Dissipative Antiferromagnetic Dynamics

Abstract

No consensus exists on the equations that govern the dissipative dynamics of antiferromagnets. In particular, from phenomenological considerations (H. Y. Yuan, Liu, et al. 2019) one expects inter-sublattice damping terms in the sublattice dynamics. These terms are not present in the Landau-Lifshitz-Gilbert equations that are commonly used to describe individual sublattice dynamics. In this thesis, we present a Caldeira-Leggett approach within the Keldysh path integral formalism to base the dissipative equations for two-sublattice antiferromagnets on a more microscopic description. From this, we argue that the dissipative dynamics are determined by the specific form of system-bath structure through which the system dissipates. We recover phenomenological equations in the quasi-classical limit and find that the presence of inter-sublattice damping terms originate from the thermal coupling between the sublattices. Remarkably, we find that in a special case the N´eel order is a dissipation-free mode. Subsequently, we argue that the existence of dissipation-free modes is a more general phenomenon and will occur in systems composed of two subsystems where the bath is shared between the two subsystems.