Magnetic excitations in the high-Tc iron pnictides

We calculate the expected finite frequency neutron-scattering intensity based on the two-sublattice collinear antiferromagnet found by recent neutron-scattering experiments as well as by theoretical analysis on the iron oxypnictide LaOFeAs. We consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J(1) and next-nearest-neighbor coupling J(2). We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J(1). Whereas magnetic excitations in the cuprates display a so-called resonance peak at (pi, pi) (corresponding to a saddlepoint in the magnetic spectrum), which is at a wave vector that is at least close to nesting Fermi-surface-like structures-no such corresponding excitations exist in the iron pnictides. Rather, we find saddlepoints near (pi, pi/2) and (0, pi/2) (and symmetry related points), which are not close to nesting the Fermi surfaces.