Neutrons have played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Neutron measurements of the phonon density-of-state are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below Tc, there is evidence compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with Tc, consistent with unconventional superconductivity of extended-s+/- wave symmetry.
Download