Abstract
Amplitude modes arising from symmetry breaking in materials are of broad interest in condensed matter physics. These modes reflect an oscillation in the amplitude of a complex order parameter, yet 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. Here we report an alternate route to realizing amplitude modes in magnetic materials by demonstrating that an 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 frustration of Heisenberg spins with uniaxial single-ion anisotropy can renormalize the interactions of a dense two-dimensional network of moments into largely decoupled, one-dimensional chains that manifest a longitudinally polarized-bound state. This bound state is driven by the Ising-like anisotropy inherent to the Mn3+ ions of this compound.
Highlights
Amplitude modes arising from symmetry breaking in materials are of broad interest in condensed matter physics
To explain the stabilization of this amplitude mode, we present a model that captures the excitation as a two-magnonbound state whose binding energy derives from an easy-axis single-ion anisotropy inherent to the orbitally quenched Mn3+ ions
Earlier neutron measurements have demonstrated that the ordered moment of α-NaMnO2 (2.92 μB) is significantly reduced from the classical expectation (4 μB)[11], suggesting substantial fluctuation effects in this material
Summary
The spin waves dispersing between the MnO6 planes (along L) are dispersionless (see Supplementary Fig. 2) as expected for the planar structure of α-Na0.9MnO2, and Fig. 2b shows that spin wave energies dispersing perpendicular to the AF chain direction in the plane (along H) are only weakly momentum dependent This demonstrates that the spin fluctuations exist as quasi-onedimensional planes of scattering in (Q, E) space, driven by the strong interchain frustration inherent to the lattice and consistent with the large magnetic frustration parameter of this compound[18]. This scan shows the large buildup of spectral. The transverse modes remain well-defined at high temperatures, reflective of the strong, inherently one-dimensional coupling and the single-ion anisotropy of the Mn moments
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