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Sm-Ba-Cu-O (SmBCO), which is a member of rare-earth barium cuprate [(RE)BCO] high-temperature superconductors, has significant potential for practical applications due to its higher critical transition temperature (Tc), higher critical current density (Jc), the so-called ‘peak effect’ characteristic at relatively high applied magnetic field and higher irreversibility field than that of the more established YBCO. The aim of this study is to investigate and overcome the obstacles in fabricating SmBCO bulk superconductors in air, to further improve their superconducting properties and, finally, to scale-up the fabrication of SmBCO single grains, therefore realising the engineering applications of this technologically important material. A modified seeding technique using an MgO-NdBCO generic seed accompanied by a buffer layer has been developed to process the SmBCO system to increase the success rate of growing single domain, bulk SmBCO superconductors in air via a top-seeded melt growth process using a conventional chamber furnace. Subsequently, the effects of doping in SmBCO bulk superconductors on the performance of SmBCO superconductors containing different dopants are discussed based on an analysis of their superconducting properties, including Tc and Jc, and on the microstructures of the samples. Furthermore, the scale-up of SmBCO bulk superconductors has been achieved by the addition of silver to the precursor powders and the introduction of a Y-123 layer beneath the SmBCO bulk pre-forms. Finally, trapped field measurements on successfully grown SmBCO bulk superconductors up to 41 mm in diameter have been performed and used to demonstrate significantly improved field trapping ability due to the optimization of the processing and composition of the SmBCO system. An Ag-SmBCO single grain of diameter 31 mm has achieved 1.033 T at 77 K, which is the highest value of trapped field reported worldwide for SmBCO samples of a similar size grown in air.

The work presented in this thesis investigates transitions between quantum states in superconducting double dots (SDDs), a nanoscale device consisting of two aluminium superconducting islands coupled together by a Josephson junction, with each dot connected to a normal state lead. The energy landscape consists of a two level manifold of even charge parity Cooper pair states, and continuous bands corresponding to charge states with single quasiparticles in one or both islands. These devices are fabricated using shadow mask evaporation, and are measured at sub Kelvin temperatures using a dilution refrigerator. We use radio frequency reflectometry to measure quantum capacitance, which is dependent on the quantum state of the device. We measure the quantum capacitance as a function of gate voltage, and observe capacitance maxima corresponding to the Josephson coupling between even parity states. We also perform charge sensing and detect odd parity states. These measurements support the theoretical model of the energy landscape of the SDD. By measuring the quantum capacitance in the time domain, we observe random switching of capacitance between two levels. We determine this to be the stochastic breaking and recombination of single Cooper pairs. By carrying out spectroscopy of the bath responsible for the pair breaking we attribute it to black-body radiation in the cryogenic environment. We also drive the breaking process with a continuous microwave signal, and find that the rate is linearly proportional to incident power. This suggests that a single photon process is responsible, and demonstrates the potential of the SDD as a single photon microwave detector. We investigate this mechanism further, and design an experiment in which the breaking rate is enhanced when the SDD is in the antisymmetric state rather than the symmetric state. We also measure the quantum capacitance of a charge isolated double dot. We observe 2e periodicity, indicating the tunnelling of Cooper pairs and the lack of occupation of quasiparticle states. This work is relevant to the range of experiments investigating the effect of non-equilibrium quasiparticles on the operation of superconducting qubits and other superconducting devices.

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.

This thesis aims to improve our understanding of intrinsic resistive switching behaviour in silicon suboxides using transmission electron microscopy characterisation and density functional theory modelling. The main new results of this thesis can be summarised as follows. In sputter-deposited silicon suboxides, oxide-wide structural reorganisation occurs during electrical stressing. This is a result of large-scale oxygen dynamics, which can result in oxygen outmigration from the oxide and electrode deformation. The fabrication of sputter-deposited silicon suboxides greatly influences device performance. Firstly, growing the oxide layer on a rougher substrate surface promotes lower electroforming voltages and greater device endurance. This is consistent with enhanced columnar microstructure in the oxide. Secondly, thin oxide layers (< 5 nm) will lead to electrode migration into the oxide layer as a result of high electric fields. This will limit the thickness of the oxide layer needed for intrinsic switching behaviour. The formation of oxygen vacancy dimers and trimers is energetically favourable at some sites in amorphous silicon dioxide, with maximum binding energies of 0.13 eV and 0.18 eV, respectively. However, neutral oxygen vacancies are immobile under room temperature operating conditions and diffuse with a mean adiabatic barrier height of 4.6 eV. In amorphous silicon dioxide, double electron trapping is energetically feasible at oxygen vacancies at Fermi energies above 6.4 eV. This greatly improves vacancy mobility; however, vacancy diffusion competes with thermal ionisation of the electrons into the conduction band. Oxygen vacancies also compete with intrinsic sites for electron trapping. This results in an inefficient diffusion process, which cannot explain the formation of a silicon-rich conductive path. These results will help guide the optimisation of future silicon suboxide-based resistive random access memory and provide new insights into the role of oxygen vacancies during the electrical stressing of silicon oxides.

I have investigated atomic-scale condensed matter, with a view towards future technology, covering two areas: point defects in a well studied and used system, Si; and a less studied semiconductor, MoS2, where I have focussed on the fundamental properties and have observed charge density waves (CDWs). Scanning tunnelling microscopy (STM) has been used over the past two decades to investigate semiconductor point defects. Here, an investigation of dangling bonds (DBs) on H-terminated Si(001) is presented. The STM tip is used to desorb H atoms to create DBs [1]. Pairs, or dimers, of DBs interact as their excited states overlap, signalled by a bright protrusion appearing between them [2]. Tip-induced band bending calculations show the dimer’s excited state comes into tunnelling range at high biases and low currents. The energy alignment with the tip Fermi level is also affected by additional DBs in the vicinity. 2D structures of DBs, including trimers and tetramers, exhibited 2D excited states. By modelling each DB with a 2D potential well holding two bound states, we could simulate the DB structure’s bound states. Using a tunnelling matrix element approximation, the bound states were combined to successfully re-create imaged quantum states. STM has been used to study correlated ground states of layered materials, including transition-metal dichalcogenides (TMDs). Here, it is used to discover and characterise the CDW of K-intercalated MoS2. This TMD is semiconducting but it is doped by the intercalating species, giving it metallic character and inducing superconductivity [3, 4]. The CDW was observed at temperatures of 78 K and below, appearing in patches localised to subsurface defects. The CDW coverage increased as the bias magnitude was decreased, however the wavevector or periodicity remained invariant; a feature of CDWs. Other evidence presented includes a gap in the density of states and the occupied and unoccupied state modulations being out of phase.

Ferroelectric materials are of technological importance for many applications and a thorough understanding of the origins of their high piezoelectric and dielectric properties is needed to optimise materials performance. One of the most widely used ferroelectrics is Pb(Zr1 xTix)O3 (PZT), which shows excellent piezoelectric response near its morphotropic phase boundary (MPB). To study PZT in large scale molecular dynamics (MD) simulations, I have developed a shell model force field that reproduces the details of the phase diagram of the PZT solid solution. The developed force field supports the temperature induced phase transitions from cubic to low symmetry phases over the whole composition range and additionally reproduces the composition driven phase transitions. This force field was subsequently used to study a variety of different effects in PZT. First, the polarisation switching in near morphotropic PZT at different temperatures was investigated, where I report a reduction of the saturation polarisation, accompanied by an exponential increase of the coercive field, as the temperature decreased. The simulations further demonstrated that the switching mechanism is fundamentally different at low temperatures, where it occurs via polarisation rotation, than at high temperatures, where domain nucleation dominates. Second, a set of simulations on near morphotropic PZT showed that the response of the local structure to electric fields highly depends on the local B-cation environment. Furthermore, I find that B-cation clustering largely influences the piezoelectric properties of the material. In a third MD study the electrocaloric effect (ECE) in the PZT compound was calculated, finding that PbTiO3 outperforms other compositions with its giant temperature change of 16 K. Lastly, density functional theory (DFT) calculations were performed to study defects and domain walls in the PZT end member PbTiO3, where we show that the presence of a couple donor-acceptor defect pair stabilises charged head-to-head and tail-to-tail 90 domain walls in PbTiO3.

In this thesis work, I employ a wide range of morphological, structural and dynamical tools to investigate how the intermolecular interactions in a variety organic semiconductors (OSCs) can influence heavily the nanoscale arrangement and govern the tolerance against external stimuli (i.e. radiation) of these promising materials and related devices. Among the experimental techniques used, this work emphasises on the use of neutron scattering, due to the strong propensity of highly hydrogenated materials to scatter these particles, and thus reveal precious information about their inner structural and dynamical features. I exploit the power of intermolecular interactions, to grow large and solvent-free single crystals of the electron-acceptor of choice of organic solar cells, the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure and quality of these crystals have been characterised by X-rays diffraction and metrological atomic force microscopy experiments. High-quality and solvent-free crystals of organic semiconductors can be a reliable platform for studying the intrinsic optical and electronic properties of these materials. The nanoscale structure and dynamics of poly (thiophene):fullerene blends has been investigate by means of neutron reflectivity and quasi-elastic neutron scattering. In particular, I show that the intercalation of fullerene particles in between polymer side-chain depends on degree of order of the polymer. Furthermore, I find that polymer:fullerene interaction has a great effect on polymer dynamics, leading to a retardation of side-chain motion. The interaction between polymer and fullerene particles and their nanoscale mutual arrangement is of great interest for improving the charge phogeneration process in organic solar cells. Finally, I studied for the first time the neutron radiation tolerance of two benchmark poly(thiophenes), observing that polymer crystallinity has a prominent role in determining the neutron radiation tolerance of conjugated polymers. Radiation hardening studies on organic electronics can pave the way to the integration of this class of devices in space and avionic applications.

Charge transport in organic semiconductors is an important current topic of research, but the exact nature of the charge transport remains an unresolved question. Experimental evidence exists to support either of two common models (band-like transport or small polaronic hopping) and various computational simulations suggest that for the standard parameter ranges for organic semiconducting devices, both of these models are likely invalid, with temperatures too high for a band-like transport model and couplings too high relative to the reorganization energy for charge localization to be assumed. There is potential for a non-adiabatic molecular dynamics method that partially separates classical and quantum degrees of freedom, such as surface hopping, to be applied to the problem. This is what I have begun with my fragment-orbital based surface hopping (FOB-SH) method. Based on Tully's famous fewest-switches surface hopping algorithm, FOB-SH simulates a condensed phase organic semiconductor with a classical molecular dynamics approach while solving the Schr\"odinger equation to directly model the behaviour of a single excess electronic charge. The latter is made computationally efficient by using an analytic overlap method to calculate the Hamiltonian off-diagonal elements. In this thesis I discuss in detail the theory behind FOB-SH, along with my first implementation and validation of the method. I present results on two systems, of two and ten `ethylene-like' molecules respectively. In the two-molecule system I calculate charge transfer rates and find that my method qualitatively agrees with standard charge transfer theory in regimes where agreement is expected, though questions are posed in regimes where standard theory becomes invalid. For the ten-molecule system I demonstrate that charge mobilities can be calculated from my simulations. I observe a thermal activation peak for low couplings and a crossover from activated hopping to band-like transport with increasing temperature, qualitatively agreeing with another similar surface hopping method. I show that FOB-SH has great potential to tackle the charge transport in organics problem.

Charge transport in organic semiconductors (OSCs) depends on a number of molecular properties, one of which is the electronic coupling matrix element for charge transfer between the molecules forming the material. They are the off-diagonal elements of the electronic Hamiltonian in the charge-localised (or diabatic) basis. The focus of this work is on the development of a method for a fast calculation of these matrix elements for OSCs. After addressing the different methods of their calculation, I present a program to estimate the off-diagonal elements of the Hamiltonian with a fast yet accurate semi-empirical method. This model approximates the off-diagonal elements of the Hamiltonian to be proportional to the overlap between the orbitals of the molecules, which are projected onto a very small basis set. The analytical results are in a reasonable agreement with accurate ab initio and fragment orbital DFT calculations and the speed-up is up to six orders of magnitude compared to DFT calculations. Following on from this, the analytic overlap method was implemented in two programs for charge carrier propagation, one based on Kinetic Monte Carlo simulation of charge carrier hopping (presented here), the other on surface hopping non-adiabatic molecular dynamics. I also show that the analytic overlap method can be used to estimate non-adiabatic coupling vectors very efficiently, which is an important quantity in surface hopping simulations.

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.

The discovery of high-temperature superconductivity in 1986 in copper-oxide materials have opened up new avenues to investigate new families of quantum materials that were previously not known. Understanding the mechanism of superconductivity in high-T[sub]c superconductors has been an important research theme in condensed matter physics, as it is believed to be essential to realize the next generation engineered materials that become superconducting at room temperature. Discovered in 2006, iron based superconductors are a new addition to the family of high-T[sub]c superconductors, these materials exhibit several interesting properties and show some vivid similarities with cuprates and other families of high-temperature superconductors. In this thesis, I will present the spin-polarized scanning tunneling microscopy (SPSTM) study carried out on the parent compound of iron chalcogenide high temperature superconductor Fe[sub](1+y)Te to investigate the bi-collinear antiferromagnetic order. Magnetic tips in this work are prepared using a novel preparation technique by picking up excess iron atoms and clusters of FeTe from the surface of the sample. Next, I will present the SP-STM results obtained in the spin glass phase of Fe[sub](1+y)SeₓTe₁₋ₓ visualizing the interplay between the short ranged bi-directional bi-collinear antiferromagnetic order and superconductivity at the atomic scale. In this thesis, I will also present the scanning tunneling microscopy and spectroscopy (STM/STS) study of the native and engineered defect bound states in the iron-pnictide superconductor LiFeAs. This study addresses the pairing symmetry of the superconducting order parameter and understanding of dip-hump features seen in STM spectra outside the superconducting gap in iron pnictide superconductor LiFeAs.

The work presented in this thesis aims to reduce cost per capita and increase the conversion efficiency of CdS/CdTe-based solar cells using multilayer graded bandgap configurations. To effect economic viability, electroplating technique was utilised as the low-cost semiconductor deposition technique. CdS and CdTe are the primary materials grown and explored in this thesis. These layers were characterised for their structural, morphological, compositional, optical, and electrical properties using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-Vis spectroscopy, photoelectrochemical cell measurement, current-voltage and capacitance-voltage measurement techniques. The precursor (thiourea) utilised in the deposition of CdS solves the problem of sulphur precipitation during electroplating. Very few publications with explicit exploration on this subject are available in the literature. Further to this, the nitrate and chloride based CdTe precursors explored is an alternative to the sulphate based norm as reported in the literature. The effect of extrinsic doping of CdTe incorporating F, Cl, I and Ga was also systematically explored to improve both the material and electronic properties of CdTe. Doping of CdTe during growth was studied by adding the above dopants to the electrolyte. The inclusion of F and Ga into the regular CdCl2 post-growth treatment (PGT) and the effect of pH were also explored with improved results as compared to the regular CdCl2-PGT. The inclusion of Ga into the regular CdCl2 post-growth treatment (PGT) and the effect of pH were also explored with the improved result as compared to the regular CdCl2 PGT. The solar cell device configurations explored include n-n+large Schottky barrier (SB), n-p, n-n-n+SB and n-n-p incorporating CdS/CdTe-base semiconductors. Based on all experimental findings as explored within the limit of this thesis, the most promising of the configurations examined are the glass/FTO/n-CdS/n-CdTe/p-CdTe/Au with thicknesses of glass/FTO/(120 nm) n-CdS/(1200 nm) n-CdTe/(30 nm) p-CdTe/(100 nm) Au. The highest conversion efficiencies observed for two separate batches were 15.3% and 18.4%. The devices with the 18.4% efficiency showed some instability and therefore require further investigation. The glass/FTO/n-ZnS/n-CdS/n-CdTe/Au configuration with thicknesses of glass/FTO/50 nm n-ZnS/(65 nm) n-CdS/(1200 nm) n-CdTe/(100 nm) Au also show promising results with the highest efficiency achieved being 14.1% owing to bandgap grading strengths.

A systematic and detailed design, building and testing of a high-sensitivity real-time magnetovision imaging system for non-destructive testing (NDT) was the purpose of the research presented here. The magnetic imaging systems developed were all based on an ultra-high sensitivity Quantum Well Hall Effect (QWHE) sensors, denoted as the P2A, which is based on GaAs-InGaAs-AlGaAs 2DEG heterostructures. The research progressed from 0D (single sensor) to 1D (linear array) to 2D (two dimensional arrays) testing modalities. Firstly, the measurement of thermal and shot noises, drift, detection limit, and dynamic offset cancellation of the QWHE sensor were studied in detail to set the framework and limitations of the fundamental QWHE sensors before their eventual use in the imaging systems developed subsequently. The results indicate that the measured data agrees well with calculations for thermal and shot noise when the input bias current is < 3 mA. The measured drift voltages of various QWHE sensors (P2A and P3A) are less than 200 µV when the sensor bias voltage is less than or equal to 2 V. A 4-direction dynamic offset cancellation technique was developed and the results show that the offset equivalent magnetic field of the QWHE sensors can be reduced from ~ 1mT to readings equal to the Earth magnetic field (~ 50 µT). Secondly, a flexible 16 × 1 array and a 32 × 2 staggered array magnetic-field scanners were designed, built, and tested. The QWHE magnetometer had a field strength resolution of 100 nT, and a measurement dynamic range of 138 dB. The flexible 16 × 1 magnetic field scanner can be used to test uneven and/or curved surfaces. This gives the flexible magnetic field scanner better inspection capabilities in both welding hump and circular pipe samples. By the staggered arrangement of two sensor arrays, a 15.4 point per inch horizontal spatial resolution can be achieved for the staggered 32 × 2 magnetic field scanner. Both direct and alternating magnetic flux leakage (DC and AC MFL) tests with the QWHE magnetometer were accomplished to obtain graphical 2-dimensional magnetic field distributions. Both the shape and the location of defects can be identified. The results show that the sensor has high sensitivity and linearity in a wide frequency range which makes it an optimum choice for AC-MFL testing and both ferromagnetic and non-ferromagnetic materials can be investigated. Thirdly, real-time 8 × 8 and 16  16 QWHE array magnetic-field cameras were designed, built, and tested. These prototypes can measure static magnetic field strengths in a 2-dimensional plane. Different shapes of magnets and magnetic field polarities can all be identified by the 8 × 8 magnetic field camera. The camera has a resolution of 3.05 mT, and a dynamic range of 66 dB (the minimum and maximum fields measurable are 3.05 mT and 6.25 mT) and a real time magnetic field measurement rate of 13 frames per second (FPS). By contrast the1616 array magnetic field camera has an improved sampling rate of 600 frame per second and with the use of an interpolation technique, a spatial resolution of 40.6 point per inch can be achieved. The minimum and maximum detectable magnetic field for this magnetic field camera are 1.8 µT and 29.5 mT respectively leading to a record dynamic range of 84 dB for high quality imaging. Finally, a novel, hand held, magnetovision system based on the real-time 16 × 16 QWHE array magnetic-field camera was developed for improved DC and AC electromagnetic NDT testing. The system uses a new super heterodyne technique for data acquisition using the QWHE sensor as a multiplier. This is the first report of such a technique in Hall effect magnetometry. The experimental results of five case studies demonstrate that the defects location and shape can be successfully measured with MFL in DC and AC magnetic field configurations including depth profiling. The major advantages of this real-time magnetic-field camera are: (1) its ease to use as a MFL testing equipment in both DC and AC NDT testing, (2) its ability to provide 2D graphical images similar to Magnetic Particle Inspection (MPI) but without its inherent health and safety drawbacks, (3) its capability to test both ferromagnetic and non-ferromagnetic materials for deep defects below the surface using low frequency alternating magnetic fields, and (4) its ability to identify materials (metals) by alternating external magnetic field illuminations, which has considerable potential in several applications such as security checking and labelling, magnetic markers for analysis, bio-imaging detection, and medical treatments amongst others.

The thermoelectric properties and microstructure of two families of misfit type layered structure cobaltites were investigated for thermoelectric applications. Firstly, Bismuth strontium cobaltite ceramics with the formulations Bi2+xSr2Co2Oy (x=0, 0.1 and 0.2), Bi1.74Sr2Co1.8Oy and Bi2Sr2Co1.8Oy were produced using solid-state reaction (MO) method. The same powders were also used to produce ceramics by Spark Plasma Sintering (SPS) fabrication technique. SEM, high resolution XRD and HRTEM techniques has been employed to characterise the microstructure and crystal structures of the ceramics. Figure of merit (ZT) was also determined from measurement of electrical resistivity, Seebeck coefficient and thermal conductivity. Together with the above, calcium cobaltite of formulation Ca3-xBixCo3O9 (x=0 and 0.3) was also produced via MO and SPS routes. The same characterisation techniques were used for characterisation of calcium cobaltite. For Bi2+xSr2Co2Oy ceramics, it was found that SPS fabrication is essential to obtain high density samples. Excess bismuth has a major role in the adjustment of the microstructure and thermoelectric properties. The room temperature microstructure contains two minor phases with compositions of CoO and Bi0.75Sr0.25O1.26. The crystal structure of the main phase was successfully indexed and refined as misfit type structure having monoclinic symmetry with I2/a space group. A high ZT of 0.12 was achieved in both x=0.1 and 0.2 MO samples. For Bi2Sr2Co1.8Oy ceramics, the microstructure contains only one minor phase, Bi0.75Sr0.25O1.26. A high ZT of 0.16 was obtained at 900 K for this composition. For Bi1.74Sr2Co1.8Oy, it was not possible to obtain high density ceramics by MO route and SPS fabrication was necessary. However, SPS sample showed a low ZT of 0.04 at 900 K.For ceramics of formulation Ca3-xBixCo3O9 (x=0 and 0.3), it was difficult to obtain high density calcium cobaltite ceramics by MO route and SPS fabrication was found to be essential. In addition to improved density, SPS produced textured microstructure. Similar to bismuth strontium cobaltite, excess bismuth played a major role in microstructure development and thermoelectric properties. Single phase and high density Ca3Co4O9 ceramics were obtained by SPS. A minor phase of Bi2Ca2Co2Oy was found in the microstructure of Ca3-xBixCo3O4 (x=0.3) samples. A high ZT of 0.25 was obtained for Ca3Co4O9 SPS samples at 900 K through improvement of power factor. In-situ synchrotron XRD in the temperature range of 300-1223 K was performed on both Bi2Sr2Co2Oy and Ca3Co4O9 to obtain their high temperature structural characteristics. The crystal structure of both compounds remains unchanged till 1223 K. For, Bi2Sr2Co2Oy the CoO and Bi0.75Sr0.25O1.26 minor phases disappear above 1073 K and a new minor phase containing (Bi-Sr-O) or (Bi-Co-O) starts forming. On heating, the lattice volume and coefficient of thermal expansion change linearly for both compounds. Thermal expansion coefficient was found to be 0.0000353-0.0000343 and 0.0000296-0.0000288 K-1 over the temperature range of 300-1223 K for Bi2Sr2Co2Oy and Ca3Co4O9 respectively.

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.

In this thesis we develop a new first-principles method for the calculation of optical absorption spectra and band structures in semiconductors and insulators at finite tem- peratures. The theoretical framework of our methodology originates back to 1950s in two pivotal research papers by F. Williams and M. Lax. Here, we expand the scope of the pioneering works by Williams and Lax, and we present a new theory of phonon- assisted optical absorption and temperature-dependent band structures. We demon- strate that our technique is highly efficient and simple to the point that a single calcu- lation is sufficient to capture temperature-dependent absorption coefficients including the effect of quantum zero-point motion. We report calculations of optical absorption spectra and of direct and indirect band gaps of Si, C, GaAs and MAPbI3 . We obtain good agreement with experiment and with previous calculations. The approach pro- posed in this thesis is highly versatile, and can straightforwardly be combined with improved descriptions of the dielectric function by including electron-hole effects via the Bethe-Salpeter and GW equations. In this thesis we also investigate the underly- ing mechanisms leading to the "inverse Varshni effect" in materials that belong to the family of metal halide perovskites. We show, using the example of MAPbI 3 , that the vibrational modes with metal-halide-metal bending or rocking character are the major cause of the band gap opening with increasing temperature. To this aim we present an approach for elucidating the physics underpinning the changes of the band gap with vibrational motion. Our methodologies developed in this thesis are simple to imple- ment in any electronic structure package as a post-processing step, having the potential to find broad applications in the ab-initio community. We anticipate that our work will open the way to predictive calculations, as well as will contribute to the better under- standing of the optoelectronic properties of solids at finite temperatures.

After more than thirty years since the discovery of high temperature superconductivity in the cuprates, their properties still lack a complete theoretical understanding. In this work, we will argue that the key role to decipher the phase diagram of these compounds lies in the physics of the Mott transition, combined with the effect of short-range order correlations. By the use of Cellular Dynamical Mean-Field Theory with the Hybridisation Expansion Continuous-Time Quantum Monte Carlo as the impurity solver, we begin examining the two-dimensional Hubbard model. The comparative analysis at half-filling of the properties of the antiferromagnetic and normal state reveals a detectable, sharp crossover in the condensation energy linked to the underlying Mott transition. Upon doping the system, the study of several parametric regimes in the presence of d-wave superconductivity reveals the role of the pseudogap to correlated metal transition, hidden under the superconducting dome. The Widom line is a line of crossovers that emerges at high-temperature from this transition. This supercritical behaviour not only determines the shape of this dome but also the maxima of $T_c$ at optimal doping as well as the driving mechanism that allows the superconductivity to occur. Furthermore, it explains how the condensation energy can change from potential-energy driven to kinetic-energy driven upon a reduction in doping. The first-order transition affects the superconducting properties, providing an organising principle for the whole phase diagram. Additionally, we investigate a more realistic model for the cuprates that includes three orbitals per unit cell, the Emery model. We compute the finite temperature behaviour of the metal to charge-transfer insulator transition driven by the interaction and of the pseudogap to correlated metal transition driven by increasing the hole carrier concentration. The features of the superconducting and normal states confirm the Hubbard model scenario, despite the large differences in microscopic details, such as the presence of oxygen and the different band structure.

The quantum Hall effect (QHE) is used realise the standard for the unit of electrical resistance, the ohm, in terms of fundamental constants h=e^2. Epitaxial graphene on silicon carbide has become the system of choice for this because of the large inter-Landau level spacing, fast energy relaxation and very robust quantum Hall state, which is owed to the charge transfer between graphene and the substrate. There are however non-equilibrium dynamics in the quantum Hall regime in graphene, which need to be understood in order to better implement the resistance standard in the future. This thesis concerns the study of this non-equilibrium dynamics in graphene, and in particular the breakdown of the QHE. In conventional semiconductor 2DEGs it is achieved by the study of the spectrum of the terahertz radiation emitted from the hot spots, where the electrons enter and leave the 2D conductor. In graphene the non-equilibrium dynamics can be investigated using the weak localisation corrections to the conductivity, which are sensitive to any time-reversal symmetry breaking perturbation, including temperature, magnetic field or ac electric field. High-frequency microwave radiation was employed to generate hot electrons and showed that the response of graphene is entirely bolometric. The response to MIR radiation (5-10 um) was studied by measuring magnetoresistance of a graphene Hall bar in a wide range of magnetic fields. At intermediate magnetic fields, the response is a combination of the direct bolometric effect and a change in the carrier density due to radiation. The cyclotron resonance did not appear as expected at the appropriate magnetic fields, which is discussed. Finally, the breakdown of the QHE in graphene was investigated by measuring cross-spectral noise power density over a 500 kHz window centred at 3.22 MHz. Comparing the spectra to those of a GaAs sample it seems that the breakdown in graphene may occur in the bulk of the graphene sample rather than in well-defined hot spots.

The miniaturisation of semiconductors is running into physical limitations. In order to further enhance computational powers, reaching the atomic or molecular level is both necessary and inevitable. At the nanometre scale, the quantum properties of materials become important. These can be used for qubits in quantum computers, which are able to process information significantly faster than traditional counterparts. One approach towards such novel nanoelectronic devices is realised via molecular magnetic materials. Single-molecule magnets, which represent the smallest possible molecular magnetic structures, are currently being tested in such devices. However, the physics of these materials is not yet completely understood. The magnetic property of electrons, known as the electron spin, plays a crucial role here. It is furthermore very sensitive to its surroundings. Interactions with neighbouring nuclear spins, for instance, lead to decoherence, which translates to a loss of stored quantum information. In order to promote the development of new elements for nanoelectronics or qubits, it is therefore essential to unravel the couplings of the spins and their surroundings, and learn how to control and manipulate them. The present work focuses on how light, as an ultra-clean technique, can be used to manipulate the molecular spin states, and how magnetic centres interact when chemically attached to a graphene nanoribbon. We employ state-of-the-art electron spin resonance techniques and SQUID magnetometry to investigate the effect of light on a new cobalt valence tautomer, where we find an enhanced blocking temperature of a metastable light-excited state. Furthermore, we examine an Fe3Cr-based single-molecule magnet, where light causes a change in the giant spin. We then progress to graphene nanoribbons, which offer an interesting semiconducting backbone for magnetic molecules. Through a comprehensive analysis of spin-spin couplings in nitronyl-nitroxide graphene nanoribbons, we eventually prove the existence of an edge state, which allows for two-qubit applications.

The world demand for energy is estimated to increase by up to 70% from 2016 to 2040. To meet this demand in a sustainable way, the power density of electric motors and generators can be increased by using superconducting materials. In particular, trapped field superconducting magnets, where the field is generated by a circulating persistent current in the sample, can create magnetic fields an order of magnitude higher than possible using conventional ferromagnets, thus increasing the power density of motors and generators. This is of great interest where added weight comes at a premium, such as for naval propulsion motors, wind generators and motors/generators for future electric aircraft. This work investigates the suitability of stacked tape layers of second generation high temperature superconductors (HTS), such as YBa2Cu3O7-x (YBCO) for trapped field applications. The present limits for trapped field magnitude have been determined, which provide a basis for the optimization of pulsed field magnetization techniques for in-situ magnetization in motors and generators. Trapped fields were increased by optimising the magnetic pulse sequence, using thermally conductive material to reduce temperature rise during pulse and changing the duration of the magnetic field pulse. Finite element method computer modelling was used to model and predict the behaviour of the trapped field magnets made of HTS tape with good agreement to experiment for both field cooling and pulsed field magnetisation. The models rely on critical current data for the HTS tape and its dependence on magnetic field and temperature. For this reason, a critical current testing facility was developed and constructed as a part of this work capable of measuring critical current up to 900 A, magnetic field of 1.5 T and down to temperatures of ~10 K in forced and dynamically controlled helium vapour flow. Lastly, first steps into scaling up by pulse magnetising an array of HTS tape stacks were made, allowing for larger overall trapped flux values. Such an array exhibits geometry, similar to what is going to be used in a functional motor prototype being developed in our research group (Applied Superconductivity and Cryoscience Group, ASCG). The work done culminated in the highest trapped field achieved to date using both field cooling (13.4 T between two stacks) and pulsed field magnetization (2.1 T above a single stack), for this type of trapped field magnet.

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 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.

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.

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.

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.

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.

The study of cuprate high-temperature superconductors has undergone a recent resurgence due to the discovery of charge order in several families of cuprate materials. While its existence is now well established, little is known about its microscopic origins or its relationship to high-temperature superconductivity and the pseudogap. The aim of the research presented in this thesis is to address these questions. In this thesis I will report on the use of spectroscopic-imaging scanning tunnelling microscopy (SI-STM) to visualise the short-ranged charge density wave (CDW) in Bi₂Sr₂CaCu₂O₈₊ₓ and NaxCa₂₋ₓCuO₂Cl₂. Building on previous measurements of the intra unit-cell electronic structure of cuprates, I introduce sub-lattice segregated SISTM to individually address the atomic sub-lattices in the CuO₂ plane with spatial phase sensitivity. Using this technique I establish that the CDW in Bi₂Sr₂CaCu₂O₈+x and NaxCa₂₋ₓCuO₂Cl₂ has a previously unobserved d-symmetry form factor, where a breaking of rotational symmetry within the unit cell is modulated periodically in space. Towards identifying a mechanism of CDW formation, I establish that the amplitude of CDW modulations in the electronic structure are maximal at the pseudogap energy-scale and that these modulations exhibit a spatial phase difference of π between filled and empty states. Together with the doping evolution of the CDW wave-vector this highlights the role of the low-energy electronic structure of the pseudogap regime in CDW formation. To elucidate the relationship between the CDW and the superconducting condensate I will introduce nanometer resolution scanned Josephson tunnelling microscopy (SJTM). In this approach the Cooper pair (Josephson) tunnelling current between a Bi₂Sr₂CaCu₂O₈₊ₓ sample and a scan-able Bi₂Sr₂CaCu₂O₈₊ₓ nano-flake STM tip is used to directly visualise the superconducting condensate. I will report the observation of a periodic modulation in the Cooper pair condensate at the same wave-vector as the CDW, the first direct detection of a periodically modulating condensate in any superconductor.

In this thesis, I will present and discuss research performed on InGaN/GaN multiple quantum well (QW) structures. The results of which were taken using photoluminescence (PL) spectroscopy and PL time decay spectroscopy. In the first two experimental chapters, I report on the effects of QW growth methodology on the optical properties of c-plane InGaN/GaN QWs. I compare structures grown using the single temperature (1T), quasi-two temperature (Q2T), temperature bounced (T-bounced) and two temperature (2T) QW growth methodologies. The T-bounced and 2T structures are observed to have gross well width fluctuations (GWWF), where the QW width varies from 0 to 100 % created when the QWs are exposed to a temperature ramp. Whereas, the 1T and Q2T structures have continuous QWs with only one or two monolayer well width fluctuations. The structures with GWWFs are observed to have a larger room temperature internal quantum efficiency (RT-IQE) at low excitation conditions i.e. below efficiency droop compared to those without. The larger RT-IQE is ascribed to several factors which include an increased radiative recombination rate, increased thermal activation energy of non-radiative recombination and reduced defect density of the QWs. The effect of barrier growth temperature is also investigated. No clear trend is observed between barrier growth temperature and RT-IQE.In the last experimental chapter I report on studies of carrier localisation in InGaN/GaN QWs using resonant PL spectroscopy. The effect of carrier localisation on the independently localised electrons and holes are investigated and the resonant PL spectrum is studied in detail. The InGaN/GaN QW structure is observed to exhibit an effective mobility edge at 12 K where delocalised carriers are created above a particular excitation energy. The emission from the resonantly excited localised states which are accompanied by the emission of a longitudinal optical phonon (resonant LO feature) is investigated as a function of temperature and excitation energy. The integrated PL intensity of the resonant LO feature is observed to quench rapidly with temperature up to around 45 K, independent of excitation energy. The integrated PL intensity of the resonant LO feature is fitted to an Arrhenius model and a thermal activation energy of ∼ 1(±1) meV is extracted. This activation energy is speculated to be consistent with the localisation energy of electrons.

In this thesis, a weak coupling formalism is developed to study superconductivity in spin-orbit coupled, multi-orbital systems. This formalism is then applied to Sr2RuO4, one of the few candidates for odd-parity superconductivity. We show that spin-orbit coupling and multi-band effects are crucial to understand the physics of this material. Depending on the interaction parameters, the order parameter can either be chiral or helical. In both cases, the gap is highly anisotropic, and has accidental deep minima along certain directions, in accordance with experiments. Focusing then on the chiral case, we show that the total Chern number is -7 instead of the usually assumed +1. This leads to drastically different predictions for the thermal and charge Hall conductances. In particular, we show that the absence of measurable charge edge currents is not incompatible with a chiral state. Finally, we study the evolution of superconductivity in Sr2RuO4 under ?100? uniaxial strain. We find a good agreement with experiments for our prediction of Tc as a function of strain. Furthermore, we find that (1) the absence of a measurable cusp of Tc at zero strain is not incompatible with a chiral state and that (2) there could be a transition to an even-parity state at larger strain close to a Van Hove singularity. We propose Hc,2/Tc2 c as a measurable quantity to identify this transition.

Persistent current joints are critical components of superconducting magnets, and improvements in their properties are key to the production of next generation devices. Despite having been made routinely for more than half a century, the science underpinning the performance of joints between NbTi wires remains poorly understood. At this critical juncture, where new jointing methods are being sought for NbTi, it is essential that we develop a sound understanding of the factors in uencing joint quality. The aim of this thesis was to identify the characteristic microstructural and superconducting properties of joints produced by the three standard jointing methods for NbTi: soldering, spot welding and cold pressing. There were three main components to this work. The first was to obtain a complete characterisation of Jc(B, T) and Bc2(T) for the raw materials from which the joints were made, which was done by standard magnetic methods. Where relevant, the results have been interpreted in terms of the basic pinning mechanisms and microstructures in the materials tested. The implications for joint performance are discussed. Secondly, a novel magnetic method was developed to measure the superconducting performance of joints over an unprecedented range of operational conditions. The method involves winding a very small NbTi coil, closing the coil with the joint of interest, and testing the sample in a conventional SQUID magnetometer. The current carrying ability of joints can be measured simply from magnetic hysteresis loops, which enabled the performance of a range of joints to be analysed in fine detail and with considerable convenience. Furthermore, owing to the extremely low coil inductance and highly sensitive SQUID magnetometer employed, current decay curves corresponding to voltages as low as 10-14 V could be obtained in very short measurement times of just 20 minutes. Unfortunately, unexpectedly large drift in the background magnetic field was found to dominate in most measurements, but nevertheless the measurements provided an extremely rapid method of proving that the joints are truly superconducting. Suggestions are made as to how the technique could be improved to permit precise measurements of joint Ic(B, T) and V-I curves in the persistent mode regime, which would enable much needed fundamental studies into superconductivity in joints. Finally, the joint testing technique was employed to measure the superconducting performance of soldered, spot welded and cold pressed NbTi joints. The results were analysed along with the raw material superconducting properties and microstructures in the joints, which were obtained by scanning electron microscopy (SEM). This allowed the performancelimiting factors in each joint to be established. As a result of this study, a new soldering/cold pressing method is proposed which may permit direct metal-to-metal bonding between NbTi materials without the need for Pb or harmful chemicals.

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.

This thesis presents the experimental study of superconducting nanowires under the influence of ferromagnetic nano-structures. Placing superconducting and ferromagnetic materials in contact with one another causes their long range orders to compete. This manifests as the leakage of superconducting properties into the ferromagnet and the suppression of superconductivity in the superconductor near the interface, known as the proximity and inverse proximity eects, respectively. The experiments presented in this thesis aim to show that the inverse proximity eect is sensitive to the magnetization of the ferromagnet, specically that the suppression is weaker if the ferromagnet has an inhomogeneous magnetization. To do this, the magnetic vortex state in sub-micron nickel disks and L-shape domain wall traps were used as the inhomogeneous magnetizations. The magnetization in the nickel disks and L-shape domain wall traps were investigated using magnetic force microscopy (MFM), in situ MFM, magnetotransport, and modelling. Aluminium nanowires were deposited over the ferromagnets and low temperature transport measurements of the hybrid structures were performed. It is found that the superconductivity in the nanowire above the disks is suppressed, creating an SNS junction. The critical current is shown to be sensitive to the magnetic history of the disks. The critical current of the entire nanowire is found to be dependent on the properties of the hybrid junction. This long-range influence has a thermal origin due to Joule heating in the hybrid junction as demonstrated by use of heat sink structures and Andreev loop interferometers. Replacing the disk with Lshape domain wall traps shows that the suppression of superconductivity is weakest when a domain wall is placed beneath the nanowire. Lastly, comparison to theory indicates the junction length is proportional to temperature. The results presented demonstrate previously unknown complexity in the behaviour of so-called proximity junctions and a step toward magnetically controlled superconducting circuitry.

The work at hand is structured into four chapters, the first of which is this introduction. An appendix explains a small experiment on a different topic of research that differs from the core of this thesis. The second chapter explains the physics of semiconductor crystals and semiconductor nanostructures. Introduces the mathematical framework to describe the exciton in semiconductor nanostructures. Chapter three outlines the mechanism of RET and is dedicated to the experimental and theoretical methods we employed during our research. We present the technique that has allowed us to perform a RET distance dependence study, the AFM equipment, the laser system and time resolved spectroscopic techniques that allowed us to investigate the exciton structure and dynamics in our material system and our results. Chapter four shows the theoretical background, the experimental methods and equipment we used to archive lasing with NCs. The theory of whispering gallery modes and photonic crystal is explained and we describe how, using NCs, silica microsphere resonator and PC nanocavity, we achieved lasing regime in both configurations. The appendix shows an evanescence field spectroscopy experiment in which tapered fibers and golden nanoparticles have been used to realize an extremely sensitive device based on evanescence spectroscopy.

In a many-body fermionic system, the suppression of continuous transitions to absolute zero can result in a low temperature quantum fluid which deviates strongly from typical metallic behaviour; unconventional superconductivity can be induced by the strange metal region surrounding the zero-temperature phase transition. In this thesis we focus on three systems which demonstrate a highly tunable phase transition, with the aim of pushing them toward the border of a zero-temperature phase transition, and potentially superconductivity. CeAgSb2 is a uniaxial 4f ferromagnet, where physical pressure or a transverse field may be used to tune the magnetic transition towards T = 0 K. Our investigations, however, did not reveal the presence of superconductivity. It is likely that the field tuned transition does not correspond to a true critical point, whilst the high pressure region may be occupied by an antiferromagnetic phase, with the true critical point at higher pressures. However, other interesting features emerge in the electrical resistivity and AC-susceptibility, along with novel thermodynamic signatures linking the magnetisation to the specific heat. The doping series Lu(1-x)YxFe2Ge2 shows an antiferromagnetic transition which is suppressed to absolute zero at a critical concentration x_c=0.2. YFe2Ge2 displays anomalous low temperature behaviour consistent with the proximity to quantum critical fluctuations, along with a superconducting transition which appears in the electrical resistivity beneath a critical temperature of T_c ~ 1.7 K. Using low temperature DC magnetisation measurements, we show that this is a bulk effect, and that the superconductivity in YFe2Ge2 is of type-II. The thermodynamic and BCS properties of the superconducting phase are analysed in line with the parameters we extract experimentally. The superconducting 3-4-13 stannides (Ca,Sr)3Ir4Sn13 show a high temperature structural transition which may be suppressed by the application of hydrostatic pressure or effective chemical pressure. A superconducting dome is found, which appears to peak near where the structural transition extrapolates to zero temperature. Anomalous exponents are seen in the electrical resistivity over a wide temperature range. We investigate the influence of pressure on the superconducting critical temperature in Ca3Ir4Sn13 and the related compound Co3Ca4Sn13, along with an analysis of the upper critical field and flux-line phenomena in Ca3Ir4Sn13 and Sr3Ir4Sn13.

The aim of this thesis was to identify a candidate material for spin-triplet superconductivity with a two-component order parameter. This unconventional superconducting state is thought to allow for exotic quantum states such as Majorana fermions. A wide survey was taken into potential candidate materials, and UAu2 was chosen for in-depth investigation. This little-studied hexagonal heavy fermion compound's unusual resistivity behaviour, combined with a series of interesting features in magnetisation and heat capacity, make it an extremely interesting material to study. The phase diagram of UAu2 was determined with measurements of heat capacity, resistivity, magnetisation and magnetoresistance on the first single crystalline samples of this material. No superconductivity was detected. Instead, a range of magnetic phase transitions were observed, which were further investigated with muon-spin relaxation experiments and time-of- ight neutron powder diffraction. UAu2 was found to undergo a transition to an incommensurate antiferromagnetic state (q1 = (1=3; 1=3; δ)) below TN = 43:5 K, but then develops signatures of weak ferromagnetism below T = 20 K. The ferromagnetism coincides with a 2q magnetic structure, with a coexistence of q1 and q2=(1/3,1/3,0). The magnetic structures of both phases were found to be most likely amplitude-modulated, with moments aligned along the crystallographic c-axis. A transition to a ferromagnetic state was observed in magnetic fields applied parallel to the c-axis. TN was found to remain almost constant in applied magnetic fields up to 9 T, while hydrostatic pressures of up to 6 kbar weakly suppress the antiferromagnetic transition temperature. The field-induced transition was found to be strongly pressure-dependent, shifting to higher applied fields with increasing pressure. The residual resistivity of UAu2 samples prepared by both the Czochralski method and quenching from the melt is relatively large, which may inhibit Cooper pairing and hence may be the reason for the absence of superconductivity in the samples investigated. Solid-state electrotransport (SSE) equipment was developed, which can induce the motion of a crystal's constituents and thereby lead to vastly improved sample quality. Refinement of UAu2 samples with SSE could be a further step in the search for spin-triplet superconductivity in this material.

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

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.

URhGe has garnered interest recently as one of the few known ferromagnetic superconductors. The superconductivity in this material appears to arise from magnetic fluctuations rather than phonons, and take a triplet form which is remarkably resistant to field. In this thesis, a number of measurements on the material are presented. Some probe the Fermiology, with strong evidence appearing for a model which as both light open sheets and heavy, small, closed pockets. The open sheets, associated with chains of real-space electron density running along the b axis, dominate the conductivity in most circumstances. Evidence for their existence arises from the general large and non-saturating magnetoresistance, and from the unusual observation of negative temperature coefficient of resistance at high fields. The closed pockets have provided a few Shubnikov-de Haas oscillations, but mostly they remain inferred from the high specific heat γ and their role in the magnetism. In order to better probe the superconductivity, a high precision low noise DC resistance measurement bridge was built using a SQUID. Along with conventional measurements, this provides evidence that the two pockets of superconductivity on the phase diagram are the same phase. The re-entrance an be understood simply as a result of magnetic field being a tuning parameter, but also suppressing bulk superconductivity through orbital limiting. The SQUID bridge allowed the detection of domain wall superconductivity linking up these two pockets. The SQUID bridge was also used to study the highly structured superconducting transition in BiPd. This material lacks inversion symmetry in its crystal structure, so is a good candidate for unusual forms of superconductivity. Here again non-bulk superconductivity is considered the most likely cause for the structure. Unusual and distinctive IV curves have been measured, and a simple model of inhomogeneous conductivity channels with different critical currents is proposed as an explanation.

This thesis presents studies of exciton diffusion in organic semiconductors measured using exciton-exciton annihilation and the measurements were performed on materials important for organic solar cells. In the conjugated polymer poly(3-hexylthiophene) (P3HT), the effect of molecular weight (4-76 kgmol⁻¹) was explored. Using exciton-exciton annihilation measurements, the highest diffusion coefficient was observed in the intermediate molecular weight region and was correlated with long conjugation lengths, higher fraction of aggregated states and more delocalised excitons within the aggregate. The results demonstrated that the molecular weight dependence is due to a complex relationship between intermolecular interactions, aggregate size and Boltzmann statistics. This thesis also includes an investigation of exciton diffusion in diketopyrrolopyrrole(DPP)-based small molecules as a function of molecular structure. Significant changes in photophysical and exciton diffusion properties were observed due to minor changes in molecular structure. Long conjugation lengths, bulky side chains or reduced steric hindrance due to absence of end alkyl chains correlated with reduced film crystallinity and reduced diffusion coefficients. The increase in disorder observed due to large conformational torsions resulted in inhomogeneous broadening of density of states and as a result exciton diffusion becomes dispersive. In this case, a slowdown of exciton diffusion is observed. This study demonstrates that enhanced exciton diffusion can be achieved by designing more rigid and planar conjugated backbones with smaller conjugation lengths. Finally, exciton diffusion measurements were used to rationalise the performance of T3 truxene oligomers as explosive sensors. Side chain lengths were found to have a subtle influence on exciton diffusion. Time-resolved PL quenching measurements were used to estimate the quencher concentration. Differences in quencher concentration were observed suggesting different interaction strengths of the quencher with the truxene oligomer which help explain the explosive sensing performance.

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.

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.

Electrohydrodynamics (EHD) is a promising research field with several trending applications. Even though the phenomenon was first observed centuries ago, there is very little research until the middle 20th century, as the mechanisms behind it were very poorly understood. To this date, the majority of research is based on the development of empirical models and the presentation of laboratory experiments. This work begins with an extensive literature review on the phenomenon, clarifying conflicts between researchers throughout the history and listing the findings of the latest research. The literature review reveals that there are very few mathematical models describing even the most important parameters of the EHD fluid flow and most are either empirical or greatly simplified. As such, practical mathematical models for the assessment of all primary performance characteristics describing EHD fluid accelerators (Voltage Potential, Electric Field Intensity, Corona Discharge Current and Fluid Velocity) were developed and are begin presented in this work. These cover all configurations where the emitter faces a plane or another identical electrode and has a cylindrical surface. For configurations where the emitter faces a plane or another identical electrode and has a spherical surface, Corona Discharge Current and Fluid Velocity models have been presented as well. Laboratory experiments and computer simulations were performed and are being thoroughly presented in Chapter 4, verifying the accuracy and usability of the developed mathematical models. The laboratory experiments were performed using two of the most popular EHD electrode configurations - wire-plane and needle-grid. Finally, the findings of this research are being summarized in the conclusion, alongside with suggestions for future research. The step-by-step development of the equipotential lines mathematical model is presented in Appendix A. Appendix B covers the mathematical proof that the proposed field lines model is accurate and that the arcs are perpendicular to the surface of the electrodes and to all of the equipotential lines.

This thesis presents experimental studies of the electronic and magnetic properties of several iron-based unconventional superconductors, primarily using the techniques of magnetotransport and torque magnetometry in high magnetic fields and synchrotron-based angle-resolved photo-emission spectroscopy (ARPES). Superconductivity in the iron-based superconductors is always found in proximity to a magnetic phase, and the details of the electronic structure and Fermi surface are also important in determining the strength of interactions, and ultimately superconductivity. This motivates the experimental studies of electronic, magnetic and superconducting properties of Fe-based superconductors presented in this thesis. First, quantum oscillation measurements using high-field torque magnetometry are used to provide a partial determination of the Fermi surface of superconducting LiFeAs. The data are compared with density functional theory calculations, finding strong mass enhancements on the observed electron bands, however the hole bands are not observed. A large portion of this thesis concerns experiments on FeSe, which uniquely has a structural transition but is not magnetically ordered at any temperature. High field magnetotransport measurements show quantum oscillations, revealing small quasi-two dimensional Fermi surfaces, and it is argued that both hole and electron pockets are observed. The low-temperature Fermi surface consisting of one hole pocket and two electron pockets is also deduced from low-field magnetotransport. ARPES studies show that both hole and electron pockets undergo a significant elongation when cooling through the structural transition at ~90 K, interpreted as the result of orbital order. Measurements of the resistivity anisotropy above the structural transition are used to show that the structural distortion is electronically-driven. By combining these data sets, a complete picture of the symmetry-broken electronic structure of FeSe is constructed. The final chapter concerns another iron-based superconductor with a more complex crystal structure, the so-called ``10-3-8" phase, and in particular finds an unusual field-induced magnetic transition.

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 .

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.

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.