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We use neutron scattering to study temperature dependent spin excitations ininsulating antiferromagnetic (AF) Rb$_{0.8}$Fe$_{1.6}$Se$_2$. In thelow-temperature AF state, spin waves can be accurately described by a localmoment Heisenberg Hamiltonian. On warming to around the Neel temperature ofT_N=500$ K, low-energy (E<30meV) paramagnetic spin excitations formLorentzian-like quasielastic peaks centered at the AF wave vectors associatedwith spin waves, while high-energy (E>50meV) spin excitations become heavilydamped. Upon further warming to above the structural distortion temperature ofT_s=524K, the entire paramagnetic excitations become overdamped. These resultssuggest that AF Rb$_{0.8}$Fe$_{1.6}$Se$_2$ is not a copper-oxide-like Mottinsulator, and has less electron correlations compared with metallic ironpnictides and iron chalcogenides.

Paramagnetic spin excitations in insulating Rb$_{0.8}$Fe$_{1.6}$Se$_2$