This thesis describes applications of the ac Josephson effect. Firstly, results are presented from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at 72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured capacitance was less than that expected from a calculation of the parallel plate shunt capacitance. The explanation was thermal noise suppression of the hysteresis and the junction saw the shunt capacitor as a distributed impedance rather than a lumped circuit element. It was found during these investigations that the influence of the SrTiO3 substrate on the intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at 4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The dispersion relation consisted of two branches, one at low frequencies with a high resonator capacitance per unit length and a high frequency branch with a low resonator capacitance per unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3, which drops above the soft optic phonon frequency. From the dispersion relation, the permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz. The ac Josephson effect was exploited to measure the permittivity of thin films of SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators. The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the 50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz. The grain boundary capacitance was used to probe grain boundary current transport. The capacitance per unit area scaled inversely with resistance area product and increased linearly with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24° bicrystals. This behaviour could not be explained by tunneling models of grain boundary current transport, and requires current flow over a fraction of the area of the grain boundary.

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This thesis starts by reviewing superconductivity in one-dimension where fluctuations cause a loss of supercurrent due to an intrinsic resistance. Solved via the Ginzburg-Landau equations, the theory of thermally activated phase slips given by Langer and Ambegaokar is outlined. In turn this leads to the investigation of superconductivity via a microscopic approach, in particular the quasi-classic green’s functions of Eilenberger. The Eilenberger equations are derived and considered in the dirty and weakly anisotropic limits which provides a simple derivation of the Ginzburg-Landau equations near the transition temperature. This prompts an extended derivation which includes the non-linear terms normally removed in deriving the Ginzburg-Landau equations. This is required for calculating effects at temperatures below the transition temperature. These quasi-classic equations of weakly anisotropic superconductors are first written for arbitrary temperature and impurity concentration then limited to the pure and dirty cases. The latter being simplified to zero temperature and solved in the context of thermally activated phase slips.

HTS magnets have been developed to generate high magnetic fields because its high critical field at low temperatures. For HTS magnets, the design of thermal stability and protection is based on understanding of its quench behaviour. However, there are few experimental and numerical results on the quench behaviour of HTS at low temperatures. This thesis work is mainly dedicated to investigate the quench behaviour of high temperature superconductor (HTS) at low temperatures by 1D and 2D numerical analysis. In addition, this work also investigates the critical current of HTS coils made from Bi/Ag2223 tape at 77K under self-field. The ANSYS implementation of a general quench model capable of handling nonlinear heat generation over a large temperature range of current sharing, e.g. for HTS at low temperatures, has been successfully validated for HTS at high temperatures and LTS with reference to predictions by the classical quench theory. The numerical model also revealed that the classical theory usually overestimates the minimum quench energy MQE as self-heating is neglected during the development of MPZ. The effective medium approximation for the coil thermal-electrical properties was also found to be sufficient for practical HTS coils. Simulation of 1D HTS conductors at low temperatures using the non-linear heat generation model revealed a different quench behaviour from that of LTS conductors. Firstly, while the minimum quench energy MQE is well defined, it is almost an order of magnitude smaller than the enthalpy of the minimum propagation zone. Hence the growth of MPZ is entirely due to self-heating while MQE is just a sufficient trigger. Secondly, for a practically defined MQE with 95% of the ”true” minimum, there is a large variation of the size of MPZ and the corresponding quench temperature. Thirdly, there appears a simple scaling between the average MPZ heat generation Gq and the temperature range (Tq − Tcs) of MPZ. 2D analysis of HTS coils showed the MQE is an order of magnitude larger than that from 1D analysis because of radial heat conduction. For practical coils, the geometry ii and boundary cooling have an important influence on their quench behaviour while the MPZs are larger and hotter. Two single pancake coils of 50mm inner diameter and 20 turns were manufactured and tested at 77K. The current carrying characteristics of HTS coils was evaluated by using the method based on the Ic-B of bifilar tape and agree well with measured results. One bifilar pancake coil was fabricated and tested at 77K. The measured critical current is 108A and about 20% larger than that of a single pancake coil.

The excitement which followed the discovery of high-temperature superconductors in 1986 was short-lived, as it became clear that their current carrying capacity (the critical current density Jc) was limited by grain boundaries (GBs). In order to reduce their detrimental effects coated conductors have been developed, in which a superconducting thin film is deposited on a polycrystalline, textured substrate. Within certain temperature and magnetic field ranges, however, GBs still limit the overall Jc. This fact motivated the present thesis, for which the electrical properties of different types of coated conductors, and in particular their GBs, were investigated. Several GBs and a single grain were isolated in a tape produced by metal-organic deposition (MOD), using a novel approach based on electron backscatter diffraction and a focused ion beam microscope. Measurements of their critical current densities for fields swept in the film plane showed the expected decrease with increasing misorientation angle at low fields. At higher fields an angle dependent crossover was found, from a GB to grain limited Jc. In order to confirm this result and put it into broader perspective, the dependence of Jc on the width of polycrystalline tracks was studied, and then explained in terms of the behaviour of the single GBs. Investigations using low-temperature scanning laser microscopy rounded out the picture, which also showed GB dissipation at certain angles and grain limitation at others. In measurements on samples produced by metal-organic chemical vapour deposition (MOCVD) characteristic differences compared to the MOD film were found. While both conductors exhibited high values of Jc, the variation with in-plane angle was significantly stronger for the MOCVD conductor, which can be explained by its sharper texture. In a track patterned perpendicular to the tape direction the phenomenon of vicinal channelling was observed, which previously was known only from films on single crystal substrates. Finally, an isolated boundary showed very high values of Jc, consistent with its low misorientation. In order to better understand how the substrate influences the superconducting properties, measurements were carried out on otherwise identical samples grown on different substrates. A tape with grains elongated along its rolling direction showed particularly good properties at all examined field orientations. This extends the previously reported result that high aspect ratios are beneficial at fields applied perpendicular to the tape plane.

The irradiation of high Tc superconducting thin films with a focused electron beam, such as that obtained in a scanning transmission electron microscope (STEM), can result in the formation of a Josephson junction. The conditions required for the formation of these Josephson junctions in YBa2Cu3O7-d and related compounds are discussed as well as the physical properties of the irradiated material. From electrical transport measurements of individual Josephson junctions it was found that these junctions have a Superconductor/Normal/Superconductor (SNS) nature. Low temperature anneal studies indicate that Josephson junctions with optimum properties can be obtained by a combination of a high electron dose and subsequent low temperature anneal. Extremely high electron doses resulted in the formation of a purely resistive region. The electrical transport in such regions with a dimension of 200 nm in the direction of current transport is shown to be compatible with variable range hopping (VRH). Barriers with the same length but a finite superconducting transition temperature showed a low bias resistance that is significantly lowered due to proximity coupling. Using purely resistive regions in combination with Josephson junctions, devices consisting of two closely spaced Josephson junctions with a third terminal connected to the shared electrode were fabricated and characterised (minimum separation 20 nm). The distinct behaviour of the Josephson critical current with applied magnetic field (Ic(B)) of these devices was found to be well described by a newly developed model, which incorporates the effect of the static redistribution current in the shared electrode on the phase distribution of the Josephson junctions. An important finding is that the behaviour of the high critical current with applied magnetic field of two closely spaced junctions was found to be consistent with a model system consisting of a closely spaced Josephson junction and a resistive barrier. A three terminal device with Josephson junctions at small separations was found to have a significantly increased trans resistance when compared with the individual resistance of the Josephson junctions it constituted of. A number of illustrative examples of device structures realised with the focused electron beam irradiation technique are also included.

Superconducting switches may nd application in superconducting magnet systems that require energy extraction. Such superconducting switches could be bypass-switches that are operated in conjunction with a parallel resistor or dump-switches where all of the energy is dissipated in the switch itself. Bypass-switches are more suited to higher energy circuits as a portion of the energy can be dissipated in the external dump resistor. Dump-switches require less material and triggering energy as a lower switch resistance is needed to achieve the required total dump resistance. Both superconducting bypass-switches and superconducting dump-switches can be thermally activated. Switching times that are comparable to those obtained with mechanical bypass-switch systems can be achieved using a co-wound heater that is powered by a capacitor discharge. Switches that have fast thermal diffusion times through the insulation can be modelled as a lumped system whereas those with slow thermal diffusion times were modelled with the full heat diffusion equation. Superconducting switches can be formed of either high temperature superconductors (HTS) or low temperature superconductors (LTS). Switches based on HTS materials allow operation at higher temperatures where the cost of cooling is less. Extracting the magnet energy and depositing the heater energy at higher temperatures will also reduce the load on the overall cryogenic system during switching and energy extraction. For magnet circuits that are based on high temperature superconductors the switch must also be formed of HTS material. Due to the approximately T3 dependence of specific heat capacity, switches that operate at higher temperatures have slower heat diffusion times and require higher triggering energies than those operating at low temperature. HTS based dump-switches and HTS based bypass-switches were tested in liquid nitrogen to show that the required switching time could be achieved at these high temperatures. The design and optimisation of superconducting switches that were formed of various superconducting materials were performed for example magnet circuits to provide reference designs of switches. These example circuits were based on selected Large Hadron Collider 600 A circuits that had a stored energy of 5.5 kJ. Superconducting switches may also nd application in magnet circuits with higher transport currents and higher energies. The scaling and suitability of the reference designs to higher energy circuits was also described.

The archetypal high temperature superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] has been extensively investigated. However, until now, little has been known about the behaviour of the magnetic vortices inside the Vortex Glass and liquid state. µSR measurements have shown a negative skewness for the field probability distributions in these regimes. Such a negative skewness has only recently been explained as being a direct consequence of three-body correlations between vortices in a similar layered superconductor. With a new understanding and knowledge of the physics of these systems, it is instructive to re-visit the superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] to explain the evolution of these three-body correlations occurring here. Comparing this with the ion-irradiated superconductors (of the same Bi₂Sr₂CaCu₂O[subscript[8+δ]] material), allows one to observe how three-body correlations between vortices evolve differently to that in the pristine material. Moreover, in the region of the macroscopic irreversibility line, entropically driven disorder exists below the matching field B[subscript[ϕ]] , whilst there is the appearance of relatively straight vortex lines at fields above B[subscript[ϕ]] . Such phenomena suggest a significant di fference in the evolution of three-body correlations compared with the unirradiated material. There has been much work conducted on the interplay between superconductivity and magnetism in materials of reduced dimensions. Work presented here on the ferromagnet/superconductor trilayer system (Permalloy/Nb/Permalloy) has shown a novel magnetic profile at the interface between the ferromagnetic and superconducting boundary, where, contrary to what is expected, the magnetism appears to be significantly suppressed at the interface before increasing towards the centre of the Nb layer.

This thesis addresses the tunnelling of charge carriers in different materials. First looking at the simplest case of electron tunnelling in metals at zero, then finite temperature, the current is obtained using the Fermi-Dirac golden rule and then the conductance is obtained. This is extended to take into account the spatial dependence of one of the metals being a tip since experimentally this is done by scanning tunnelling microscopy where a tip traces over the surface of a sample. The next step is to look at tunnelling between a metal and a semiconductor, again the current is found. Semiconductors can be doped and the effect this has on tunnelling is examined. Next superconductors are introduced. The purpose of my research has been to look at the tunnelling spectra of high-temperature superconducting cuprates for both extrinsic (metal-superconductor) and intrinsic (superconductor-superconductor) tunnelling. The main features seen experimentally with cuprate tunnelling are identified and then a theory capable of explaining these features is discussed. The theory is compared to experimental results and we find good agreement.

High-pressure techniques have become increasingly important in the synthesis of ceramic and metallic solids allowing the discovery of new materials with interesting properties. In this research dense solid oxides have been synthesised at high pressures, and structural investigations have been conducted using x-ray and neutron diffraction. The perovskite LaPdO3 has been synthesised at pressures of 6{10GPa. Neutron diffraction studies have been carried out from 7{260K to investigate any structural distortions, particularly related to the possibility of charge order at low temperatures. No reduction in symmetry associated with charge ordering has been observed; the material appears to remain metallic with only one unique Pd site down to 7K. LaPdO3 adopts the GdFeO3-type Pbnm structure. The PdO6 octahedra exhibit a tetragonal distortion throughout the temperature range with a shortening of the apical Pd{O bonds of 2:5% relative to the equatorial bonds. Attempts to prepare analogues of the perovskite containing smaller rare earths have resulted in multi-phase samples, and further RPdO3 perovskites remain inaccessible although there is evidence for a small amount of the perovskite phase in the products of synthesis attempts with neodymium. Three new oxypnictide superconductors, RFeAsO1 xFx (R = Tb, Dy and Ho) have been synthesised at 7{12GPa. The materials are isostructural with other recently discovered iron arsenide superconductors and have Tc's of 52:8 K, 48:5K and 36:2K respectively, demonstrating a downturn in Tc in the series for smaller R. Systematic studies on TbFeAsO0.9F0.1 and HoFeAsO0.9F0.1 show negative values of dTc=dV in contrast to those reported for early R containing materials. Low-temperature neutron diffraction measurements on both materials, and synchrotron studies on HoFeAsO0.9F0.1 reveal no tetragonal to orthorhombic transitions as observed in early R-containing materials with lower doping levels. Magnetic reflections are evident but they are shown to be from R2O3 and RAs impurities with TN's of 5:5K for Tb2O3, 6:5K for HoAs and 1:7K < TN < 4K for Ho2O3. The implications of these results for superconductivity in the iron arsenides are discussed.