A completely new family of high-temperature superconductors was recently discovered, based on iron compounds of the form RFeAsO where R is a rare-earth element. Like the well-known cuprate superconductors it is a layered system, but with highly-conducting layers consisting of Fe-As rather than Cu-O. The superconducting transition temperature depends on the level of doping and the rare-earth element but can reach over 50 K. Despite over 20 years of work, there is still no consensus on the mechanism behind high-temperature superconductivity, and these new materials offer the opportunity to look for unifying properties in a completely new class of materials.
The ISIS muon instruments have been used to investigate the superconducting and magnetic properties of the samarium series of compounds, which show the highest superconducting transition temperatures. Measurement has been made of the magnetic penetration depth in the superconducting vortex state and studies made of the complex interplay of magnetism and superconductivity in this new system.
The successful use of inelastic chopper spectrometers such as MAPS for studies of the cuprate superconductors has led to their high demand for investigations of these new materials. One group from the University of Tennessee have made measurements on Merlin looking at how the crystal field peaks change through Tc in the Ce based compounds. Another group from Japan have seen spin density waves in the parent compound LaFeAsO.
A group from Oak Ridge and Argonne National Laboratories have seen a resonance peak in superconducting Ba0.6K0.4Fe2As2 that disappears at Tc. All these groups have benefited from the huge count rate that Merlin delivers to study the dynamics at a large number of temperatures.
Research date: December 2008
AJ Drew, FL Pratt et al, Phys Rev Lett 101 (2008) 097010
S Chi et al, arXiv:0807.4986v1, Phys Rev Lett (accepted)
Ishikado et al, J Phys Soc Japan (accepted)
Christianson et al, arXiV:0807.3932v1, Nature (accepted)