calculated. The study is extended to look at excitons in the presence of an inhomogeneous magnetic field. The behaviour of excitons in the presence of a inhomogeneous magnetic field, is found to depend on the type of magnetic field, and is shown to be different for a magnetic field created by a microscale and nanoscale ferromagnetic disk and a ferromagnetic strip.

Existing as much more than just a gemstone or refractory material, boron-doped diamond is a semiconductor with enormous potential as an electronic material. Diamond will form the basis of future high-power electronic devices, radiation-hard electronics and radiation detectors. It is also a highly effective electron emitter and a unique, biocompatible material for biomedical devices. Still, many questions remain surrounding the versatile, easy to grow polycrystalline form of boron-doped diamond. What is the surface atomic structure of these films after growth? How uniform is the boron-induced conductivity? How and why does the work function vary across the films? All of these properties affect how diamond electronic devices can be designed, fabricated and used. In this thesis we investigate nanoscale variation in properties across the surfaces of a number of differently grown boron-doped diamond films under ultra-high vacuum and evaluate the potential impact of the changes in these properties to surface electronic applications of diamond. Kelvin probe force microscopy results demonstrate significant variation in work function across the diamond surface, with a step change in work function of 0.8 eV measured between hydrogen and oxygen surface terminations, and variations across a single diamond surface are calculated to correspond to a 310% change in dopant concentration . The effect of this dopant variation is demonstrated by conductive atomic force microscopy studies in which entire crystallites exhibit insulating behaviour, with significant variation in conduction also observed across the surface. Finally, scanning tunneling microscopy studies of the diamond surface demonstrate that nanoscale roughness on large microcrystals is caused by the existence of a layer of nanocrystalline diamond at the surface. These nanocrystals persist throughout the growth process and exhibit many different surface reconstructions. The implications of all of these discoveries are discussed, with possibilities and suggestions for further work given also.

This thesis presents the results obtained from de Haas-van Alphen experiments in ironbased superconductors. Measurements of the quasi particle mass in the quantum critical system BaFe2(Asl- xPxh are presented, which show strong enhancement towards the critical composition Xc = 0.3. This is found to be in good agreement with the prediction of a diverging behaviour of the effective mass close to a quantum critical point. Further results obtained on the lower and upper superconducting critical field of this system will be presented, which are found to contradict the expectations from Ginzburg-Landau theory based on results of the quasi particle mass and London penetration depth. However we can reconcile the different experimental findings on superconducting and normal state properties, in this quantum critical system, by considering a significant contribution from Abrikosov vortex core states. The importance of understanding the normal state electronic structure and interactions is shown to be essential for an understanding of the superconducting ground state of a quantum critical system. Further we will show a detailed de Haas-van Alphen study of the il1-type iron-based superconductor LiFeP and its isovalent partner LiFeAs. To understand the formation of nodes on the superconducting gap structure in systems with little change in their Fermi surface topology, is essential for the formulation of a microscopic theory of the pairing mechanism in pnictide-superconductors. 'While we find both systems to be close to the geometric nesting condition, we are able to point to different possible scenarios of the origin of nodes based on quasi particle mass enhancement. Extending the study of quasi particle mass and its relation to the superconducting properties to the stoichiometric high-temperature cuprate superconductor YBa2 CU4 Os, we aim to study the Fermi surface evolution under hydrostatic pressure. As the system has a very stable oxygen stoichiometry which does not allow it to be doped by oxygen ordering, we use hydrostatic pressure to tune the system unexpectedly leading to an increase in the superconducting critical temperature with almost no change to the quasi particle mass .

Microcrystalline indium(III) selenide was prepared from a diphenyl diselenide precursor and a range of chloroindate(III) ionic liquids via a microwave-assisted ionothermal route. A mixture of nano- and micro-sized zinc(lI) selenide materials were also prepared using the same ionothermal procedure and chlorozincate ionic liquids. Influence of the reaction temperature and variation of the cation and the anion of the ionic liquid on the product morphology and composition were investigated. All products formed were characterised using PXRO, SEM and EOX. Additional characterisation was carried out using Raman spectroscopy and photoluminescence measurements. The investigation into the production of indium(III) selenide, zinc(II) selenide and gallium(III) selenide semiconductor materials using conventional heating methods was subsequently carried out and the products were characterized using SEM, EOX and ESI-MS. Electrochemical investigations into some of the stable homogeneous liquids that were formed between the chloroindatel chlorzincate ionic liquids with the diphenyl diselenide with conventional heating, have been carried out. An additional selenium precursor, selenium tetrachloride, was also investigated with the depositions analysed using SEM and EOX. Finally, the ternary compound copper indium selenide (CIS) has been prepared as nano- and micro-sized materials through colloidal synthesis using an indium(III) selenide precursor and copper(I) chloride via a microwave-assisted ionothermal route. The crystal structures of three intermediate structures were determined after formation through an ionothermal procedure utilising metal-containing imidazolium ionic liquids and a selenium precursor with conventional heating. A comparative study into the use of microwave irradiation over conventional heating with different ionic liquids on the stoichiometry of the resulting products was carried out. The influence of the reaction temperature, reaction time, order of addition of reagents and variation of ionic liquids on the final products was investigated, which have been characterized using PXRO, SEM and EOX. I

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.

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.

The dilute nitrides on GaAs (e.g. Ga(In)NxAs1-x where typically x < 5%) have provided a large amount of interesting physics and led to the development of improved telecommunication lasers. In the last 5 years, novel dilute nitride materials (such as GaN(As)P) have grown in popularity due to applications such as monolithic growth on silicon and improved device characteristics in red-amber-yellow solid state illumination. The Band Anti-Crossing model, using a single weighted nitrogen level (found through photocurrent spectroscopy), is found to successfully predict the band gap energy in GaNP/GaP and GaNAsP/GaP and bulk GaInNAs/GaAs with nitrogen composition, temperature and high hydrostatic pressure. An investigation over a range of host materials found that the coupling parameter can be modelled in terms of the energy separation between the host conduction band minimum and the nitrogen level. GaNAsP/GaP single quantum well lasers are a realistic possibility for an efficient laser source grown on silicon for monolithic optoelectronic integrated circuits. Large threshold current densities of over 0.75kA/cm2 at 80K and 890nm and large temperature variations in the threshold current are shown to be due to non-radiative loss processes. Low characteristic temperatures of the threshold current and differential efficiency (T0=60K and T1=30K at 130K) are found. The sub-linear dependence of the spontaneous emission vs current curve and increasing threshold current with high pressure reveal this loss process is carrier leakage. It is shown that the leakage is not into the X-minima of the GaP barriers, but associated with nitrogen levels in the GaP barriers. If this leakage path can be eliminated, a low room temperature threshold current density should be achieved. (In)GaNP/GaP LEDs are studied for use in red-amber-yellow illumination applications. Through high pressure measurements, it is found that 90 +/-10% of the current flowing through a GaN0.006P0.994 bulk LED is lost to carrier leakage at room temperature and pressure, at a current density of 40A/cm2. When using an In0.14Ga0.86N0.006P0.994 quantum well (of thickness 100 A) active region, the electron potential barrier is increased and carrier leakage accounts for 50+/-15% of the device current density (40A/cm2) at room temperature and pressure. At 100K it was found the leakage reduces to below 10% at current density of 40A/cm2 in all devices. If the leakage path can be reduced, these devices may offer better device characteristics than current red AlInGaP LEDs. Initial investigations on bulk GaInNAs avalanche photodiodes show that with an increase of nitrogen concentration and reduction in band gap, the breakdown voltage increases, consistent with the nitrogen suppressing electron impact ionization as predicted by theory. The pressure coefficient of the breakdown voltage in GaInNAs was found to be larger in magnitude and opposite in sign (dVbd/dP=+5.5+/-0.5 x10-3kbar-1) than the pressure coefficient of the breakdown voltage in GaAs, showing that nitrogen induces a profound change in the carrier scattering mechanisms, which may provide low-noise, high sensitivity detectors at telecommunication wavelengths.

This work studies the use of Ga0.12-0.i6In0.88-0.84Sb quantum wells grown by molec­ular beam epitaxy (MBE) on a highly mismatched substrate for use in light emit­ting diodes (LEDs) and lasers emitting in the 3-4 /μm spectral range. Quantum well samples were grown at Lancaster which had abrupt interfaces and showed room temperature photoluminescence (PL) emission between 3.6 and 4.0 /μm. Trans­mission electron microscope (TEM) imaging revealed a very high defect density of more than 10[10]10 cm-2 in the buffer layer and due to this, Shockley-Read-Hall (SRH) recombination was found to dominate the temperature quenching of the PL emission. Despite the structural problems with the material, the PL quenching performance compared well with other materials designed for this spectral range. Modelling of the quantum wells found that a conduction band offset ratio of 80% gave the best agreement with the experimentally determined transition energies. LEDs fabricated at Lancaster emitted in pulsed mode up to room temperature at 3.6 μm and with an efficiency of 34%. At room temperature SRH recombination was found to dominate the total recombination up to 350 mA drive current and this was also reflected in the temperature quenching of the LED output. From the fitting of the temperature dependence of the LED efficiency the SRH recombination centres were calculated to be 30-50 meV from the centre of the band gap. Fitting the room temperature LED emission spectra revealed that the emission comprised of transitions involving the first two heavy hole states as well as holes in the valence band of the barrier. The emission spectra from the edge of the LED mesa contained amplified spontaneous emission modes which were attributed to radial modes formed due to current crowding under the LED top contact. Lasers fabricated by QinetiQ were examined and from the gain spectra the internal loss was found to be -94 cm- 1 . This was attributed to the high defect density in the structure. The devices emitted at 3.2 /μm at 130 K and from po­larisation measurements it was found that the emission was completely polarised in the TE mode corresponding to emission from the heavy hole band. The lasers tested failed to reach operating temperatures above 130 K due to a sharp increase in the threshold current. An analysis of the temperature dependence of the thresh­old current provided evidence for hole current leakage as the cause of the increase in threshold current between 80 and 130 K.

In this work the Path Integral quantum Monte Carlo (PI-QMC) method has been used to study exciton complexes in semiconductor nanostructures. This powerful technique allows for coulomb correlations in these complexes to be correctly treated, and at the same time allowing for nite temperature simulations in an arbitrary external potential without the need for complicated trial function or basis set information. Quantum dots and rings were modelled using both analytic potentials, and by potentials derived from atomistic models of these structures, including strain and piezoelectric e ects. The e ect of strain and the piezoelectric potential on quantum rings is explored, and rings are shown to have a unique strain and piezoelectric pro le which directly impacts observables. This unique piezoelectric potential in quantum rings is exploited by use of vertical electric elds, to induce a novel lateral switching of the exciton and biexciton probability distributions when the direction of the applied eld is switched. Calculations of in-plane polarizability suggest the switching would be observable experimentally. The diamagnetic susceptibility of quantum rings and dots are investigated, and accurate reproduction of experimental results are shown { which require the proper treatment of coulomb correlations. Finally, the transition between a bound and anti-bound biexciton in a core/shell Type-II colloidal quantum dot, with increasing shell thickness is for the rst time theoretically shown. Excellent agreement with experimental results are seen, and these results are contrasted with previous perturbative results which miss this transition from the literature.

A method for computing electron momentum densities and Compton profiles from ab initio calculations is presented. This is employed, together with the momentum density spectroscopy known as Compton scattering to investigate the electronic structure of MgCNi3. A method for computing the positron state within a material is also presented. In our method for computing the electron momentum density, reciprocal space is divided into optimally-shaped tetrahedra for interpolation, and the linear tetrahedron method is used to obtain the momentum density and its projections such as Compton profiles. Results are presented and evaluated against experimental data, showing good agreement, and demonstrating the accuracy of our method. For the intermetallic superconductor MgCNi3 , high-resolution x-ray Compton scattering experiments were combined with electronic structure calculations to study a sample with the composition MgCO.93 Ni2.85. Our calculations indicate that the electronic structure, whilst smeared by disorder, does not drastically change in the presence of vacancies, and provide an explanation for some of the discrepancies between measurements of single crystals and polycrystals. Compton scattering measurements were used to determine a Fermi surface in good agreement with that of our supercell calculation, establishing the presence of the hole and electron Fermi surface sheets that are necessary for the proposed two-gap model for the superconductivity. We identify significant smearing of certain parts of the Fermi surface when C and Ni vacancies are present. To calculate the positron state, we have implemented two component density functional theory in the limit of vanishing positron density. We present calculations of the positron lifetime, affinity, and of the momentum density of annihilating electron-positron pairs, for several materials, using a wide variety of electron-positron correlation and enhancement schemes, finding excellent agreement with previous calculations and experimental results. Possible limitations of the method are found in describing positrons localised in vacancies .

This thesis investigates the properties of entanglement in one-dimensional many-body systems. In the first part, the non-equilibrium dynamics following a sudden global quench are exploited for the purpose of generating long-range entanglement. A number of initial states are considered. It is shown that the dynamics following the considered quench can be mapped to the problem of a state transfer. The quench can then be optimised by exploiting the literature about quantum state transfer to generate maximal long-range entanglement and maximal block entropy. In the second part of the thesis, a spin chain emulation of the two-channel, Kondo (2CK) model is proposed. Studying the local magnetisation and susceptibility we show that the spin-only emulation truly represent the two-channel Kondo model and extract the Kondo temperature. A detailed entanglement analysis is presented. Using density matrix renormalisation group (DMRG), which allow for real space analysis, Kondo temperature and Kondo length are evaluated. An entanglement measure, namely the negativity, as well as the Schmidt gap are used as possible order parameters predicting the critical point. An extensive analysis of the block entropy of the system is presented for different limiting values of Kondo coupling. A universal scaling of the impurity contribution to the entropy is found and the 2CK residual entropy is extracted. The last part explores quench dynamics in Kondo systems using time-dependent DMRG. For a quench in the Kondo coupling a travelling and breathing clouds are ob-served. A measurement-induced dynamics lead to an oscillation between an effective singlet and triplet states of the impurity and the Kondo cloud. Kondo temperature can be extracted from the frequency of the oscillation.

Zintl-type materials are of great interest in the field of thermoelectrics as their structures lend themselves to independent optimization of properties. For this reason, three distinct series, that can all be described using a Zintl-type model, were prepared using standard solid state reactions. A detailed investigation of X1-xX’xNiSn (X = Ti, Zr, Hf) half-Heusler compositions was undertaken, results of which are presented in Chapters 3-5. These materials are well established as promising thermoelectrics but progress in increasing efficiencies has been hindered by a lack of understanding of the structure and irreproducibility of properties. The work presented herein therefore aimed to provide a detailed analysis of the structure of these compositions. This was achieved primarily by neutron powder diffraction which has not previously been used in the study of these materials. Complimentary electron microscopy analysis was also carried out to gain information on the structure over various length-scales and thermoelectric property measurements were performed. Significant multiphase behaviour was discovered in all compositions where x ≠ 0, 1. Our data indicates that the compositional variations occur over long length-scales and has no significant impact on the thermoelectric properties. In addition, 0-3% excess Ni, located on interstitial sites in the half-Heusler structure was found in all samples, regardless of the synthesis method used. This has not previously been acknowledged in the literature and is difficult to identify without neutron diffraction. Three TiNiMySn series (M = Ni, Co, Cu) were subsequently prepared. Up to 8% interstitial metal was successfully introduced to the TiNiSn structure and had a doping effect on the thermoelectric properties. The introduction of interstitial metals was therefore found to be a new route to controlling the electronic properties of these promising thermoelectric materials. TiNiX (X = Si, Ge) and RMnSbO (R = Nd, La) were considered as new materials to the field of thermoelectrics as described in Chapters 6 and 7, respectively. Both series showed some promise for thermoelectric applications, with a large Seebeck coefficient found in NdMnSbO and low resistivity values displayed by the TiNiX compositions. In addition, detailed X-ray and neutron powder diffraction experiments and measurements of their magnetic properties were undertaken. Large magnetoresistances were found in TiNiX and incommensurate magnetic ordering was uncovered in NdMnSbO.

This dissertation describes low-temperature electronic transport measurements on semiconductor structures of restricted dimensionality. The experiments fall into two sets. The first concerns anisotropic magnetotransport measurements and electron focusing in a varying external magnetic field. These are performed using MBE-grown high mobility two-dimensional electron gases formed on (311)B GaAs substrates. The second is a study of magnetic field induced insulator-quantum Hall liquid transitions performed on GaAs-AIGaAs heterostructures in which a number of InAs monolayers are inserted in the centre of a GaAs quantum well. The sample structures were characterised by STM, TEM, STEM, and AFM. Interest in electron transport on high-index GaAs surfaces is increasing, especially since the advent of patterned substrate regrowth. An anisotropic mobility in orthogonal directions seems to be universal for electron gases grown on (311)B-oriented GaAs substrates. The anisotropy depends on the two-dimensional electron gas carrier density, but mobilities are always higher in the [233] direction. The interface roughness scattering is a possible cause of the mobility anisotropy. The electron focusing results demonstrate that the effective mass and Fermi surface are isotropic even through the mobility is anisotropic. An explanation is proposed based on interface roughness scattering. In the second part, a magnetically induced direct transition from an insulating state at zero magnetic field to quantum Hall effect states with Hall resistance Pxy = h/2e2 and Pxy = h/e2 and back to an insulating state at higher field is observed. The phase boundaries are plotted as a function of disorder and magnetic field using two methods, firstly the temperature independent Pxx points and secondly the maxima in CJxx. This experimental phase diagram is related to the disorder induced collapse of spin splitting in the lowest Landau level obtained from activation energy studies.

In-situ surface x-ray scattering measurements of the electrode/electrolyte interface have been made along with complementary electrochemical and ultra-high vacuum measurements. Using these techniques, two main systems 'have been studied; bimetallic surfaces which have shown enhanced activity for the oxygen reduction reaction (ORR) and single-crystal surfaces to probe the effect of temperature on the interfacial structure and surface reactions. The Pt3Ni(111) electrode surface has been found to be the most highly catalytically active surface ever detected for the ORR, 1/202 + 2H+ + 2e- = H20. , In-situ surface x-ray scattering measurements of this surface revealed that the Pt concentration profile oscillates over three atomic layers, whereby the oute'rmost atomic layer was found to be pure Pt, followed by a Ni-rich second layer and a slightly Pt-rich third layer, before reaching the bulk occupation value. It is the concentration profile which causes the surface' electronic structure of Pt3Ni(111) to be significantly different from the atomically identical surface of pure Pt(lll). The surface electronic structure of Pt3Ni(111) acts to optimise conditions for the adsorption of reactant species involved in the ORR. Density-functional theory calculations have indicated that 670-atom octahedral Pt3Ni nanoparticles, where each surface plane is [111] orientated, would be thermodynamically stable and have approximately the same Pt concentration profile as the Pt3Ni(111) single crystal, which, if synthesised, would prove to be an improved catalyst for practical application in fuel cells. The solution/substrate temperature plays an important role in determining the molecular adsorbate structure of CO (COad ) formed on the Pt(111) surface. At electrode potentials approaching hydrogen evolution, 0.05 V (versus the reversible hydrogen electrode), the (2x2) and vII9 CO-structures coexist at high temperature (rv45°C). This is due to a significant negative shift in the onset of OH adsorption and subsequent partial oxidation of COad through the LangmuirHinshelwood reaction (COad +OHad --+ CO2 +H+ +e-). The second investigation into the role of temperature in electrochemical systems has been made on the surface reconstruction of the AU(100) and Au(111) electrodes. Surface reconstruction involves the rearrangement of surface atoms and can be directly monitored by in-situ surface x-ray scattering measurements. The AU(100) reconstruction is greatly enhanced at high temperature, rv45°C, however, the Au(lll) reconstruction is unaffected by temperature changes. The apparent discrepancy is due to the initial step by which the respective reconstructions form. As observed in the CO/Pt(l11) system, the onset of OH adsorption is shifted negatively with increased temperature. The negative shift increases the nucleation sites of hex-strings, the initial step in forming the Au(100) reconstruction, which causes an enhancement of the reconstruction.

This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures. The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples. The investigated AlxGa₁₋xN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlxGa₁₋xN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlxGa₁₋xN:Si samples showed the incorporation properties of Si in AlxGa₁₋xN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects. The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InxAl₁₋xN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated. The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InxAl₁₋xN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs.

This thesis examines the interaction between superconductivity and inhomogeneous ferromagnetism. Through careful engineering of the interface, it is possible to unlock a new spin aligned triplet Cooper pair, which is capable of penetrating and modifying the magnetisation of a ferromagnet in proximity to a singlet, s-wave, BCS, superconductor. This triplet state is the building block for the new class of super-spintronic devices. Two candidate ferromagnetic systems in which to study the spin aligned triplet are considered. Firstly, the rare-earth ferromagnet erbium is fabricated using sputter deposition. Neutron diffraction measurements show the retention of the conical magnetic state in the thin film form for the first time. This conical state makes it an ideal candidate material for triplet Cooper pair generation. Placing erbium next to superconducting niobium has a drastic effect on the critical temperature of the superconductor, causing a suppression and oscillation of Tc with erbium thickness. In addition the remanent state of erbium at a single thickness can be used as a control to switch the niobium from the superconducting state into the normal state. The second system studied is the superconducting spin valve. In this system the inhomogeneity is engineered in a multi-layer structure using exchange biased Co. To study the nature and extent of the triplet Cooper pair in this structure, large scale facility techniques are employed to look for expected changes to the magnetic state of the heterostructure, with the onset of superconductivity. Surprisingly, no observation directly attributable to the triplet Cooper pair was observed. Instead a new type of induced ferromagnetism in a normal metal coupled to the superconducting spin valve was discovered.

Specific heat measurements are presented on the unconventional superconductors BaFe2(Asl-xP xb YBa2Cu408 and Lio.gM060 17 · The development of a novel highprecision AC micro-calorimeter is also presented. An inexpensive and simple to fabricate SiN membrane-based AC micro-calorimeter was developed and successfully calibrated to an accuracy of 1 % between 6 K and 200 K. An appropriate thermal model was developed and the device shown to be capable of resolving transitions in heat capacity with an absolute accuracy of 5%, with good resolution of transitions down to 10 pJ /K. The total heat capacity of a sample at any given temperature can be determined to within 10%. Specific heat measurements of the supel'conducting transition in BaFe2(Asl_xP xh show that the quantum critical fluctuations renormalising the electron effective mass are robust to both temperature and magnetic field and that inter-band scattering is dominant giving equal renormalisation across the Fermi surface. The upper critical field has been shown to contradict theoretical predictions through a lack of enhancement around the quantum critical point, possibly due to mixing of antiferromagnetism in the vortex cores with the superconducting state. With reference to lower critical field measurements, the condensation energy is used to infer a strongly renormalised vortex core energy that indicates a direct enhancement of the superconducting state due to quantum criticality. The heat capacity of YBa2Cu408 was studied with magnetic fields applied along each of the orthogonal crystallographic directions. An enhancement in the density of states along the chain direction of 0.52 mJ mol- 1 K-2 was found in relatively low fields «1 T) consistent with the quenching of the superfluid density in the unhybridised region of the chain Fermi surface. Two values of the electron effective mass are deduced, mi = 3.9 ± OAme and m2 = 3 ± 0.3me, in excellent agreement with quantum oscillations. A large increase in the electron effective mass is seen on approach to the pseudogap end point up to a value of m* ~ 9.9me by analysing published data in a similar fashion. The condensation energy of Lio.gM06017 was determined and with reference to magnetoresistance data shows a breaching of any reasonable Pauli-limiting field thereby invoking an unconventional superconducting state. The superconducting critical temperature, the normal state specific heat, and the size of the superconducting transition are found to loosely correlate between different samples. Hall effect measurements on the same samples reveal that the strong upturn seen in the Hall effect cannot be a result of carrier loss thus refuting a density wave gap. A long-localisation scenario is proposed as an alternative explanation. This study also suggests that the sample dependence in Lio.gM06017 is a doping effect at all temperatures, likely due to the removal of electrons by excess oxygen. The Hall coefficient is observed to change sign at low temperatures and moderate fields.

As a multipotent tool for scientific exploration, semiconductor nanoparticles, or quantum dots (QDs), have gained enormous interest in nanoscience in the past two decades. The research presented here focused on cadmium telluride (CdTe) QDs: novel synthetic methodologies were used to prepare previously inaccessible nanomaterials based on CdTe QDs. -- CdTe/CdSe/ZnSe core/shell/shell QDs were prepared by a one-pot synthesis. The resulting QDs exhibited near infrared emission, were readily dispersed in aqueous media and applied to deep tissue imaging where emission through the skin indicated the gradual transition of the QDs via the lymphatic tract. -- Using a different synthetic approach, CdTe QDs, which were dispersed in organic media, were exposed to mercury cations in a toluene/methanol solution, resulting in CdHgTe nanoalloy formation. The optical characteristics of the resulting materials were substantially red-shifted from those of the original CdTe QDs. Structural changes were also examined and the influence of the addition of metal cations to other colloidal QDs. -- The organometallic compound Cd(TeC6H5)2 was synthesised and used as a single-source precursor for CdTe QDs. Products isolated after thermal decomposition of the single-source precursors showed strong emission of various wavelengths depending on the reaction time. The underlying chemistry on QDs formation was investigated, and CdTe/ZnS QDs were prepared using only single-source precursors. -- To make the QDs useful in biology, the surface of organically synthesised CdTe/ZnS QDs was modified with an amphiphilic protein (hydrophobin) to phase transfer the particles into aqueous solution. The QDs exhibited bright emission after phase transfer, and were applied to cell imaging in order to examine the validity as a fluorophore and the influence on a cell.

This thesis tells the story of the Balkanization of the theory community in High Temperature Superconductivity (HTS) and of the many roles experimental evidence has been playing in the battles there. In the twenty-five years that followed the discovery of HTS, the Condensed Matter Physics (CMP) community has experienced extreme difficulty in trying to reach a consensus on a 'final' theory. I will explore some of the reasons for such dissent, starting from testimonies that I collected through personal interviews with HTS physicists. I will focus on the way experiments actively contribute to the formulation of theories. I claim that there is a tension between the different methods and aims of two scientific traditions as they implement the contribution from experiments. This tension will be illustrated through the discussion of several episodes from the history of Superconductivity and CMP research. In particular the paradigmatic quarrels between two of the major players in the history of superconductivity, physicists PW Anderson and B Matthias, will be presented to explore the meeting of theoretical and experimental driving forces and their impact on the evaluation of theories and research programmes. I will also argue that the ambiguity in the theories of evidence employed by the warring camps in HTS allows each of them to claim empirical adequacy for itself and deny it to the opponents, and I shall raise questions about whether the standards of evidence employed there are consistently applied and grounded.

Ca(1-x)Nd2x/3TiO3 and MgTiO3-Ca0.61Nd0.26TiO3 composite ceramics were prepared by the mixed oxide route and characterised in terms of their structure, microstructure and properties. Ceramics sintered at 1450-1500oC achieved better than 95% of the theoretical density. X-Ray diffraction (XRD) revealed that Ca(1-x)Nd2x/3TiO3 ceramics were single phase for all compositions. For x ≤ 0.39 the structure was Pbnm with lattice parameters of a = b = √2ac and c = 2ac and a tilt system of a-a-c+. Compositions with x ≥ 0.48 could be better described by a C2/m structure with lattice parameters of a = b = c = 2ac. Scanning electron microscopy (SEM) revealed that the ceramics had grain sizes in the 5-70 μm range with abnormal grain growth for Nd3+ rich compositions. Images revealed that the twin domains in CaTiO3 were needle shaped and on addition of Nd3+ the domain morphology becomes more complex. The needle domain morphology returns for Ca0.43Nd0.38TiO3. High resolution electron microscopy (HAADF-STEM and electron diffraction) was used to probe cation-vacancy ordering (CVO) in the lattice. It was found that there was no CVO for x < 0.48 whilst at x = 0.48 there was evidence of a transition to a short range CVO. A transition to long range ordering is almost complete for the Ca0.1Nd0.6TiO3. The structural characteristics of Ca(1-x)Nd2x/3TiO3 ceramics as a function of temperature were investigated using in-situ XRD and Raman spectroscopy. All compositions were found to have the same structure across the entire temperature range. The Raman spectroscopy as a function of temperature indicated a possible transition with similar characteristics to a Curie temperature in a ferroelectric ceramic. The transition temperature was dependent on the cation ordering with the ceramics with greatest degree of disorder having the lowest transition temperature. The microwave dielectric properties of the samples were measured by a cavity resonance method in the 2-4GHz range. The relative permittivity (εr) was found to decrease from 180 for CaTiO3 to approximately 80 for Ca0.1Nd0.6TiO3 with an exponential dependence between the composition and the property. The temperature coefficient of resonant frequency (τf) ranged from +770ppmK-1 for CaTiO3 to +200ppmK-1 for Ca0.1Nd0.6TiO3. The Q x f for CaTiO3 was found to be 6000GHz and this increased to a maximum of 13000GHz for Ca0.7Nd0.2TiO3. After the Ca0.7Nd0.2TiO3 composition, the Q x f decreased to approximately 1100GHz for Ca0.1Nd0.6TiO3. The εr and τf were found to be mainly dependent on the composition of the ceramics whilst the Q x f value was more complex being dependent on the width of the twin domains in the grains. CaTiO3 samples fabricated by spark plasma sintering at 1150oC and above achieved better than 95% of the theoretical density. XRD revealed only a single phase with an orthorhombic Pbnm structure at room temperature and a tilt system of a-a-c+. SEM confirmed that the samples were single phase with grain size between 500nm-5μm. Transmission electron microscopy (TEM) of specimens sintered at 1150oC showed evidence of both (011) and (112) type domains. The τf of the ceramics was shown to be dependent on the volume of the unit cell, in agreement with the Bosman-Havinga equations. The ceramic sintered at 1150oC showed improvement in the Q x f value compared to samples prepared by conventional sintering. The structure, microstructure and properties of composite ceramics based on the MgTiO3-Ca0.61Nd0.26TiO3 system were investigated. Optimum properties were achieved at a composition of 0.8MgTiO3-0.2Ca0.61Nd0.26TiO3 with τf = -0.1ppmK-1, Q x f of 39000GHz and εr of 25.4. XRD revealed the presence of 3 phases including Ca0.61Nd0.26TiO3, MgTiO3 and MgTi2O5. The grain size of the ceramics was typically 5μm. The Q x f value was sensitive to the cooling rate and these changes could be related to changes in the vibrational properties of the lattice through changes in the lattice parameters.

Measurement of the I(V ) characteristics of single molecules is the first step towards the realisation of molecular electronic devices. In this thesis, the electronic transport properties of alkanedithiol (ADT) and alkylthiol-terminated oligothiophene molecules are investigated under ultra high vacuum (UHV) using a scanning tunnelling microscope (STM). Two techniques are employed that rely upon stochastic molecular bridge formation between gold STM tip and substrate; a novel I(V; s) method is proven to be a powerful alternative to the well-known I(s) method. For ADTs, three temperature-independent (180 - 390 K) conduction groups are identified, which arise from different contact-substrate coordination geometries. The anomalous reduction of conductance at small chain lengths reported by other groups for non-UHV conditions is far less pronounced here; all groups closely follow the anticipated exponential decay with chain length, β = (0.80 ± 0.01) Å ¹, until a small deviation occurs for the shortest molecule. Thus, the likely explanation for the anomalous effect is hydration of thiol groups. The I(V ) curves were fitted using a rectangular tunnel barrier model, with parameters in agreement with literature values; m = (0.32 ± 0.02) m, φ = 2 eV. For the oligothiophene molecules, one common temperature-independent (295-390 K) conduction group was identified; the conductance decays exponentially with molecular length, with different factors of β = (0.78 ± 0.15) Å ¹ and β = (0.16 ± 0:04) Å ¹ for length changes to the alkylthiol chains and thiophene backbone, respectively. An indented tunnel barrier model, anticipated from the physical and electronic structure of the molecules, was applied to fit the measured I(V ) curves; φ1 = φ3 = 2 eV, φ2 = 1.3 to 1.6 eV, m = 0.17 to 0.24 m. These UHV measurements provide an important baseline from which to better understand recent reports indicating hydration-dependent, and hydration-induced temperature-dependent, transport properties.

Charge transport is the one the most fundamental concepts in organic semiconductors. The key quantity that characterises this transport behaviour is carrier mobility. The ability to transport carriers in a fast and unimpeded nature in organic devices such as Organic Photovoltaics (OPV) or Organic Light Emitting Diodes (OLED) is a key parameter for building more efficient devices. Significant steps have so far been taken to understand and model this phenomenon, however there are still many questions that need to be answered. One such fundamental question is the role of excited states on the charge transport properties of organic materials which historically has been ignored. This thesis aims to investigate the transport properties of two of the most widely used organic materials, N,N′-bis-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPB) and are N,N’-diphenyl-N,N’-bis 3-methylphenyl-1,1’-biphenyl-4,4’-diamine (TPD). We demonstrate how excitons are generated in a single organic layer OLED devices and how traditionally hole transport materials are capable of fast long range electron transport. We provide a comprehensive analysis of the charge transport properties of both materials with respect to the Gaussian Disorder Model (GDM) and demonstrate how both types of carriers can easily be transported in these materials. We then investigate the effects of exciton generation on the transport properties of the materials and propose some numerical modeling to analyse the effects of such excited states and the distribution of energetic traps in our system. We show that the swing of carrier mobility in either direction depends on the interplay and dominance of each mechanism (triplet/carrier interaction and trap filling). We also investigate the effects of 5 removing excited states from our device by deliberately introducing impurities via doping of a phosphorescent molecule to alter their concentration. Finally we propose some future direction that one can take to model charge transport behaviour in disordered organics based on the experimental work discussed in thesis.

The focus of this work is to investigate the effects of applying an external magnetic field to a solidifying liquid metal melt. The principle is that thermoelectric currents that are naturally inherent to solidification processes will interact with this magnetic field, resulting in a Lorentz force. This force will exist in a microscopic region in the vicinity of the solidification front, generating microscopic fluid flow in the liquid region which can significantly effect the mechanism of dendritic growth. The work contained in this thesis provides an initial insight into the complex behaviour of this process, through the use of numerical models. To model the soldification dynamics, an enthalpy based model for dendritic growth in a supercooled melt is used in 2-dimensions and extended into 3-dimensions. The dendrite is defined as being equiaxed in nature and, for purely diffusion driven growth, numerical calculations show a good agreement with other methods under similar growth parameters. To investigate the effects of fluid dynamics, dendritic growth is tested under forced convection conditions and significant morphological changes occur. The incident tip velocity is increased and the downstream tip velocity is decreased; in agreement with many other authors investigating similar situations. In the presence of a magnetic field the Lorentz force will form in planes perpendicular to the direction of the magnetic field. Due to the morphology and anisotropy of the surface temperature, the nature of the flow is dependent on the relative orientation of the magnetic field and the crystallographic orientation of the lattice. Using a low magnetic field strength approximation, thus removing the non-linear and resistive terms in Navier-Stokes equation, the resulting fluid velocity is arbitrarily small so that convective transport is negated. At some time, when the morphological features of a dendrite are apparent, steady state simulations show the flow fields that exist with different orientations of the magnetic field. The results are compared to an analytic solution for the Lorentz force, which is described by reducing the morphology of a dendrite to a sphere and assuming that the surface temperature is equivalent to the anisotropy in the surface energy. When the thermoelectric currents are large and the magnetic field strength is substantial the convective transport, non-linear and resistive terms become significant. The problem is purely 3-dimensional and it is shown that classical 2-dimensional boundary conditions lead to stagnant conditions. A 2-dimensional quasi 3-dimensional approximation is proposed and, with the magnetic field orientated in the (001) direction, the effect of heat and solute redistribution through convection on the crystal morphology is modelled. Two significant morphological changes occur; the first is a deflection of the dendrite tip and the second is the initiation of secondary branching into the incident flow. The deflection is caused by circulations at the tips of the dendrite; the circulations continuously provide a region of higher free energy on the incident side while lowering it on the other. The net effect is a bias of growth in the direction of incident flow. The increase in secondary branching, in a similar fashion to the deflection, is caused by both a circulation at the tip and also a global circulation around the entire dendrite, destabilising the incident interface and initiating secondary growth. To qualify the quasi 3-dimensional approximation, a moving mesh technique is developed that tracks a single tip of 3-dimensional growth and the similar morphological features are observed in comparison to the quasi 3-dimensional case. Finally a discussion into possible extensions of this work is proposed and preliminary results for grain growth in the presence of a magnetic field are given.

This thesis comprises a theoretical study of the dynamics of indirect excitons in coupled quantum wells at low lattice temperatures. The results of numerical simulations of the exciton photoluminescence pattern are presented and compared to available experimental data. The in-plane transport of quantum well excitons created by laser excitation is modeled using a non-linear drift-diffusion equation. Combined with a model of exciton relaxation thermodynamics, a complete description of the evolution of the exciton density and temperature is built. The optical decay of indirect excitons is included in the modeling. This is used to make predictions of the spatial photoluminescence patterns which have been observed experimentally. The transport of dipole orientated excitons via externally applied electrostatic potentials is also studied. The drift-diffusion equation is adapted to include the inplane electric field. This is done for some specific forms of the potential landscapes such as a linear potential energy gradient and a propagating lattice. These correspond to some recent experiments for which results are available. The combined theoretical and experimental studies reveal a deeper insight into the transport properties of indirect excitons. Finally, the external ring structure in the indirect exciton emission pattern is studied. Its formation is modeled using a set of coupled transport equations for electrons, holes and indirect excitons. The Coulomb interactions between all three species are incorporated in the model. It is shown that these interactions lead to an instability in the external ring and are responsible for its fragmentation into a periodic array of islands which has been observed experimentally.

In this thesis the development of new analysis methods that study the carrier distributions in quantum dots (QDs) directly from experimental measurement of spontaneous emission and gain spectra are described. These were applied to three InAs QD structures that are nominally identical except for the doping type in the active region, one p-doped, one n-doped and one left un-doped for comparison. The effect that carrier localisation within individual dots had on this temperature dependence of the carrier distribution under injection was studied and related to key aspects associated with laser device performance. The nature of QD occupation in the three samples was determined through measurement of the carrier temperature (TC) of the electrons populating the QD states. It was found that the un-doped samples QDs were in thermal equilibrium with the bulk lattice down to 200 K. Below this temperature the sample’s QD states become decoupled from the lattice and at 60 K QD occupation was shown to be random. The p-doped sample was shown to be non-thermal between 300 K and 200 K where at 150 K the occupation of QDs became random. The TC was observed to decrease for this sample below 200 K and this was attributed to fewer dopants ionising as the temperature decreased. The n-doped sample was also shown to be non-thermal between 300 K and 200 K with the QD occupation becoming random at 100 K. In all three samples, above 300 K, the measured TC was lower than that predicted by a Fermi-Dirac distribution and this was attributed to the these QDs having a large number of closely spaced hole states leading to a size dependence of the number of these states. This means an individual ΔEf exists for a given set of dot sizes. So emission from an ensemble of dots is “smoothed” across different ΔEf levels leading to a reduction in the apparent TC. These results have a significant effect on the threshold current densities of these samples and suggest that the differences observed due to doping will not be reproduced by calculations assuming a quasi-thermal equilibrium across the QD structure. The temperature dependence of the shift in gain peak energy was determined for the un and p-doped samples. This showed that the blue-shift of the gain peak due to state-filling in un-doped QD structures is independent of temperature, at a given value of peak gain, over the temperature range studied (200 K to 350 K). In the pdoped sample however, the state filling is temperature dependent at any fixed gain with a shift of 8meV observed between 200 K and 350 K. This was attributed to the wide electron state distribution and the lowering of the electronic quasi-Fermi level by the p-doping. This renders p-doped materials unsuitable for any technology application where gain peak wavelength temperature stability is required for efficient operation.

A custom built thermal evaporator equipped with in situ electrical transport probes and an electromagnet, designed to investigate magnetic thin films and nanostructures, was constructed and calibrated. Magnetoresistance measurements were used to characterise a 20 nm thick film grown in 2 nm steps and measured in situ as a function of film thickness. It was found that the thin film had a smaller than expected anisotropic magnetoresistance (AMR) signal of 0.024%. It was suggested that an oxide formed at each 2nm thick layers during the growth phase altered the conductivity of the film and caused the measured AMR to be anomalously small. Lateral spin valves fabricated from a range of ferromagnetic and normal metal components were investigated. NiFe/Au/NiFe lateral spin valves were the most thoroughly investigated to determine the spin diffusion length in the Au, the spin polarisation of NiFe and the injection efficiency at the NiFe/Au interface. Lateral spin valves fabricated from NiFe/Al/NiFe and utilising tunnelling contacts were also investigated and a pure spin current detected. Other devices, including a non-local lateral spin valve dual spin injection structure, were fabricated and measured. Nanomachining using diamond coated silicon nitride atomic force microscope (AFM) tips was employed to modify nickel iron (NiFe) nanowires. The modifications to nanowires in this way subsequently altered the observed domain wall motion in the wires. AFM nanomachining was found mostly to increase the coercive field of the nanowires owing to the formation of a pinning site for domain walls. Magnetoresistance measurements were used to study the effect of machining nanowires of varying widths and thickness. Theoretical predictions regarding the change in coercive field due to machining were larger than those experimentally measured. Domain wall anisotropic magnetoresistance (DW AMR) was also studied as a function of width for two thicknesses of nanowire (10nm and 20nm). Deviation from existing theoretical models was observed consistently for both wire thicknesses. A dependence of the DW AMR on the proximity to the phase boundary between different domain wall types was observed for each thickness of nanowire studied.

In this thesis the transport of light through disordered, densely packed semiconductor nanowire mats is studied. It is found that the extremely high photonic strength of these samples leads to corrections to the traditional diffusion picture of light transport due to mesoscopic interference. Such effects are characterized by large intensity fluctuations and correlations, and it is found the transport is dominated by only a few independent transmission channels, close to the Anderson localisation regime. In addition to the strongly scattering nanowire samples, comparatively weakly scattering samples of ZnO are investigated, demonstrating mesoscopic effects in a less exotic, isotropic multiple scattering material. Control is obtained over the transmission by a combination of shaping the incident wavefront and harnessing the intrinsic nonlinearity of the semiconductor with ultrafast optical excitation. Through these techniques, a bright focus at an arbitrary point through the nanowires is created which can be modulated by up to 60% in a demonstration of a reconfigurable photonic switch.

The main focus of the dissertation is description, modeling and understanding of the mechanisms underpinning electroluminescence from quantum wells. The dissertation contains original contribution of methodological and phenomenological character. We have described in detail the eight band model within the envelope function approximation(EFA) using the Löwd in perturbation method used for band structure calculations. Although not novel, a detailed derivation of this is rarely done in the literature. We have derived a theoretical expression for electroluminescence spectral emittance based entirely on quantum mechanical model, unlike the more usual semi classical models used in semiconductor physics. The final expression for the spectral emittance has a different dependence compared to the semi classical expression, namely the prefactor in the newly derived expression is proportional to 2 . We use the combination of 8 band EFA method and the newly derived expression for spectral emittance to interpret experimental measurements on unpolarized spectral emittance from several InSb/AlxIn1-xSbquantum wells. We do that using slightly novel procedure and identify several transitions unreported in InSb/AlxIn1-xSb material system up to now. In simplified models these are regarded as forbidden. We show that in 8 band EFA model there aren’t any forbidden transitions. Instead all transitions are allowed and we discuss the product of momentum matrix elements and 2D density of states, to which we refer as "generalized selection rule", as the quantity which determines the strength of the individual transitions in different energy ranges. Furthermore we discuss three groups of mechanisms which determine various properties of the electroluminescence spectrum. These groups are entirely general to electroluminescence from all sorts of quantum wells. They are: (i) band structure embodied in the "generalized selection rules" ; (2) broadening effects and (3) statistical effects. Very important are the effects of structure inversion asymmetry (SIA) on the "generalized selection rules" and the spectral emittance, which we describe and explain. Finally we discuss aspects of two other major themes related to the two characteristic properties of InSb:(i) the broken space inversion invariance and (ii) the relativistic correction of spin-orbit coupling.

Since 2004, graphene attracts intensive attention from scientists and engineers all over the world. During the last decades, the research relates to graphene and other 2 dimensional (2D) materials are rapidly increasing. Approximately, ten thousand journal papers have been published after the discovery of graphene in relative topics widely spread. On the other hand, the simple graphene properties research is nearly completed. Researchers turn their attention to other 2D materials or Van der Waals heterostructures. By increasing the liberty and knowledge of 2D materials, the Van der Waals heterostructures can start to build something on this 2D wander land. In this thesis the Van der Waals heterostructures is based on graphene and some other well known 2D materials such as hexagonal boron nitride (hBN) to study fundamental physics and possible applications in near future. In this thesis, three published papers which are related to Van der Waals heterostructures have been included. The electronic properties of encapsulated graphene on different 2D crystals have been investigated by the capacitance spectroscopy. Several 2D crystals have been tested as a substrate such as MoS2, WS2, mica, LiNbO3…etc. The quality of encapsulated device is correlates the interface self-cleaning. Follow with the fundamental physics study employed by a simple Van der Waals heterostructure. Graphene and hBN is lattice aligned within 2 degrees in difference and creates a new superlattice structure which just like moire pattern happens while two similar patterns overlapped. The basic electronic properties do not vary at near Dirac point. Away from the first generation Dirac point, the superlattice structure affects the band structure in higher carrier concentration. In this paper, aligned graphene-hBN capacitors have been demonstrated to discover more fine details of these many-body interactions in this superlattice structure. The final part is related to twist controlled graphene-graphene resonance tunneling transistors. A Van der Waals heterostructure is constructed by two aligned graphene stripes with a thin layer of hBN as a spacer. The electrons are tunneled from one stripe to another graphene stripe while a bias voltage applied. The resonance tunneling is occurred when two graphene flakes are aligned at certain bias voltage. In this paper, we contribute the resonance tunneling to momentum conservation of tunnelling electrons. Theory simulation is highly agreed with our experiment results.

Дисертація на здобуття наукового ступеня доктора філософії за спеціальністю 141 – Електроенергетика, електротехніка та електромеханіка (14 – Електрична інженерія). – Національний технічний університет "Харківський політехнічний інститут" Міністерства освіти і науки України, Харків, 2020. Об’єктом дослідження є імпульсне магнітне поле, що утворюється масивними одновитковими соленоїдами при магнітно-імпульсній обробці металевих заготовок. Предметом дослідження є профілі масивних одновиткових соленоїдів, що забезпечують заданий розподіл імпульсного магнітного поля на поверхні оброблюваної металевої заготовки. В дисертаційній роботі вирішена науково-практична задача визначення профілів масивних одновиткових соленоїдів за допомогою аналітичних розв’язків задач аналізу імпульсного магнітного поля, що утворюється джерелами елементарної форми. Дослідження виконано за допомогою фундаментальних положень теоретичної електротехніки, математичної фізики, чисельних методів аналізу та сучасних інформаційних технологій. У вступі обґрунтовано актуальність теми дисертації, визначені задачі дослідження, показано зв’язок роботи з науковими програмами, планами, темами, наведено дані про наукову новизну, практичне значення, апробацію результатів та публікації. У першому розділі проведено огляд конструкцій полеутворюючих систем для магнітно-імпульсної обробки металів та аналіз відомих методів визначення їх форми. Детально розглядаються два підходи до вирішення цієї задачі. Перший базується на ітеративному або аналітичному підборі параметрів полеутворюючої системи, другий – на вирішенні задачі продовження поля з граничної поверхні. Обґрунтовано необхідність розвитку методів, заснованих на використанні аналітичних розв’язків задач аналізу для джерел елементарної форми, обрано напрями досліджень, поставлені основні задачі дисертаційної роботи. У другому розділі запропоновано метод визначення форми масивних одновиткових соленоїдів для створення заданого розподілу азимутальної складової індукції магнітного поля на поверхні циліндричної та плоскої заготовки, що засновується на використанні систем елементарних джерел зі струмами, котрі розташовані поблизу цих поверхонь. При цьому середовище поза провідниками вважається непровідним і немагнітним, а заготовка замінюється ідеальним надпровідником: нескінченно довгим циліндром або півпростором. Розглядаються три випадки. У першому елементарні джерела – це кільця зі струмами нескінченно малого перетину, що розташовуються співвісно внутрішньому циліндру, у другому – такі ж співвісні кільця, але розташовані паралельно плоскій границі нижнього півпростору, у третьому – осі зі струмами, розташовані паралельно нижньому півпростору. Наведено формули для розрахунку індукції магнітного поля та магнітного потоку, що створюються такими джерелами. Варіацією геометричними параметрами елементарних джерел та струмами, що в них протікають досягнуто відповідності утвореного поверхневого розподілу індукції та заданого. Для підбору оптимальних параметрів системи застосовано метод градієнтного спуску. Для визначення шуканого контуру профілю масивного соленоїда побудовано силові лінії магнітного поля систем елементарних джерел, що забезпечують найменшу похибку відтворення заданого розподілу. Правильність визначення точного контуру профіля соленоїда підтверджується за допомогою методу інтегральних рівнянь. У третьому розділі запропоновано апроксимацію точного контуру профілю масивного одновиткового соленоїда багатокутником, що дозволило значно спростити його проєктування і виготовлення. Досліджено розподіли індукції магнітного поля та їх розбіжності із заданим для соленоїдів точного та апроксимованих профілів. Розраховано та порівняно індуктивність системи соленоїд – циліндр. Детально розглянуто розподіли поверхневої густини струму на крайках апроксимованих соленоїдів і визначено вплив радіуса скруглення та величини кута, що скруглюється, на її максимальні значення. Розрахунки третього розділу виконано за допомогою чисельного розв’язання інтегрального рівняння відносно поверхневої густини струму. У четвертому розділі експериментально досліджено розподіли індукції плоскомеридіанного магнітного поля, що створюється масивним одновитковим соленоїдом поблизу циліндричної поверхні заготовки. Для цього із латуні було виготовлено соленоїд, контур профілю котрого отримали за допомогою методу, який запропоновано в дисертації. Точний контур профіля масивного соленоїда, котрий отримали за допомогою системи дев’яти елементарних кільцевих джерел, було апроксимовано шестикутником. Соленоїд розміщувався на спеціальному стенді співвісно з мідною оболонкою, яка імітувала заготовку. У проміжку між соленоїдом та оболонкою розташовувався індукційний перетворювач, за допомогою якого вимірювали відносну індукцію в контрольних точках поблизу поверхні оболонки. Через соленоїд пропускали імпульси струму від низьковольтного генератора, котрі мали форму експоненційно згасаючою синусоїди. Частота імпульсів змінювалась в діапазоні (40÷225) кГц зміною ємності батареї конденсаторів в генераторі. Наведено відносні розбіжності між виміряними та заданими розподілами, які при всіх варіантах імпульсу не перевищують 6 відсотків по всій довжині оброблюваної поверхні. Результати досліджень дозволили отримати низку наукових результатів: - уперше для визначення форми одновиткового масивного соленоїда, що забезпечує заданий розподіл імпульсного магнітного поля на циліндричній поверхні металевої заготовки при магнітно-імпульсній обробці, застосовано функцію Гріна; - уперше запропоновано апроксимацію складного криволінійного контуру профілю масивного соленоїда контуром багатокутника, що дозволило суттєво спростити його проєктування та виготовлення; - отримало подальший розвиток застосування функцій Гріна для визначення профілів масивних соленоїдів, що забезпечують заданий розподіл плоскомеридіанного та плоскопаралельного магнітного поля на плоскій поверхні металевої заготовки; - достовірність теоретичних результатів, отриманих у дисертації, підтверджено вимірюваннями відносних розподілів індукції магнітного поля, що створюється масивним соленоїдом, поблизу поверхні циліндричної заготовки на стенді для фізичного моделювання; - Результати досліджень використано в НТУ "ХПІ" при виконанні науково-дослідних робіт на кафедрі інженерної електрофізики.
The thesis is submitted to obtain a scientific degree of Doctor of Philosophy, specialty 141 – Electricity, electronics and electrical engineering (14 – Electrical engineering). – National Technical University “Kharkiv Polytechnic Institute” of the Ministry of Education and Science of Ukraine, Kharkiv, 2020. The object of research is the pulsed electromagnetic field, which is created using massive single-turn solenoid in the process of electromagnetic forming of metal workpieces. The subject of research are profiles of massive single-turn solenoids which generate a given distribution of magnetic field at the surface of a metal workpiece. The scientific and practical task of determining the massive single-turn solenoid profiles is solved using analytical solutions of analysis problems for pulsed magnetic field which is created by elementary sources. The problems of scientific research were solved using fundamental concepts of theoretical electrical engineering, mathematical physics, numerical methods of analysis and modern informational technologies. The introduction substantiates the relevance of research tasks showing connection between the work and scientific programs, plans, themes. The information on the scientific novelty and practical value the obtained results are stated. The first chapter provides an overview of known systems for generating electromagnetic pulsed field and methods which are used to determine its shape. Two different approaches are emphasized. The former is based on iterative or analytical adjustment of field-generating system parameters. The latter is the use of the solution of the problem of field continuation from boundary surface. The further development of the methods which are based on analytical solutions of analysis problems for elementary field sources is justified, the direction of research is chosen and objectives are formulated. The second chapter presents the method aimed to determine the shape of massive single-turn solenoids which generates a given distribution of tangent component of magnetic induction at the surfaces of cylindrical or sheet metal workpieces in the process of electromagnetic forming. The method is based on using the solutions of analysis problems for systems which consist of current carrying conductors of elementary shape that are placed near the boundary surfaces, whereas environment outside the conductors is supposed to be nonconducting and nonmagnetic. The ideal skin-effect approximation is used, according to which we suppose that currents flow within the infinitely thin surface layers. With accordance to the approximation the cylindrical workpiece is replaced by ideal superconductive cylinder of infinite length, the flat workpiece is replaced by superconductive half-space. There are three cases described. In the first case elementary sources is represented by annular current carrying conductors which are places axially with the inner cylinder. The cross section of the annular conductors is infinitely small. In the second case there are the same annular axisymmetric conductors but placed above the flat boundary of superconducting half-space and are parallel to it. In the third case elementary sources are straight current carrying axes which are parallel to each other and to the lower superconducting half-space. The formulas for magnetic induction and magnetic flux for that systems are stated. Compliance with given boundary distribution of magnetic induction is achieved by varying of system parameters using gradient descent optimization method. Field lines for the system of elementary sources, that provides the smallest differences between given and obtained induction distributions, were built and used to determine the exact profile of massive single-turn solenoid that generate given magnetic induction distribution. The correctness of exact profile was verified using the method of integral equations. In the third chapter approximation of exact massive single-turn solenoid profile, which significantly simplify its design and manufacturing, is proposed. Magnetic induction distributions, which are generated by the solenoids, are calculated and errors due to the approximation are investigated. Inductance of the solenoid-cylinder system is compared for different accuracy of approximation and different outer radius of the solenoid. Surface current density distributions at the solenoid contour are shown. An influence of radius of rounding of sharp edges is considered and relation between the angle of rounded edge corner and maximum surface current density is described. Calculations of the third chapter were performed using a numerical solution of the integral equation with respect to surface current density. In the fourth chapter an experimental research on relative induction distributions of axisymmetric magnetic field that is created using brass massive single-turn solenoid at the surface of cylindrical workpiece is stated, whereas the shape of the solenoid was determined according to the method which is described in the thesis. The exact profile contour was chosen by one of the field lines which cover the system of nine annular conductors. With accordance to the approximation method the exact profile contour was replaced by six-sided polygon. The solenoid is placed at the special installation axially with a cylindrical copper shell. Induction sensor is placed into the gap between the solenoid and the shell to measure relative induction at reference points near the shell boundary. The solenoid is connected to low voltage pulse generator which creates the pulses of exponentially damped sine wave. The frequency of the pulses is changed in the range of (40÷225) kHz varying the capacitance of the generator battery. Relative differences between measured and given induction distributions are shown and do not exceed 6 percent at all reference points within working area for every variant of pulse frequency. The research results have allowed obtaining a number of scientific results: - for the first time, an approach which is based on the use of Green’s function to determine a shape of massive single-turn solenoid for generating a given magnetic induction distribution at cylindrical workpiece surface is proposed for electromagnetic forming; - for the first time, approximation of curvilinear contour of massive single-turn solenoid profile by a polygon, which allow to significantly simplify its design and manufacturing, is researched; - the use of Green’s functions is developed for determining of massive singleturn solenoids which generates given distribution of axisymmetric or plane-parallel magnetic field at the flat surface of sheet metal workpiece; - theoretical results obtained in the dissertation are confirmed by measurements of relative distributions of magnetic induction which is created by massive single-turn solenoid at the cylindrical boundary of metal workpiece using the installation for physical modeling; - the results of the research were used for research work at Engineering electrophysics department of National Technical University “Kharkiv Polytechnic Institute”.

In this work, I detail the reconstruction and upgrades performed on the axial cavity ion trap in the ITCM group at the university of Sussex, and the measurement of the coupling of multiple ions to the cavity mode. This enables the optimal coupling between the ions and the cavity by adjusting the ions position in the radial and axial positions. This covers new ground in extending the optimal coupling beyond two ions which is of great importance for experiments with several ions in an optical cavity. The thesis outlines the background theory of light-matter interaction and cavity QED, before describing the physical ion trap hardware and its assembly. A description of the laser and cavity systems is provided, including techniques for locking both to stable references. A number of novel measurement techniques for measuring and maximising the stability of the ions and cavities are presented, including micromotion minimisation, spectroscopy, magnetic field compensation using the ground state Hanle effect, and Raman spectroscopy. These techniques enable the measurement of crucial parameters of the atomic transitions and the cavity. The work culminates in a description of the optimisation of the coupling between ion strings and the cavity first by adjusting the radial trap position by means of variable capacitors attached to RF electrodes, and then axially by means of adjusting the endcap potentials and therefore the spacing between ions to obtain the greatest localisation while still positioning the ions close to the antinodes of the cavity field.

The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy.

This thesis discusses experiments that have been performed to investigate ultrafast acoustoelectric effects in semiconductor devices. Current commonly employed techniques to generate ultrafast acoustic pulses and detect them with spectral resolution require a powerful pulsed laser system that is bulky, expensive and complicated. If the acoustic pulses could instead be generated and detected by electrical methods, picosecond acoustic techniques could become more readily available as a tool for other users. This thesis focusses on the electrical detection of acoustic pulses with spectral resolution. In many of the key experiments described in this thesis a picosecond strain pulse was generated optically on the opposite face of the sample to the semiconductor device of interest. The strain was generated either in a thin Al film thermally deposited on the sample surface, or directly in the GaAs substrate. Acoustic phonons generated by this method propagated across the substrate to the device. Transient voltages across the semiconductor device caused by the incident phonons were detected using a high frequency real-time oscilloscope. The first evidence of heterodyne mixing of coherent acoustic phonons with microwaves was obtained, for frequencies up to about 100 GHz. First, it was confirmed that Schottky diodes can produce a fast transient voltage in response to an incident acoustic wavepacket. The detection process occurs at the semiconductor-metal interface, and is due to the deformation potential. Bow-tie antenna fabricated directly onto the GaAs substrate proved to be ineffective at coupling microwaves from free space to the Schottky diode. A waveguide-coupled beam-lead Schottky diode provided by e2v had a sufficient response to the incident microwaves to proceed with the mixing experiments. The microwave local oscillator signal was mixed with a tunable narrow frequency band acoustic signal that was produced using a Fabry-Perot etalon external to the laser cavity. The intermediate frequency components were in the range of 1-12 GHz, which could be detected on the oscilloscope. Mixing was performed using both the fundamental frequency acoustic wave and the second harmonic generated in the sample. Semiconductor superlattices were also investigated as electrical detectors for ultrafast acoustic pulses. In this case, the transient voltage measured across the device contained an unexpected contribution in the form of a peak with a width of approximately 2 ns. This signal is too slow to be caused by a strain pulse and too fast for a heat pulse. It is proposed that this peak is caused by long-lived phonon modes from the centre of the mini-Brillouin zone being confined in the superlattice due to Bragg reflections. The peak caused by confined phonons and the two peaks caused by heat pulses also present in the detected signal were investigated for a range of experimental conditions. This allowed comparisons to be made to previous works. A similar superlattice structure had a very different response to the incident acoustic wavepacket. The polarity of the transient voltage detected was inverted and there was no evidence of an electronic response to the confined phonon modes, which would have been present in both samples. It is proposed that the barriers of the NU1727 superlattice sample are thicker than expected, and this strongly affects the electron transport through the structure. This thesis shows that semiconductor devices can be suitable for the electrical detection of ultrafast strain pulses. For this technique to reach its full potential, it is also necessary to be able to generate these strain pulses electrically. A step recovery diode has been considered for this purpose as part of the suggested future work.

Lattice mismatched heterostructures grown by Interfacial Misfit (IMF) technique, which allows the strain energy to be relieved both laterally and perpendicularly from the interfaces, are investigated. However, electrically active defects are created at the interface and away from the interface with energy levels deep in the bandgap of the host materials. These defects dramatically affect the optical and electrical properties of the devices. In this thesis, an investigation of deep level defects is carried out on GaSb/GaAs uncompensated and Te compensated heterostructures grown by the IMF method using DLTS, Laplace DLTS, I-V, C-V, C-F and C-G-F measurements. Furthermore, the effect of thermal annealing treatments on the defect states is also studied on both types of samples. It was found that the well-known EL2 electron trap is commonly observed near to the interface of both uncompensated and Te compensated GaSb/GaAs IMF samples. However, several additional electron defects are detected in Te compensated samples. Rapid thermal annealing performed on uncompensated samples resulted in the annihilation of the main electron trap EL2 at a temperature of 600 oC. On the other hand rapid thermal annealing and conventional furnace annealing were carried out on Te compensated samples, and it was observed that rapid thermal annealing process is more effective in terms of defects reduction. The density of interface states is determined from C-G-F and forward bias DLTS measurements. Te compensated samples exhibit the highest density of interface states and have additional hole traps as compared to uncompensated samples. The electrical properties of p-i-n GaInAsSb photodiodes grown on uncompensated and Te compensated GaSb/GaAs templates on GaAs substrates using special growth mode are investigated. The non-radiative defects which could have detrimental effects on the performance of these photo diodes are studied here for the first time. Both electron and hole defects are detected, and their capture cross-section measurements reveal that some of defects originate from threading dislocations. The double pulse DLTS measurements are performed and the concentration distributions of the detected defects are determined.

The nature of ultrafast acoustic strain generation and effects in III-V semiconductor-based nanostructures is explored in this thesis via experimental observations that are supported by theoretical analysis. Specifically, coherent phonon generation processes in bulk gallium arsenide (GaAs) are investigated through remote hypersonic detection using a double quantum well-embedded p-i-n diode, after which strain-induced effects in a double barrier quantum well resonant tunnelling diode are examined. Finally, preliminary studies on acoustic modulation of a double barrier quantum dot resonant tunnelling diode are also considered, with recommendations for future experimentation. It was experimentally observed that the transduction of strain in bulk GaAs produces an initial acoustic wavepacket that is strongly asymmetric with a heavily damped leading edge. This was determined to be due to photogeneration of a supersonically expanding electron-hole plasma near the irradiated GaAs surface. Coupled with its propagation from the free surface, the plasma generates stress and therefore strain in the system that is caused by a combination of the deformation potential and thermoelasticity; the former and latter are shown to be dominant for low and high optical excitation densities, respectively. These acoustic waves cannot escape the plasma until it has decelerated to subsonic velocities, which is achieved in a finite time, thus resulting in the observed asymmetry and damped leading edge. This finite acoustic escape time was reduced at high optical excitation densities due to plasma expansion limitation by increased non-radiative Auger recombination of electron-hole pairs. This conclusion is substantiated by analytical expressions derived from the inhomogeneous wave equation, and analysis of the spatially- and temporally-expanding plasma density based on the deformation potential mechanism only. Numerical simulations based on these expressions are fitted to the experimental data, and the thermoelasticity contribution at high excitation densities is deduced from a non-linear deviation of the electron-hole recombination rate and a change in the duration of the leading edge. This contribution expressed a square-law behaviour in the former parameter, which is attributed to non-radiative Auger processes. Strain-induced effects on a double barrier quantum well resonant tunnelling diode resulted in the detection of current modulation on a picosecond timescale only when the device was biased within its resonance region, with the largest modulations at the resonance threshold and peak biases. Through analysis of the device structure and stationary current-voltage characteristics, it is demonstrated that the observed current changes are due to variations of the resonant tunnelling rate caused by acoustic modulation of the confined ground state energies in the diode itself. Numerical analysis of the tunnelling rates provided excellent agreement with the experimental data, particularly when comparing charge transfer rates, where the limited temporal response of the experimental device could be ignored. Furthermore, the charge transferred at the resonance threshold and peak has a set polarity regardless of optical excitation density, and therefore the device possesses “rectifying” behaviour. As such, it has been demonstrated that, by exploiting this acoustoelectronic pumping effect, control of picosecond charge transfer in a resonant tunnelling diode or its application as a hypersonic detector are possible. In closing, the mechanisms for strain generation in bulk GaAs and the utilisation of the acoustoelectronic pumping effect in a double barrier quantum well resonant tunnelling diode are both exhibited in this work, and provide promising evidence and novel hypersonic detection methods for future research into ultrafast acoustic effects in semiconductor nanostructures.

This thesis is concerned with the development of condition monitoring for future design of high temperature superconducting (HTS) power apparatus. In particular, the use of UHF sensing for detecting PD activity within HTS has been investigated. Obtained results indicate that fast current pulses during PD in LN2 radiate electromagnetic waves which can be captured by the UHF sensor. PD during a negative streamer in LN2 appears in the form of a series of pulses less than 1 μs apart. This sequence cannot be observed using conventional detection method due to its bandwidth limitation. Instead, a slowly damped pulse is recorded which shows the total amount of charge transferred during this period. A study into PD streamer development within LN2 has been undertaken that reveals the characteristics of pre-breakdown phenomena in LN2. For negative streamers, when the electric field exceeds a threshold value, field emission from the electrode becomes effective which leads to the formation of initial cavities. Breakdown occurs within these gaseous bubbles and results in the development of negative streamers. For positive streamers, the process is much less well-understood due to the lack of initial electrons. However, from the recorded current pulses and shadow graphs, the physical mechanism behind positive streamer development is likely to be a more direct process, such as field ionisation, compared with the step-wise expansion in the case of negative streamers. The mechanisms that cause damage to solid dielectrics immersed in LN2 have been investigated. Obtained results indicate that pre-breakdown streamers can cause significant damage to the solid insulation barrier. Damage is the result of charge bombardment and mechanical forces rather than thermal effects. Inhomogeneous materials, such as glass fibre reinforced plastic (GRP), tend to introduce surface defects which can create local trapping sites. The trapped charges when combined with those from streamers can create much larger PD events. Consequently, damage observed on GRP barriers is much more severe than that on PTFE barriers under similar experimental conditions. Thus, design of future HTS power apparatus must consider this degradation phenomenon in order to improve the reliability of the insulation system.

Dilute magnetic semiconductors are an important family of materials that have many potential applications in spintronics; (Ga,Mn)As, (In,Ga,Mn)As and (Ga,Mn)N are of major interest. This thesis investigates dierent aspects of these, using the synchrotron radiation techniques of x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD), supported by superconducting quantum interference device (SQUID) magnetometry and magnetotransport measurements. A large anisotropic XMLD signal is observed for the Mn L-edge in (Ga,Mn)As. In unannealed (Ga,Mn)As, an apparently reduced Mn magnetic moment is commonly observed. It is thought to be related to compensation of both carriers and magnetic moment, caused by interstitial Mn. This issue is investigated using combined data from XMCD, XMLD and SQUID magnetometry. The findings suggest that substitutional and interstitial Mn form `non-magnetic' pairs which do not have a preferred spin orientation. (Ga,Mn)N is studied by x-ray absorption and field-dependent XMCD at the Mn L-edge. Two distinct Mn congurations are identified: Mn2+ is prevalent towards the surface with nearly paramagnetic behaviour, while a weakly ferromagnetic Mn2+/Mn3+ mixed valence exists within the bulk. The weak ferromagnetism, often observed in (Ga,Mn)N, is attributed to coupling between the impurities by the double exchange mechanism. Finally, XMCD is used to measure the orbital polarization of As 4p states of (III,Mn)As materials. These states correspond to those of the holes involved in the itinerant exchange interaction in ferromagnetic semiconductors. The coupling between the localized d states of the magnetic impurities and the valence band p states of the host is demonstrated by an anisotropy in the orbital moment of these states. This is experimental confirmation of the origin of the magnetocrystalline anisotropy in dilute magnetic semiconductors.

This thesis presents the work done to characterise two superconducting materials. We study BiPd, a non-centrosymmetric superconductor which is theoretically expected to show signs of spin singlet and triplet mixing due to the strong spin-orbit scattering of its composing elements. We map the field-temperature superconducting phase diagram along two crystal directions using Small Angle Neutron Scattering (SANS), magnetisation and \(µ\)SR measurements and determine the microscopic parameters defining the superconducting state. We also uncover a rare behaviour displayed in low-\(k\) superconductors, the Intermediate Mixed State, which causes domains of vortex lattice with constant spacing to coexist with Meissner domains at low applied fields. Finally we show evidence that, unlike what was expected, the superconductivity in BiPd behaves conventionally. The second material studied is Nb3Sn, widely used to produce large magnetic fields in various devices such as MRI machines. We investigate the superconducting state of several polycrystalline samples with different tin concentrations, as recent evidence point towards a lack of change of the upper critical field with varying Sn doping, in contradiction with older measurements that see a drop in H\({c2}\) associated with the apparition of a structural (martensitic) crystalline transition. Using SANS, we show that these recent results were likely not measuring the bulk state of Nb3Sn and that we find large variation of H\({c2}\) with Sn concentration. We also present indications that the vortex lattice is influenced by non-local effects at large fields by measuring the change in the vortex lattice structure with field. Lastly, our measurements are consistent with a full single gap behaviour in Nb3Sn.

AC susceptibility is an important characterisation technique measuring the time dependent magnetisation and dynamics of a magnetic system. It is capable of yielding information on thermodynamic phase transitions, relaxation processes and losses in a variety of interesting magnetic and superconducting materials. In particular it is a powerful probe of the mixed state of superconductivity providing insight into the ux dynamics at play and determination of a number of physical properties such as the critical temperature Tc, field Hc and characteristic length scales. Application of pressure can tune materials through multiple phases and interesting phenomena. The thesis describes the design of a calibratable susceptometer in a piston cylinder pressure cell, achieving AC susceptibility measurements of the same accuracy as a SQUID magnetometer but under pressure. This is used to make measurements on an electrostatically doped capacitance device, a single chain magnet and a heavy fermion superconductor. These studies are summarised below. Electric double layer (EDL) devices provide a means of continuous tuning through a materials phase diagram by applying an electric field, including inducing superconductivity. Application of pressure in tandem with electrostatic doping could improve the efficiency of these devices and provide a second tuning parameter. An EDL capacitor was constructed and measured with the above susceptometer aiming to shift the Tc of a doped high temperature superconducting cuprate La1:9Sr0:1CuO4. The Tc shifts proved irreproducible already at ambient conditions. Indeed during the course of this research further experimental evidence emerged in the literature indicating EDL devices may very well work due to electrochemical doping rather than electrostatic, possibly accounting for the lack of repeatability. Work therefore focused on mapping the ionic liquid DEME-TFSI's glass-liquid phase diagram over the 1 GPa pressure range, rather than extending the study of the EDLC device to high pressure. Single chain magnets (SCM) are an interesting class of material consisting of a one-dimensional molecular magnet chain manifesting magnetic hysteresis and slow relaxation best characterised by AC susceptibility. The susceptometer was used to study the SCM [Co(NCS)2(pyridine)2]n to investigate the effect of pressure on its characteristic magnetic relaxation time and energy barrier. A secondary signal appears at ~0.44 GPa which is attributed to the development of an additional structural phase that has been independently observed in X-ray crystallographic measurements. The heavy fermion superconductor U6Fe has the highest Tc ~4 K of all the U-based compounds and large critical fields of ~10-12.5 T, depending on direction, which increase on initial application of pressure. It exhibits a coexisting charge density wave (CDW) below 10 K making it a promising candidate for the modulated superconductivity of the theorised Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. A feature at 110 K is also evident in Mossbauer, resistivity and specific heat measurements, the origin of which has not yet been clearly identified. Evidence for the FFLO state was sought by mapping the upper critical field Hc2 along with the peak effect through AC susceptibility measurements up to pressures of 1 GPa. The data is accounted for by an evolution of collective pinning and superconducting parameters, with no clear evidence for an FFLO state although an enhancement of the reduced field is observed.

The gradual increase in global warming and environmental pollution has made low-carbon technologies an urgent need for the whole world. Superconducting technology, which is known for its extremely high conductivity and high power density, is capable enough to provide novel solutions, contributing to the future smart grid, thus aiding the power industry towards the realisation of a low-carbon and green planet. In recent decades, several industrial applications using superconducting technology have been developed. Of them, particularly in the power industry, a range of superconducting applications including superconducting magnetic energy storage (SMES), superconducting motors/generators, superconducting cables and superconducting fault current limiters (SFCLs) have been developed. Among them, SFCLs are one of the most promising and are successfully being implemented in power distribution networks. SFCLs exhibit low impedance during normal operation and gain considerable impedance under a fault condition, providing a new solution to the increasingly high fault current levels. However, most of the SFCL projects are limited to low-voltage and medium-voltage levels, there are very few successful operational trials of high voltage SFCLs. This thesis, for the first time, studies the comprehensive characteristics of solenoid type SFCLs based on second generation (2G) high temperature superconductors (HTS), which may be successfully implemented in power grids with high voltage levels. The main contributions of this work include three aspects: 1) proposing an innovative method for simulating the AC losses of the solenoid coils and an electro-magneto-thermal model for simulating the SFCL’s current limiting property; 2) comprehensive and in-depth comparison study concerning the application of the two types of non-inductive solenoid coils (braid type and non-intersecting type) in SFCLs both experimentally and numerically; and 3) the first and thorough discussion of the impact of different parameters such as pitch and radius of coils to the overall performance of braid type SFCLs and the validation of the braid type SFCL concept with a 220 V/300 A SFCL prototype. Based on these experimental and simulation works, the thesis provide strong guidance for the development of future non-inductive solenoid type SFCLs based on 2G HTS, which are promising for high voltage level power grid applications.