Magnetic Correlations in Superconducting Iron Arsenides

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Since February 2008 a new family of superconductors has been developed based on a series of compounds containing layers of iron and arsenic atoms or alternatively iron and selenium atoms. These compounds exhibit superconductivity (i.e. they lose all their electrical resistance) at relatively high temperatures (at up to 56 K as at November 2008). The current thinking is that the mechanism of superconductivity is not the same as that found in simple elements and compounds, such as lead and magnesium diboride, in which the vibrations of the crystal lattice cause the electrons to lower their energy by binding into pairs which can move through the crystal without resistance. Rather, the new iron arsenide and selenide superconductors exist in close proximity to magnetic phases, which suggests that magnetic interactions might be responsible for superconductivity.The purpose of the proposed work is to investigate the relationship between superconductivity and magnetism in selected members of the iron arsenide family of compounds. So far, all compounds found with a particular structural arrangement of iron and arsenic atoms have been successfully turned into superconductors through control of their electron count, even though there is considerable variation in the crystal structures. This is achieved by making small variations in the chemical composition through substitutions, e.g. by replacing some of the iron toms by cobalt. This chemical flexibility potentially makes it possible to tune the physical behaviour of the materials in many different ways, which may be exploited to perform systematic experiments to study the interplay between magnetism and superconductivity. The applicants have recently contributed new compositions to this field, such as LiFeAs and NaFeAs, and are well placed to embark on a systematic investigation of the magnetic and superconducting properties of these materials. In this project a graduate student will perform a series of studies using a range of neutron and X-ray scattering techniques to gain access to the fundamental magnetic behaviour on an atomic scale. The particular focus will be on the use of neutron spectroscopy to probe the magnetic dynamics in the non-superconducting compounds close in composition to the superconductors, and to probe the nature of the superconducting energy gap in the superconducting members. The proposed studies at the ISIS, ILL and Diamond facilities will build on the very recent work by the applicants in this field and will provide a coherent research programme for a graduate student who will gain experience of chemical synthesis, physical property measurements and extensive use of Central Facilities.