Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering

The discovery of a new family of superconductors containing layers of iron arsenide, the iron oxypnictides, has stimulated much interest, largely because they combine promisingly high transition temperatures with a mechanism of superconductivity rather similar to that of that the high-temperature copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase is suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation that is predicted to occur when the superconducting energy gap has opposite sign on different parts of the Fermi surface. Now inelastic neutron scattering observations of the iron arsenide Ba0.6K0.4Fe2As2 reveal a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, demonstrating that the superconducting energy gap in these materials has unconventional symmetry.