No description available.

No description available.

No description available.

No description available.

Dimensionality is a determinant factor in the behaviour of interacting electron systems. The cuprates and the organic superconductors are examples of systems of reduced dimensionality with anomalous normal state correlations. This thesis presents a study of one and two-particle correlations, namely, the single particle spectral function and the pair propagator, for electron systems with low dimensionality. The single particle spectral function is probed by angle resolved photoemission experiments. Theories of photoemission are presented. One dimensional metals belong to the Luttinger liquid universality class, sharing the low energy properties of the Luttinger Model. A study of the spectral and Green's functions of this model is given. The spectral functions reflect the Fermi liquid breakdown due to the anomalous correlations and spin-charge separation which are typical of this model. The upper critical field <I>H</I><SUB>c2</SUB> of a type-II superconductor is a probe of anomalous normal state pair correlations. We study the weak coupling superconducting instability equation for a simple quasi-one dimensional system, the two-dimensional highly anisotropic tight-binding model for each typical filling. The magnetic field in conjunction with the underlying lattice induces a dimensional crossover which leads to divergent mean field <I>H</I><SUB>c2</SUB> curves in the strong anisotropy limit. We study the suppression of the mean field superconducting instability of a clean weak coupling BCS superconductor in a finite magnetic field at a 2D van-Hove singularity. The absence of a velocity scale is shown to lead a spectacular deviation from the Fermi liquid BCS result, such as upward curvature and a strong enhancement of the upper critical field relative to <I>T</I><SUB>c</SUB>, <I>H</I><SUB>c2</SUB>(0) ˜ <I>T</I><SUB>c</SUB><SUP>√2</SUP> in contrast with the conventional BCS scaling <I>H</I><SUB>c2</SUB>(0) ˜ <I>T</I><SUB>c</SUB><SUP>2</SUP>. The consequences for the superconducting transition of a Luttinger liquid-like spin-charge separated normal state are explored. We show that spin-charge separation leads to an enhancement of the superconducting critical temperature compared with the conventional BCS result. The spatial decay of the pair propagator is shown to contain a logarithmic correction relative to the free Fermi gas result in a finite interval between the spin and charge thermal lengths which leads to deviations from the usual BCS parabolic-like <I>H</I><SUB>c2</SUB> behaviour.

No description available.

The development of bulk superconductors for engineering applications, where large critical current densities (<I>J</I><SUB>c</SUB>'<I>s</I>) must be supported over large areas and at relatively high fields and temperatures, is one of the key issues in current superconductivity research. Melt-textured YBa<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>7-δ</SUB> (YBCO) offers the greatest potential for such applications, however the limited size of single grains which can be grown and the limited <I>J</I><SUB>c</SUB> supported by grain boundaries (GBs) have presented major hurdles. This thesis describes the fabrication of joins between YBCO grains, which opens up the possibility of producing larger samples of any shape and orientation. The joining technique uses a barium-cuprate 'liquid phase' released from the grains themselves above 920°C to mediate the epitaxial growth of YBCO across the interface between two grains. The microstructure and composition of joins fabricated with different orientations were investigated using optical, transmission electron and secondary electron microscopy. Extensive electrical measurements were made in order to determine the electromagnetic properties of the joins. Measurement of resistance as a function of temperature revealed information about the GB formed at the join. The maximum field dependent temperatures at which a <I>J</I><SUB>c</SUB> could be supported by different joins was determined (the 'irreversibility line') and compared directly with that of the adjoining grain material. The results have shown that joins with well defined GBs are formed, generally behaving like those found in other bulk and thin-film YBCO samples. Further optimisation of the joining technique is required, however great promise is shown for scale-up of the process to produce samples suitable for engineering applications.

No description available.

No description available.

No description available.