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

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

Part I of this dissertation is concerned with the problem of the magnetic field dependence of the surface impedance of superconductors, with particular reference to tin. In chapter 1 the predictions of the simple theory for the behaviour of the surface impedance for different frequencies and fieid configurations are described and compared with the results of previous experiments. Chapter 2 deals with the experimental side of the present work, where a frequency of 170 Mc/s was used, and in chapter 3 the results obtained are discussed. In chapter 4 the attempts which have been made to account theoretically for the experimental observations are reviewed. Part II of the dissertation is concerned with another problem in superconductivity, namely the behaviour of superconducting systems partitioned by thin barriers of substances which in bulk are not superconducting. The theory of such systems is developed, the consequences investigated in some detail and the present experimental situation reviewed.

The optical properties of InAs/GaSb heterostructures under applied magnetic fields are studied in experimental and theoretical detail. The InAs/GaSb system is a type-II "crossed-gap" system, where the valence band edge of GaSb lies higher in energy than the conduction band edge of InAs. This leads to a region of energy above the InAs conduction band where conduction and hole states mix. Thin-layer superlattices remain semiconducting due to confinement effects, but thick-layer superlattices experience charge transfer which leads to intrinsic carrier densities approaching 10¹² cm^-2 per layer. Existing multi-band modeling techniques based on the <strong>k·p</strong> formalism are discussed, and a method of solving superlattice band structure (the "momentum-matrix" technique) is presented. The quantizing effects of the superlattice layers and applied magnetic fields are investigated, and the selection rules for optical transitions are derived. Standard cyclotron resonance (CR) is used to study effective masses in InAs/GaSb structures. The heavy hole mass is found to be strongly orientation-dependent, with a mass in the [111] orientation reduced 25% from the [001] mass. The electron mass is found to be roughly isotropic with respect to growth orientation, but shows variation with the InAs width due to quantum confinement effects. CR of InAs/GaSb heterojunctions display hitherto unexplained oscillations in linewidth, intensity, and effective mass. A model is proposed which explains the oscillations, based on the intrinsic nature of the InAs/GaSb system. CR is performed on an InAs/GaSb heterojunction using a free-electron laser, where due to the high intensities (on the order of MW/cm²) the absorption process saturates. This saturation allows for a determination of non-radiative relaxation lifetimes, and through the energy dependence of these lifetimes the magnetophonon effect is observed, allowing a direct measurement of LO-phonon-assisted energy relaxation rates. Coupling is introduced into the standard CR experiment, either by tilting the sample with respect to the magnetic field, or by applying a metal grating to the surface. These coupled CR experiments have striking qualitative results which allow for determination of subband separation energies and coupling matrix elements. Photoconductivity experiments are performed on thin-layer (semiconducting) superlattices, showing optical response at far-infrared wavelengths (5-20 μm). The results are compared with <strong>k·p</strong> calculations. One sample is processed for vertical transport, in which conduction occurs perpendicular to the superlattice layers. Strong optical response from this sample indicates the viability of InAs/GaSb-based far-infrared detectors. The momentum-matrix technique is used to predict optimum parameters for semiconducting superlattices with band gaps in the far-infrared. Semimetallic structures are studied via a multi-band self-consistent model, with results corroborating with and extending previous work. Intrinsic structures under applied magnetic field are modeled theoretically for the first time.

This thesis presents a study on several series of unconventional Fe-based superconductors; namely, derivatives of NaFeAs and LiFeAs, as well as molecular-intercalated FeSe. Primarily using muon spin rotation (SR), but also x-ray diffraction (XRD) and magnetic susceptibility measurements, the nature of both magnetic and superconducting phases within these systems is studied. Particular attention is focussed on how these states compete or coexist with one another. The aforementioned experimental techniques are first used to explore the phase diagram of NaFe1xCoxAs. This phase diagram includes regions of long-range antiferromagnetism and short-range order, that both coexist with superconductivity. Magnetism is gradually destroyed, primarily through a diminishment of the size of the ordered moment, as superconductivity is enhanced by Co substitution. This interplay is explored in detail. By contrast, superconductivity in LiFeAs cannot be enhanced by transition metal substitution, suggesting that it is intrinsically optimally-doped. I investigate this conclusion by studying the evolution of the penetration depth in superconducting compositions of LiFe1xCoxAs and LiFe1xNixAs, and comparing these data to those from other electron-doped systems. I also study an unusual and emergent magnetic phase in Li1yFe1+yAs. This work suggests that LiFeAs supports a superconducting phase that resembles those in other Fe-pnictides, but is uniquely close to an additional magnetic instability. I then move on to the study of a recently discovered series, based on FeSe intercalated with ammonia and various metals. I study both the penetration depth and the intrinsic magnetic phases in these systems using SR and compare them with other compounds based on FeSe. I find that these intercalated systems support spacially separated regions of dynamic magnetism and superconductivity and I discuss how much these phases depend on the precise chemical details of the intercalated layer. Finally, I return to the experimental study of NaFe1xCoxAs, extending the range of techniques employed by using high-field magnetometry and high-pressure SR. These studies reveal new features of this system that were not accessible using low-field and ambient-pressure methods. In particular, I show that the magnetic moment size in NaFeAs unexpectedly increases with pressure, suggesting that the electronic structure of this compound may be unique amongst known Fe-based superconductors.

This thesis describes inelastic neutron scattering (INS) experiments on several iron-based materials. The experiments were primarily designed to investigate the link between magnetic dynamics and superconductivity. The work contributes to evidence that magnetic fluctuations influence or are influenced by superconductivity. It is demonstrated that the INS response of a material, in conjunction with theoretical models, can provide valuable information about both superconductivity and magnetism. I measured the magnetically ordered parent-compound SrFe2As2 to investigate the nature of magnetism in iron-based systems. Comparison of the data to models based on both itinerant and localised magnetism showed that an itinerant model offers the best description of the data. LiFeAs is a superconductor that shows no magnetic order, however I was able to distinguish a magnetic signal in its INS spectrum. The signal is consistent with the magnetic resonance observed in several other iron-based superconductors. This indicates that LiFeAs likely hosts an s± gap symmetry. I investigated two iron-phosphide systems, LaFePO and Sr2ScO3FeP, and in this case I was unable to identify any magnetic scattering. Comparison to LiFeAs showed that any signal in LaFePO is at least 7 times weaker. These results suggest that magnetic fluctuations are not as influential to the electronic properties of iron-phosphide systems as they are in other iron-based superconductors. In CsxFe2−ySe2 I found two independent signals that appear to be related to phase-separated magnetic and superconducting regions of the sample. I showed that fluctuations associated with the magnetically ordered phase are consistent with localised magnetism, and do not respond to superconductivity. The second signal, however, increases in intensity below the superconducting transition temperature Tc = 27K, consistent with a magnetic resonance. This could be indicative of a pairing symmetry in CsxFe2−ySe2 that is distinct from most other iron-based superconductors. Finally, the molecular intercalated FeSe compound Li0.6(ND2)0.2(ND3)0.8Fe2Se2 revealed strong magnetic fluctuations. Again the signal was consistent with a magnetic resonance responding to Tc = 43 K. The results suggest that Lix(ND2)y(ND3)1−yFe2Se2 is similar to the superconducting phase of CsxFe2−ySe2, placing constraints on theoretical models to describe the molecular intercalated FeSe compounds.

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.

Les matériaux diélectriques sont présents dans de nombreux dispositifs en microélectronique. Ces derniers peuvent être soumis à de fortes contraintes électriques impactant leur durée de vie. Le stress électrique peut en effet provoquer le claquage du diélectrique ou la modification des performances des composants par accumulation de charges. Dans ces travaux de thèse, différentes méthodes de caractérisation et d'analyse physique ont été utilisées pour étudier la structure des échantillons et identifier les mécanismes en jeu dans le processus d'accumulation de charges dans des couches minces de nitrure de silicium. Puis un code de simulation modélisant les phénomènes de transport de charges dans les isolants a été développé. Le modèle prend en compte des phénomènes de transport par effet tunnel et par effet thermique, dans le volume du diélectrique et aux interfaces isolant-métal. Il permet d'étudier l'évolution de grandeurs physiques (courants, charge, champ électrique) en fonction du temps et de la profondeur dans la couche mince diélectrique. Des résultats de mesures sur des composants capacitifs ont pu être reproduits grâce aux simulations. Cet outil permet d'estimer l'intérêt d'un matériau diélectrique relativement à la fiabilité de composants capacitifs. Il peut également être utilisé en amont afin de définir un matériau aux propriétés idéales pour l'application visée ou aider au dimensionnement de dispositifs en microélectronique. / Dielectric materials can be found in numerous devices in microelectronics. They can be subjected to significant electrical stress, which impacts their lifetime. Indeed, this electrical stress can lead to dielectric breakdown or modify the component performances by charge storage. In this work, several characterization methods and physical analysis have been used in order to study the samples and identify mechanisms involved in charge transport in silicon nitride thin films. Then a simulation code has been developed to model charge transport phenomena in insulators. This model takes into account tunnel and thermal effects in the dielectric and at the dielectric-metal interfaces. The temporal and spatial evolution of physical quantities (currents, charge, electric field) in the dielectric film are calculated. Measurement results on capacitive components can be obtained thanks to simulations. This simulation tool allows testing dielectric materials according to capacitive component reliability. It may be used to define optimal properties for materials depending on applications or to assist in device design in microelectronics.

Η παρούσα εργασία αναφέρεται στη μελέτη της ηλεκτρικής εκκένωσης τύπου κορώνας και ιδιαίτερα στα ηλεκτρικά και υδροδυναμικά (ηλεκτρικός άνεμος) φαινόμενα που τη συνοδεύουν. Μια εκκένωση κορώνας είναι το φαινόμενο που δημιουργείται σε ένα αέριο διάκενο μεταξύ δύο ηλεκτροδίων διαφορετικής ακτίνας καμπυλότητας, όταν αυτά βρίσκονται υπό την επίδραση διαφοράς δυναμικού μερικών kV. Η εκκένωση κορώνας συνοδεύεται από την εμφάνιση ηλεκτρικών, οπτικών, ακουστικών, χημικών, αεροδυναμικών (ηλεκτρικός άνεμος) και ραδιοηλεκτρικών φαινομένων. Ο ηλεκτρικός άνεμος είναι ένα φαινόμενο το οποίο δημιουργείται από τη μετάδοση της ορμής των ιόντων στα ουδέτερα μόρια του αερίου οπότε παρατηρείται μία κίνηση του αερίου από το ηλεκτρόδιο με μικρή ακτίνα καμπυλότητας, προς το άλλο ηλεκτρόδιο. Το κύριο αντικείμενο της εργασίας αυτής είναι η μελέτη του φαινομένου της θετικής κορώνας, μεταξύ μιας μη συμμετρικής διάταξης ηλεκτροδίων, στον ατμοσφαιρικό αέρα. Στην άνοδο τοποθετήθηκαν μια και δύο ακίδες της ίδιας ακτίνας καμπυλότητας ενώ χρησιμοποιήθηκαν τρεις διαφορετικές κάθοδοι. Οι πειραματικές μετρήσεις είχαν ως στόχο τον προσδιορισμό της χαρακτηριστικής του μέσου ρεύματος της εκκένωσης συναρτήσει της τάσης διακένου, των παλμών του ρεύματος για διάφορες τιμές της τάσης , τον προσδιορισμό της κατανομής και της χαρακτηριστικής της ταχύτητας του ηλεκτρικού ανέμου καθώς και της χαρακτηριστικής της θερμοκρασίας του ηλεκτρικού ανέμου. Πρέπει να αναφέρουμε ότι οι μετρήσεις πραγματοποιήθηκαν για συνεχή και παλμική τάση λειτουργίας. / In this thesis, the effect of the positive corona discharge was studied. Especially, electric and hydrodynamic phenomena (corona wind) were studied. By the term “corona discharge”, one generally refers to the ensemble of phenomena which occur in a gaseous medium in the vicinity of conductors of small radius of curvature, subjected to intense, but not disruptive, electric fields. The main part of this thesis is experimental procedures that were held between a non-symmetrical set-up of electrodes in atmospheric air in order to, initially define the V-I characteristics and study the current impulses for different voltage values. Furthermore, we studied the velocity distribution and velocity characteristics of corona wind. In the last part of this work the temperature characteristics are present. It is important to mention that the experimental measurements were carried out on continuous and pulsed voltage.

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.