Condensed Matter Seminars

The iron pnictides have attracted great interest following the discovery of superconductivity up to 50K and because of their superficial similarities with the cuprate superconductors. In both systems, superconductivity emerges when longrange antiferromagnetic order is suppressed by doping or pressure. This indicates that spin correlations are intimately connected with superconductivity and may be involved in the pairing mechanism, in contrast to ordinary superconductors where the pairing is provided by phonons. Elastic and inelastic neutron scattering provides a most direct probe of both the longrange spin order as well as the strong spin fluctuations remaining when the longrange order has been suppressed, allowing detailed studies of how these spin correlations evolve as the superconducting state is approached and entered as a function of doping and temperature. Here, I will discuss what we have learned about the iron arsenides from such neutron scattering studies across the whole doping range. Particular focus will be given on the observation of a resonant spin excitation, how its sensitivity to the superconducting order parameter proves that superconductivity in these compounds is indeed unconventional, and how its doping dependence provides further clues to the origin of pairing in these compounds.