Superconducting Gap Structure and Symmetry in Fe Based Superconductors

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

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.

Planned Impact

Who will benefit from the research? The RA on the grant, the students who come to work with us, the schools who interact with our department through the Schools Liaison office, the wider academic community, the general public and potentially government policy makers. See also the academic beneficiaries summary and also the dissemination and exploitation section of the main case for support. How will they benefit? The RA will benefit from career development opportunities, and the UG and PG students will benefit from educational developments. The outreach program aims inspire and enthuse school children and the British economy will benefit if we manage to recruit more of our young people into scientific or technology based careers. The wider public will be engaged with our work through the media coverage of our highest impact results, our own web pages, exhibitions that we will arrange through the science museum or the Imperial College exhibition space. The general public will benefit by being better informed about the process of long term research and the potential benefits of that research to society. Government bodies will benefit by knowing that state of the art leading science is taking place in British universities. What is the plan to ensure that they benefit? This is covered in part on the management section of the main grant and also in detail in the impact plan document. The plan for the RA will be to assess progress on career development at an annual appraisal (personal review and development plan). For the UG and PG project work, in order to attract students we will publicise widely, in order to make sure they benefit from the association with the grant we will supervise them closely. Most of the UG and PG project work in our group results in at least one high impact journal publication and students will directly benefit from that in terms of their own career development. We will continue to engage with the school outreach office so that school children benefit from access to our laboratories during Open Day visits and also are enthused by our school visit lectures. The plans to ensure that the general public benefit will be by engagement with our Media office, the Institute of Physics, the National Laboratories, the EPSRC and the Science and Technology Policy Research

Publications

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Description This proposal was about exploration of a new exciting family of superconducting material systems. It placed the Imperial group at the heart of these new developments. This proposal aimed 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. During the course of the grant we utilised 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.



The project made the following impact:



1. Understanding the vortex pinning landscape in these materials

2. Studying the superconducting gap structure and symmetry

3. Examining the evidence for magnetic boson coupling
Exploitation Route Pnictide superconductors offer high transition temperature and high upper critical fields with relatively low anisotropy - from a practical point of view they may offer a useful alternative to HTS or MgB2 superconductors in the longer term for power applications. Currently the challenge is to understand the underlying mechanism that provides the 'glue' for Cooper pairing at high temperatures.
Sectors Energy

URL http://www.imperial.ac.uk/people/l.cohen/research.html
 
Description Our findings have not been used in a translational sense. Our work helps provide a groundwork of fundamental understanding which in the longer term will lead to applications or certainly increased knowledge.
First Year Of Impact 2012
Sector Electronics
Impact Types Cultural