Professor Xiong Qihua and collaborators from Singapore and the Netherlands have directly observed strong coupling between magnons and phonons in the two-dimensional antiferromagnetic crystal FePS3 - Department of Physics, Tsinghua University

### Abstract A team of researchers led by Professor Qihua Xiong from Tsinghua University, in collaboration with Professor Pinaki Sengupta from Nanyang Technological University and scientists from the National High Magnetic Field Laboratory in the Netherlands, has achieved a significant breakthrough in the study of magnetophonon interactions within two-dimensional (2D) antiferromagnetic crystals. The study, published in *Physical Review Letters* (DOI: 10.1103/PhysRevLett.127.097401), reports the direct observation of strong coupling between magnons (quantized spin waves) and phonons (quantized lattice vibrations) in FePS3, a 2D van der Waals crystal. #### Key Events and Findings - **Observation of Strong Coupling**: The team utilized Raman scattering spectroscopy under strong magnetic fields to observe the strong coupling between magnons and optical phonons in FePS3. This coupling resulted in the formation of new quasi-particles with characteristics of both magnons and phonons. - **Magnetic Field Effects**: By applying an out-of-plane magnetic field, the researchers observed that the magnon energy levels split due to the Zeeman effect, creating two branches with opposite spin angular momenta. When the external magnetic field exceeded 12 Tesla, one of these magnon branches resonated with an adjacent optical phonon branch, leading to strong coupling. - **Theoretical Model**: A theoretical model based on the single-ion model and the magnetoelastic Hamiltonian was developed to accurately describe the dispersion relations of the coupled quasi-particles under magnetic fields. - **Spin Angular Momentum Transfer**: The study also confirmed that the spin angular momentum of magnons is transferred to the coupled phonons, resulting in the emergence of phonon spin states, as evidenced by circularly polarized Raman spectroscopy. #### Scientific and Practical Implications - **Scientific Significance**: The discovery provides new insights into the behavior of magnons and phonons in the terahertz (THz) frequency range within antiferromagnetic materials. This is particularly important because antiferromagnetic magnons have intrinsic THz frequencies, which offer a natural advantage over the gigahertz (GHz) frequencies of ferromagnetic magnons. - **Application Potential**: The strong coupling observed in FePS3 suggests the potential for developing new methods to control magnon properties such as frequency and amplitude. Given that FePS3 also exhibits 2D semiconductor characteristics and photonic responses, it could serve as a central platform for integrating magnon-based devices with traditional photonic devices, opening up a wide range of applications. #### Reviewer Feedback The work was highly praised by the reviewers, with one noting, "The observation of clear and complete anticrossing curves using magnetic fields as high as 30 Tesla looks like a textbook example." This indicates the robustness and clarity of the experimental results. #### Funding and Support The research was supported by the National Key Laboratory of Low-Dimensional Quantum Physics and Tsinghua University's startup funding. #### Figures and Illustrations - **Figure 1**: (a) Schematic diagram showing the interaction between Fe atom magnetic moments and lattice vibrations in FePS3 crystals under a magnetic field; (b) Magnetic field strength of FePS3 crystals as a function of temperature, with a Néel transition temperature of approximately 116 K; (c) Temperature-dependent Raman spectra of FePS3 crystals; (d) Two-dimensional spin lattice of FePS3 in the antiferromagnetic phase. - **Figure 2**: (a) Raman spectra of FePS3 crystals under a magnetic field, with the two magnon branches labeled as M↑ and M↓; (b) Trends in the Raman peak positions of phonons and magnons as a function of magnetic field; (c) Calculated magnon energy-momentum dispersion relation at 0 Tesla; (d) Calculated magnon energy-momentum dispersion relation at 15 Tesla. This breakthrough not only advances the fundamental understanding of magnetophonon interactions in 2D antiferromagnetic materials but also paves the way for the development of novel magnon-based devices and integrated systems. The findings can be extended to various low-dimensional antiferromagnetic materials and structures, highlighting the broad scientific and technological impact of this research.