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This thesis describes measurements of the optical and d.c. electrical conductivity of a family of high-temperature superconductors. For this purpose, an optical spectroscopy laboratory was constructed, incorporating a commercial infrared spectrometer and a homemade continuous-flow optical cryostat. The performance of the experimental set-up was studied through comparison with recent, high-quality, reflectance data on heavily over doped La2-xSra;Cu04 (LSCO) with x = 0.28, which was measured by the Timusk group at McMaster University, Canada. Excellent agreement, to within 1%, was found and thus, a working experiment was realized. The system is capable of measurements at temperatures n the range 12 - 350K, from the far-infrared to the near-infrared (~ 40 - 10,000cm ⁻¹). The overall experimental uncertainty in the reflectance data does not exceed ±0.5%.

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Using a high resolution susceptometer based on a self-resonant tunnel diode circuit, the temperature dependence of the London penetration depth λ(T) was measured in the cuprate superconductor YBa2Cu4O8 and in two iron-based superconductor materials (LaFePO,FeSe0.5 Te0.5). The first project investigated the role of CuO chains in the underdoped high- Tc cuprate YBa2CU4O8 in relation to the anisotropy observed in the in-plane components of the superfluid density [1][2]. The components of λ(T) relative to the three crystallographic axises were measured. An in-plane anisotropic response was revealed by a clear enhancement of the superfluid density along the chain direction (b-axis) at low temperature. A small DC magnetic field applied perpendicular to the CuO chain revealed a strong suppression of this extra screening current density. In terms of coupling between plane and chain bands, the result can be interpreted within a proximity-model with superconductivity induced in the intrinsically normal chains by the superconducting planes through the "proximity-effect' [3] [4] In the second project we studied the symmetry of the order parameter in iron- based superconductors. The linear T dependence of λ observed in LaFePO strongly suggested the presence of nodes in the gap function. The linearity evolved into a T2 dependence upon isovalent substitution of Y atoms on the La site supporting the conclusion of the existence of nodes where the gap changes sign. In contrast λ(T) in FeSe0.5Te0.5 followed a higher power-law T" with n=2.2±O.1. Such behaviour can be taken as evidence for strong pair breaking within a sign-changing pairing state either with or, more likely according to other experimental results (thermal conductivity in particular), without nodes. Heat-capacity measurements were also performed on this material revealing a field dependence close to the transition which resembles the high- Tc cuprates behaviour with evidences of strong thermal fluctuations. Interestingly such fluctuations appear to have unusual anisotropic effects in FeSe0.5Te0.5.

The measurement of specific heat provides unique information concerning the superconducting state. It has the advantage over many other techniques as in that it is a bulk thermodynamic probe and as such is relatively insensitive to the presence of minority impurity phases. For this thesis, two calorimeters were designed, built and calibrated to measure ae specific heat of various exotic superconductors in the form of sub-milligram single crystals.

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Bulk high temperature superconductors have increasing potential to be used in a number of devices including motors, levitated transportation, flywheels, cables, transformers and fault current limiters, etc. To optimize the design of such systems, it is necessary to predict their behaviours. The research presented in this thesis aims to fulfil this need by implementing a range of critical state models using finite element method in order to calculate field distributions, current densities, trapped magnetic fields and AC losses. A numerical solver of critical state is proposed which is based on a set of Maxwell’s equations using magnetic field as the known. The equations are used with <i>E-J</i> constitutive law and are incorporated with commercial finite element software to give fast and accurate solutions. Four case studies surrounding the application of using superconducting materials as wires and trapped field magnets are investigated. The phenomenon of MgB­₂ wires carrying transport current is investigated in order to determine the AC loss in the wires. Two finite element models based on different assumptions are introduced to solve the AC losses in YBCO coated conductors both in self field and external field conditions. The crossed field effects on bulk high-temperature superconductors are studied. A bulk YBCO single domain is premagnetized with the applied field parallel to their shortest direction and then subjected to several cycles of the application of a transverse magnetic field parallel to the sample surface plane. The decay of magnetization is investigated both experimentally and numerically. The remagnetization process is studied by applying rotational field to a bulk YBCO which has been partially demagnetized by a transverse field. It is shown that all principal features of the experimental data can be reproduced quantitatively or qualitatively by using the model proposed in this thesis.

Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic flux sensors known. In order to achieve a high field resolution, an antenna is coupled to the bare SQUID. Flux focusing, direct and flux transformer coupling schemes were investigated. Off-axis pulsed laser deposited YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB> films with protective SrTiO<SUB>3</SUB> (STO) cap layers on 24° STO bicrystal substrates were patterned into SQUID structures. A multilayer technology with YBCO and STO layers was used to fabricate trilayer flux transformers. The flux transformers was then coupled inductively to the bare SQUID in a flip-chip arrangement. Characterisation equipment was developed to measure SQUID parameters. Noise measurements with a bias reversal technique were employed to reduce the 1/f component, which dominates the noise at low frequencies. All noise data were obtained at 77 K. For a flux focusing SQUID, a flux noise level of φ<SUB>n</SUB> = 2.8 μφ<SUB>0</SUB>Hz<SUP>-1/2</SUP> at 10 kHz and φ<SUB>n</SUB> = 11.5 μφ<SUB>0</SUB>Hz<SUP>-1/2</SUP> at 1 Hz with bias reversal was achieved. The field sensitivity of the flux focused SQUID was limited by the small effective area, A<SUB>eff </SUB>= 0.83 x 10<SUP>-3</SUP> mm<SUP>2</SUP>, but this type of device was successfully integrated into the IRC cold sample scanning SQUID microscope which was used to image currents in wires and ferromagnetic link samples. Direct and flux transformer coupling schemes increased the effective areas to 0.23 mm<SUP>2</SUP> and 1.08 mm<SUP>2</SUP> respectively. The best white field sensitivity measured corresponded to 105 fTHz<SUP>-1/2</SUP> for the directly coupled SQUID and 42 fTHz<SUP>-1/2</SUP> for the flux transformer coupled SQUID. Planar, first-order directly coupled gradiometers were investigated as part of the collaboration with Oxford Instruments plc and a white gradient field sensitivity of 2.1 pTcm<SUP>-1</SUP>Hz<SUP>-1/2<I> </I></SUP>in an unshielded environment was achieved.