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This dissertation expands the scope of Z-pins' application in composites by investigating their unconventional uses. The mechanical behaviour, electrical performances, and magnetic properties of Z-pinned laminates are examined, respectively. In addition to exploring the well-established fracture toughness and in-plane properties of Z-pinned flat panels, the study experimentally investigates the through-thickness tensile strength of Z-pinned curved laminates under four-point bending. The effects of Z-pins at different volume fractions on the strength and failure mode are analysed, comparing the results to unpinned specimens. High fidelity finite element models of both unpinned and carbon-fiber Z-pinned specimens are developed in Abaqus/Explicit to provide insights into the experimental observations and different failure mechanisms introduced by Z-pins. Moreover, the study investigates the electrical (and consequential thermal) behaviour of Z-pinned laminates under localised in-plane and through-thickness fault currents. The effects of Z-pin material and volume fraction on the current conducting path and temperature are discussed through experiments and micro structural observations. Furthermore, traditional fibre-reinforced composites are magnetically inert, which limits their potential applications in certain fields. Therefore, the study explores the influence of carbon-fibre and ferromagnetic Z-pins on the global magnetic properties of composites. The effects of pin material, misalignment, and volume fraction on the in-plane and through-thickness magnetic properties are experimentally studied, with further insights provided by numerical modelling conducted in COMSOL. In conclusion, this dissertation contributes to the wider understanding and utilisation of Z-pins in composites, potentially enabling the development of novel and practical applications.

Despite the additional experimental difficulty required for high pressure measurements, the ability to tune materials with applied pressure opens up a remarkable phase space of novel structures, stoichiometries and exploration through the rich phase diagrams of condensed matter systems. Exploring the interplay of these ordered phases is vital in understanding the underlying superconductivity present in many of these such materials. For the field of high temperature superconductivity, the hydride family of superconductors have the highest experimentally verified critical temperatures, but are only currently stable and accessible with experiments in the megabar pressure range. As the current best candidates for room-temperature superconductivity, research is vital in characterising these hydride compounds at extreme conditions. This thesis covers a variety of compounds and experimental techniques, but grouped under the topic of superconductors under pressure. In particular, these measurements focus on characterising the pairing gap, a key parameter in understanding the nature of superconductivity in all superconductors. Firstly, hydrostatic pressure up to 8 GPa has been used to suppress the charge-density wave phase in the transition metal dichalchogenide NbSe₂, where measurements of the magnetic susceptibility track the superconducting critical temperature with applied pressure. Here, the onset temperature of the charge density wave (CDW) and superconducting phases (SC) show a clear anticorrelation, revealing a competition for electronic density of states between these two phases. Magnetisation measurements also show a sharp increase in the lower critical field with applied pressure that cannot be solely explained by a SC-CDW competition, but instead indicates unconventional superconducting behaviour in NbSe₂. Fits to the temperature-dependent superfluid density are used to measure a superconducting gap magnitude as a function of pressure. Secondly, measurements of two elemental superconductors are presented under extreme conditions. Raman measurements on elemental sulphur observe a clear asymmetric excitation between 80 and 150 GPa that can be attributed to the CDW amplitude mode. This mode is observed to undergo a weakly first-order phase transition at the SIV-SV structural phase boundary, with additional phonon modes visible below 150 GPa used to map the series of structural transitions that occur above ambient pressure. Electrical measurements on the same sample reveal a sharp increase in the superconducting critical temperature Tc at high pressure that, contrary to previous interpretations, is shown to not be related to a structural transition. Instead, fits to the temperature-dependent electrical resistivity provide evidence for a pressure-induced electronic transition from a single to multiband superconductor. Likewise, tunneling spectroscopy measurements on elemental lanthanum are used to probe the superconducting gap structure at 90 GPa, where nonlinear IV features indicate that Fmmm La is a strong-coupled multigap superconductor. Lanthanum hydride samples with critical temperatures of 70 - 80K have been synthesised from evaporated lanthanum films and ammonia borane as a hydrogen donor. These samples shed light on high-Tc metastable phases in this system, and measurements of the upper critical field Hc₂ are presented on stable phases down to low temperature. An unusual H₂(T) is observed, which can be well described by an extremely strong electron-phonon coupling. X-ray diffraction measurements on a similar Tc synthesised lanthanum hydride sample are analysed, which shows that the formation of stable low symmetry LaH₁₀ structures are responsible for the observed critical temperatures. Preliminary tunneling spectroscopy measurements on LaHx are also introduced, observing clear signatures of superconductivity with evidence for multiple superconducting gaps. Finally, the synthesis of H₃S is shown via elemental sulphur and the hydrogen donor ammonia borane, where a sharp superconducting transition at 197 K is observed at 155 GPa. Low temperature Raman spectroscopy measurements observe a novel excitation at 80-90 meV that is associated with the quasiparticle peak at the superconducting pairing gap energy. Comprehensive spatial and temperature dependencies of this excitation link this peak to the synthesised superconducting H₃S. In sharp contrast to conventional BCS superconductivity, the temperature dependence of this excitation shows evidence for preformed electron pairs above Tc, with H₃S in the BCS-BEC crossover regime.

In this thesis, electrical transport measurements were performed on superconducting, charge density wave, transition metal dichalcogenide compounds and hydride superconductors under extreme pressures. Firstly, Shubnikov-de Haas quantum oscillations and the masses of electrical carriers were measured in TiSe₂. Hydrostatic pressure was used to suppress the charge density wave, which resulted in several clear frequencies emerging and no indications of quantum critical mass enhancements were observed. Secondly, the interaction between the charge density wave and superconductivity in NbSe₂ was investigated. By suppressing the charge density wave with hydrostatic pressure, it was shown that there is no quantum critical enhancement in superconductivity around the critical point because the transition becomes 1st-order. Comprehensive magnetotransport measurements analysed within a four-band model suggest the presence of a pseudogap state coexisting with and outside of the charge density wave, which is further supported by Shubnikov-de Haas oscillations observing Fermi arcs generated by the pseudogap. This pseudogap state is also suppressed with pressure alongside the charge density wave and both states are destroyed at 4.4GPa. Measurements of the high temperature superconductors sulfur, yttrium, and lanthanum hydrides are presented alongside measurements on their precursor elements. Sulfur trihydride was synthesised from ammonia borane and observed to have similar superconducting properties to previous work, whilst Raman measurements on elemental sulfur observed a charge density wave amplitudon coexisting with superconductivity. Novel thin-film deposition techniques were used to synthesise lanthanum and yttrium hydrides, which promise to aid future syntheses of predicted room-temperature superconducting ternary hydrides. Superconductivity in yttrium thin-films was measured to 164GPa and agree with predictions for an Fddd crystal structure, which is believed to be universal for rare-earth elements. Finally, a low symmetry lanthanum superhydride was synthesised with a drastically lower superconducting temperature than the high-symmetry cubic phase observed previously.

In this thesis, the results of four electronic transport studies of correlated electron systems performed under hydrostatic pressure, at low temperatures and in the presence of high magnetic fields are presented. Firstly, magnetoresistance and Hall effect measurements in Tl2Ba2CuO6+δ, an overdoped cuprate, are presented. At low magnetic fields, a comparison between the evolution of the in-plane transport properties with a suppression of the superconducting transition temperature tuned by both chemical doping and the application of hydrostatic pressure. At high magnetic fields and in a region of doping-temperature phase-space in which conventional transport theory has previously been deployed to accurately describe the transport properties of highly overdoped cuprates, a highly unconventional magnetoresistance has been revealed. Definitive evidence of two charge sectors is presented where the first is orbital in nature whilst the second is highly unconventional and likely incoherent. Secondly, a high-field Hall effect study of YBa2Cu3O7−x, an underdoped cuprate, is presented. The evolution of the superconducting transition temperature and the temperature at which the Hall coefficient changes sign, an indicator of a Fermi surface reconstruction due to the onset of charge order, are tracked as a function of hydrostatic pressure. The first is used as a measure of the strength of superconductivity whilst the second is used as a proxy for the strength of charge order. The implications with regard to the interplay or competition between the two phases are discussed. Thirdly, a comprehensive study is presented of the in-plane magnetoresistance under pressure of FeSe1−xSx with sulphur concentrations that span the nematic phase. Nematicity is suppressed with pressure. At the critical pressure Pc at which the nematic transition is suppressed to zero temperature (at the nematic QCP), the in-plane resistivity measured at 35T exhibits hallmarks of quantum criticality. Specifically, the high-field resistivity is highly linear down to lowest temperatures and a growing region of T2 behaviour is observed at lowest temperatures as P is increased beyond Pc. Finally, a quantum oscillation study of the nodal-line semi-metal ZrSiS has been performed under hydrostatic pressure. At ambient pressure, unconventional breakdown phenomena have been observed. Analysis of the Shubnikov de-Haas oscillations at both ambient and elevated pressures has been performed to elucidate the nature of the Fermi surface and the origin of the aforementioned breakdown phenomena.