Normal and superconducting state electronic structure of iron based superconductors

Superconductivity is a fascinating phenomenon giving rise to quantum coherence over vast distances (several hundred kilometres of wire). It also has many valuable practical applications - basically anything which uses electricity can in principle benefit from superconducting technology. The most obvious applications are lossless power transmission cables, very high efficiency power transformers and generators. These applications have been promised since the early days of superconductivity at the start of the 20th century, but are only now becoming practical 20 years after the discovery of materials which superconduct above the boiling point of liquid nitrogen (77 Kelvin). The discovery and refinement of new superconducting materials benefits immensely from improving our understanding of their fundamental physics - most importantly the reason for the formation of the coherent superconducting state. Despite more than twenty years of research there is still no consensus as to the mechanism of high temperature superconductivity in the famous copper oxide (cuprate) materials. The discovery just over one year ago of high temperature superconductivity in material containing the element iron was very surprising as the magnetism normally associated with iron is highly detrimental to the formation of the superconducting state. These materials are of great interest from a point of view of both the fundamental physics and potential applications. The former stems from several key similarities (and differences) with the cuprate superconductors - perhaps making these materials the 'Rosetta stone' that can be used to understand how electronic mechanisms can produce a high temperature superconducting state. The latter from the fact that some materials continue to superconduct even when subjected to the world most intense magnetic fields in excess of 60T.The research in this proposal aims to further our understanding of high temperature superconductivity and in particular iron-based superconductors by pursuing two related research paths. One path will address the nature of the superconductivity itself. By measuring the influence of temperature on the density of superconducting electrons, through measurement of properties such as the magnetic penetration depth (i.e., the fundamental ability of a superconductor to screen out an applied magnetic field) we can learn the microscopic properties of the electrons which carry the superconducting current. The other path will be to study the properties of the 'normal' metal state. By applying large magnetic field, the superconductivity can be suppressed (effectively turned off) and then we can study the non-superconducting normal metallic state of the materials. In sufficiently high field, characteristic periodic oscillations of the magnetisation as a function of field (the de Haas-van Alphen effect) reveal the exact momenta of the electrons that carry current and how this momentum varies as a function of direction. This determination of the 'electronic structure' of a metal is akin to knowing its DNA. By studying how this normal state electronic structure varies in the different materials and how this influences the superconducting properties it will be possible to build up a theoretical picture of the mechanism of high temperature superconductivity.

As this is essentially fundamental research the economic benefit will likely be in the medium to long term (5 - 20 years). As this field is very new (around 1 year old) the timescale for applications is presently unknown as it depends strongly on the detailed properties of these newly discovered materials which are only just now being explored. Understanding of the fundamental principles of superconductivity will be of benefit to technologists working to develop application of superconductors. The economic impact of superconductivity has been predicted to increase significantly in the first few decades of the 21st century. Its progress is critically dependent on improvements in the characteristics of the materials, which goes hand-in-hand with improved understanding of their fundamental properties. A more immediate benefit of the research is the training of the post-doc and student associated with the project. The experimental and analytical skills learn will be of benefit to a wide section of UK industry. In particular, several UK companies (e.g., Siemens magnet technology / Oxford Instruments) have a direct need for graduates with a strong background in experimental physics at cryogenic temperature and high magnetic fields.