High-energy damping by particle-hole excitations in the spin-wave spectrum of iron-based superconductors

Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site, as well as itinerant electrons from the degenerate dxz and dyz orbitals. The local moments interact with each other through antiferromagnetic J1-J2 Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund coupling. We employ the fermionic spinon representation for the local moments and perform a generalized random-phase approximation calculation on both spinons and itinerant electrons. We find that in the (π,0) magnetically ordered state, the spin-wave excitation at (π,π) is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with a previous study using the Holstein-Primakoff transformation. In the paramagnetic state, the particle-hole continuum keeps the collective spin excitation near (π,π) at a higher energy even without any C4 symmetry breaking. The implications for recent high-temperature neutron scattering measurements will be discussed.