Spin Waves and magnetic exchange interactions in insulating Rb0.89Fe1.58Se2

Superconductivity in alkaline iron selenide AFe$_{1.6+x}$Se$_{2}$ (A = K, Rb, Cs) may have a different origin from the sign reversed s-wave electron pairing mechanism, because they are insulators near x = 0 and form a blocked AF structure that is completely different from the iron pnictides. We use neutron scattering to map out spin waves in the AF insulating Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$. A comparison of the fitted effective exchange couplings using a local moment Heisenberg Hamiltonian in Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$, (Ba,Ca,Sr)Fe$_{2}$As$_{2}$, and iron chalcogenide Fe1.05Te reveals that their next nearest neighbor (NNN) exchange couplings are similar. Therefore, superconductivity in all Fe-based materials may have a common magnetic origin that is intimately associated with the NNN magnetic exchange interactions, even though they have metallic or insulating ground states, different AF orders and electronic band structures.