Fermi Surface Reconstruction in Cuprate High Temperature Superconductors

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Superconductors have the potential to revolutionise the way the world uses electricity. There are already many practical applications of these materials, ranging from energy transport to uses in medical diagnosis, communications and mass people transport. However for more wide-ranging impact we need to discover materials which have even better properties than are already known today. In order to tune these properties and to guide the search for new materials, knowledge of the fundamental physical reasons why these materials are superconducting is highly desirable. Although this is known for so-called conventional materials, mostly discovered before 1980, an understanding of the superconducting mechanism responsible for copper oxide based high temperature superconductivity, discovered in 1986, is still lacking. The research in this proposal aims to advance our understanding of the electronic structure of copper oxide high-temperature superconductors. We believe this knowledge will provide a major step forward in the world-wide quest to understand and hence improve these materials.We are proposing a wide ranging programme which will study the thermodynamic and quantum coherent properties of extremely well ordered samples of these materials. In less well ordered samples, the signatures of the fundamental symmetry-breaking phase transitions may be smeared out, making them invisible to experiment. Also quantum coherence effects which give unique information about the electronic structure are made unobservable by disorder. We will use techniques developed over the last twenty years to grow highly ordered single crystal samples and study their behaviour under the world's highest available magnetic fields of up to 100 T (which is roughly 2 million times larger than the earth's field) at temperatures less than one degree above absolute zero. From these measurements we will discover how the Fermi surface, which characterises the momentum distribution of the current carrying electrons in the material, evolves with electron concentration. This will give unique and important information to guide the development of a theory of superconductivity in these materials.