Superconducting Gap Structure and Symmetry in Fe Based Superconductors

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This is a proposal about exploration of a new exciting family of superconducting material systems. The work described here is strategically important in the UK and internationally. It places the Imperial group at the heart of these new developments. This proposal aims to enhance understanding, of the newly-discovered Fe based superconductor materials, in particular to unravel the superconducting gap structure and symmetry and its relationship to doping and crystal structure. In the eighteen months since high Tc pnictide materials were discovered, the area has become fundamentally extremely rich. Many families of material have been discovered, the similarities and differences to high Tc cuprates superconductors have become clearer. Most importantly high quality pnictide crystals have now become readily available. We plan to utilise the specialised facilities and expertise available to us to make a high impact investigation of the fundamental and applied properties of these systems, exploiting our excellent collaborative links to the best international crystal growing groups around the world. Serendipity has played a major role in the discovery of new superconductors since 1986 - an indication of the difficulty we have in predicting the behaviour of relatively simple inorganic compounds, despite the development of powerful analytical tools for electronic structure. Indeed, only for the simplest binary compound, MgB2, is there is a consensus on the understanding of the superconducting mechanism, and even there it has proved impossible to engineer the phase so as to further enhance the superconducting transition temperature Tc to any significant degree. One reason for MgB2 being such a difficult compound to improve is the paucity of other phases sufficiently similar in structure and chemistry to allow tuning of its properties. In contrast, in the eighteen months since the initial discovery of oxypnictide superconductivity, several different - but closely related - families of compounds have emerged. Consequently, there is a much larger chemical parameter space to be explored, within which it should be possible to gain a much improved understanding of the role of magnetic order in competing with superconductivity. On the other hand, in comparison with the cuprate superconductors, the latter have so many structure types and are often so difficult to prepare with high quality, that their complexity is a serious impediment to understanding. To give one example, the cuprate superconductor that has been most intensively commercialised to date, the BaSrCaCuO 2223 phase, has never yet been grown as a sizeable high-quality single crystal for fundamental studies.We suggest therefore that a continued exploration of the oxypnictide families is likely to improve our understanding of electronic properties of materials at a rapid pace, and perhaps even to improve our predictive abilities. This grant will build on our substantial preliminary investigations using Andreev point contact spectroscopy, that have been conducted on polycrystalline materials. This work gives us confidence in our methodology and our planned future activities. With access to high quality single crystals that are now readily available, we believe that we will be able to make significant possibly step change impact in this field both in terms of gap structure and symmetry and in terms of the role of phonons in the superconducting mechanism.