Elementary excitations in magnetically ordered systems with orbital degeneracy

Abstract

The generalized Holstein-Primakoff transformation is used to develop a quantum flavor wave theory for spin systems with orbital degeneracy. Elementary excitations of ordered ground states consist of spin, orbital, and spin-orbital waves. Spin and spin-orbital waves couple to each other due to orbital anisotropy and Hund's rule, resulting in modes observable by inelastic neutron scattering. In the $\mathrm{SU}(4)$ limit, flavor waves are dispersionless along one or more directions, and give rise to quantum fluctuations of reduced dimensionality.