Amplitude Mode in the Planar Triangular Antiferromagnet Na0.9MnO2
Rebecca L. Dally, Yang Zhao, Zhijun NMN Xu, Robin Chisnell, M. B. Stone, Jeffrey W. Lynn, Leon Balents, Stephen D. Wilson
Amplitude modes that arise from continuous symmetry breaking in materials are of broad interest as condensed matter analogs of the Higgs boson in particle physics. These modes reflect an oscillation in the amplitude of a complex order parameter, yet they are typically unstable and decay into oscillations of the order parameter's phase. This renders stable amplitude modes rare, and exotic effects in quantum antiferromagnets have historically provided a realm for their detection - one where effects such as singlet formation and the proximity to a quantum critical point stabilize propagating oscillations of the magnetization's magnitude. Here we report a new route to realizing amplitude modes in magnetic materials by demonstrating that a classical antiferromagnet on a two-dimensional anisotropic triangular lattice (α-Na0.9MnO2) exhibits a long-lived, coherent oscillation of its staggered magnetization field. Our results show that geometric frustrations of Heisenberg spins with easy-axis anisotropy can renormalize the interactions of a dense two-dimensional network of moments into decoupled, one-dimensional chains that manifest a longitudinally polarized bound state. This presents a novel avenue for realizing amplitude modes in classical spin system and suggests an intriguing new material for exploring topological excitations such as solition bound states.