Discovery in February 2008 of a new class of superconductors based on Fe-As combinations has spawned enormous activity as materials groups race to find the right combinations that will push the superconducting temperature as high as possible. In some of these, the critical temperature is now above 50 K, but the combinatorial possibilities for four or more elements are so vast that exploring the chemical landscape has only just started.
The new Fe-As materials share many similarities with the copper oxide superconductors uncovered by Nobel Prize winners Bednorz and Müller in 1986. The iron arsenides and the copper oxides both have layered crystal structures and superconductivity arises in each when an antiferromagnetically ordered phase has been suppressed by chemical doping. Superconductivity seems to be connected to magnetic behaviour, but quite how this works continues to be a mystery.
The latest generation of neutron spectrometers at ISIS, Maps and Merlin, enable atomic level magnetism in these materials to be mapped in great detail. They give the most complete information on the fluctuations in space and time of the magnetism and open the way for the most exacting tests of superconductivity theories.
Although the first iron arsenide superconductors were based on doped variants of RFeAsO, where R is a rare-earth element, there has been considerable interest in a new series of tetragonal compounds based on AFe2As2 (where A is barium, strontium or calcium), in which superconductivity is induced either by doping the A site with potassium or sodium or by applying pressure.
Since superconductivity in the Fe-As family is suspected to come from a magnetic pairing mechanism between electrons, Diallo et al. opted to explore the non-superconducting parent compound CaFe2As2 in a tour-de-force experiment with 400 co-aligned crystals grown from tin flux with a total mass around 2 grams. The material was found to be best described as an itinerant three-dimensional antiferromagnet [Diallo et al., Phys. Rev. Lett. (2009) 102, 187206].
Meanwhile, experiments on another material, polycrystalline Ba0.6K0.4Fe2As2, found that a resonant spin excitation vanished at the superconducting temperature [Christiansen et al., Nature (2008) 456, 07625]. A similar resonant excitation is a universal feature in the copper oxide superconductors and this experiment has demonstrated that the symmetry of the superconducting energy gap differs from that of the copper-oxide based materials.
What is becoming clear is that static magnetism persists well into the superconducting regime for both material classes. Proximity to magnetic order and low-energy magnetic fluctuations may be the key ingredients of high-temperature superconductors. With two classes of materials, and maybe more to come, the race is now on to find the right theory to explain the experimental data and maybe win a second Nobel Prize for high temperature superconductivity.