Finite temperature magnon dynamics and interactions in a spin-1 chain

Quasiparticles are constructs for simplifying the complicated many-body description of solids. Their dynamics and mutual effective interactions provide rich information about real world material properties. We probe dynamics of magnon quasiparticles in a ferromagnetic spin-1 Heisenberg chain with easy-axis anisotropy and analyze the effect of magnon-magnon interactions on its thermal properties. This model is relevant for recent terahertz (THz) optics experiments on NiNb2O6 by Chauhan et al. [Phys. Rev. Lett. 124, 037203 (2020)], where a tunable, field direction sensitive, temperature-dependent shift of the dynamical magnetic susceptibility was reported. We perform numerical and visual analyses of the corresponding Kubo formula by utilizing matrix elements between various n and n+1 magnon states, identifying the most active transitions at a given temperature. Using a combination of exact diagonalization and density matrix renormalization group calculations, our study reveals that single-ion uniaxial anisotropy results in field dependent magnon attraction or repulsion. Attraction stabilizes composite bound states of magnons, and we propose ways of detecting them in future experiments.