Structure of LiFeAs (Pitcher et al); spin waves in CaFe2As2 from magnetic exchange parameters measured on Merlin (Zhao et al); phase diagram of SmFeAsO1-xFx (Drew et al).
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The recent discovery of superconductivity in several iron- and arsenic-based compounds with critical temperatures as high as 55K has caused great excitement because they are the first non-copper-based high-temperature superconductors.
Just like the cuprate superconductors, they are formed by doping a layered parent material, and there is strong evidence that magnetism has a central role in the mechanism responsible for the superconductivity. There are, however, many differences, and researchers are using a gamut of experimental probes to unravel the origins of the superconductivity in these materials.
Experiments performed at ISIS are making a significant impact. Muon measurements from SmFeAsO1-xFx showed enhanced magnetic fluctuations near the superconducting transition, and have revealed a region of coexisting superconductivity and iron magnetism. The existence of a magnetic resonance that appears only in the superconducting phase of Ba0.6K0.4Fe2As2, one of another family of iron pnictides, and which is strongly reminiscent of the cuprate superconductors, was discovered on Merlin. The bandwidth of the spin fluctuations of parent materials BaFe2As2 and CaFe2As2 have been measured on Merlin and Maps, revealing excitations that extend to almost 200 meV. Such measurements help in enabling a detailed model of the magnetic behaviour to be produced. Finally, examples of structural studies are those on the oxygen-free iron pnictides LiFeAs and NaFeAs, which show superconductivity without doping.
Research date: September 2009
Further reading: A Christianson et al., Nature 456 (2008) 930; RA Ewings et al., Phys Rev B 78 (2008) 220501(R); J Zhao et al., Nature Physics 5 (2009) 555; S Diallo et al., Phys Rev Lett 102 (2009) 187206; AJ Drew et al., Nature Materials 8 (2009) 310; MJ Pitcher et al., Chem Comm. 2008 5918; DR Parker et al., Chem Comm (2009) 2189.
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