Deterioration of Spin-Orbit Transitions in Mott Insulating CoO

Despite a myriad of measurements spanning several decades, the low energy magnetic excitations of the classical Mott insulator CoO in the Néel regime are still poorly understood. As a result of the strong molecular-field induced entanglement of various j_eff manifolds, the establishment of a clear model for the low energy magnetic excitation spectrum of this deceptively simple monoxide has proven intractable using conventional spin wave approaches so far. Having extracted estimates for the exchange constants via the dilute monoxide Co_0.03Mg_0.97O (Sarte et al. PRB 98, 024415 (2018)), we have employed a random phase-type approximation in the method of Green's functions to model the rich low energy magnetic excitation spectrum of antiferromagnetically ordered CoO. The multi-level spin wave model successfully accounts for the temporally sharp spin-orbit transitions consistent with orbital ordering observed near the magnetic zone center. However, the model fails to account for higher energy transfers, where well-defined spin waves are replaced by energy and momentum broadened excitations, characterized by steeply dispersive columns of scattering. The failure of the model and breakdown of spin-orbit excitations are discussed in terms of coupling to a higher energy process.