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

TL;DR

This study investigates how spin excitations in insulating Rb$_{0.8}$Fe$_{1.6}$Se$_2$ evolve with temperature, revealing damping behaviors that differentiate it from other correlated electron systems like cuprates and iron-based superconductors.

Contribution

The paper provides detailed neutron scattering analysis of temperature-dependent spin excitations in Rb$_{0.8}$Fe$_{1.6}$Se$_2$, showing it is not a Mott insulator and has weaker electron correlations.

Findings

Low-energy spin excitations form quasielastic peaks near the Neel temperature.

High-energy spin excitations become heavily damped at elevated temperatures.

Paramagnetic excitations become overdamped above the structural transition temperature.

Abstract

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.