Paramagnetic spin excitations in insulating Rb$_{0.8}$Fe$_{1.6}$Se$_2$
  We use neutron scattering to study temperature dependent spin excitations in insulating antiferromagnetic (AF) Rb$_{0.8}$Fe$_{1.6}$Se$_2$. In the low-temperature AF state, spin waves can be accurately described by a local moment Heisenberg Hamiltonian. On warming to around the Neel temperature of T_N=500$ K, low-energy (E<30meV) paramagnetic spin excitations form Lorentzian-like quasielastic peaks centered at the AF wave vectors associated with spin waves, while high-energy (E>50meV) spin excitations become heavily damped. Upon further warming to above the structural distortion temperature of T_s=524K, the entire paramagnetic excitations become overdamped. These results suggest that AF Rb$_{0.8}$Fe$_{1.6}$Se$_2$ is not a copper-oxide-like Mott insulator, and has less electron correlations compared with metallic iron pnictides and iron chalcogenides. 
Miaoyin Wang, Xingye Lu, R. A. Ewings, Leland W. Harriger, Yu Song,   Scott V. Carr, Chunhong Li, Rui Zhang, Pengcheng Dai
cond-mat.supr-con cond-mat.str-el