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Abstract

Emergent cooperative motions of individual degrees of freedom, i.e.
collective excitations, govern the low-energy response of system ground states
under external stimulations and play essential roles for understanding
many-body phenomena in low-dimensional materials. The hybridization of distinct
collective modes provides a route towards coherent manipulation of coupled
degrees of freedom and quantum phases. In magnets, strong coupling between
collective spin and lattice excitations, i.e., magnons and phonons, can lead to
coherent quasi-particle magnon polarons. Here, we report the direct observation
of a series of terahertz magnon polarons in a layered zigzag antiferromagnet
FePS3 via far-infrared (FIR) transmission measurements. The characteristic
avoided-crossing behavior is clearly seen as the magnon-phonon detuning is
continuously changed via Zeeman shift of the magnon mode. The coupling strength
g is giant, achieving 120 GHz (0.5 meV), the largest value reported so far.
Such a strong coupling leads to a large ratio of g to the resonance frequency
(g/{\omega}) of 4.5%, and a value of 29 in cooperativity
(g^2/{\gamma}_{ph}{\gamma}_{mag}). Experimental results are well reproduced by
first-principle calculations, where the strong coupling is identified to arise
from phonon-modulated anisotropic magnetic interactions due to spin-orbit
coupling. These findings establish FePS3 as an ideal testbed for exploring
hybridization-induced topological magnonics in two dimensions and the coherent
control of spin and lattice degrees of freedom in the terahertz regime.