Properties of Spin-Waves in Iron Pnictides*

The spectrum of spin density-wave (SDW) fluctuation at zero temperature is studied within a multiband Hubbard-like Model. It is assumed that the formation of the SDW is driven by a short range interaction in a particle-hole channel. The mean-field solution reveals that the ground state is an itinerant, antiferromagnetically ordered phase, with staggered magnetic moment modulated at a wave vector M and a partially gapped Fermi surface, consisting of several disconnected pockets. A familiar Hubbard-Stratonovich transformation is used to obtain the effective action for this SDW state and the quantum fluctuations around the mean-field SDW solution are analyzed for realistic band structure of iron pnictides. In particular, we find that the damping is greatly enhanced by the inter-pocket decay of a spin-wave when its momentum reaches a critical value. We compare our theoretical results with the neutron scattering experiments and discuss interplay of SDW and superconductivity.