Coherent strong-coupling of terahertz magnons and phonons in a Van der Waals antiferromagnetic insulator

(submitted), (2021)

Coherent strong-coupling of terahertz magnons and phonons in a Van der Waals antiferromagnetic insulator

Qi Zhang, Mykhaylo Ozerov, Emil Vinas Boström, Jun Cui, Nishchay Suri, Qianni Jiang, Chong Wang, Fangliang Wu, Kyle Hwangbo, Jiun-Haw Chu, Di Xiao, Angel Rubio, Xiaodong Xu

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

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Authors thank Dr. Yuan Wan for valuable discussions. This project is mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0012509). QZ’s work is partially supported from MOST of China (Grant No. 2020YFA0309200) and Fundamental Research Funds for the Central Universities (0204-14380184). The FIR measurement was performed at the NHMFL which is supported by the NSF (Grant number DMR1644779) and the State of Florida, US. First principle work was supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence ’Advanced Imaging of Matter’ (AIM), Grupos Consolidados (IT1249-19) and SFB925 “Light induced dynamics and control of correlated quantum systems”. AR acknowledges support from the Max Planck-New York City Center for Non-Equilibrium Quantum Phenomena. The Flatiron Institute is a division of the Simons Foundation. Bulk crystal growth is supported by NSF MRSEC DMR-1719797 and the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF6759 to JHC.

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