Magnon sound waves

Strong interactions between particles can lead to emergent collective excitations. In the solid state, these phenomena have been extensively established in electronic systems, but are also expected to occur for gases of neutral particles like spin waves (magnons) in a magnet. In a regime where magnons are strongly interacting, they can form a propagating density oscillation - in analogy to hydrodynamic sound waves in water - with characteristic low-frequency signatures. While such a mode has been predicted in theory, particularly for spin isotropic magnets like Van der Waals CrCl3, its signatures have yet to be observed experimentally. In this talk, I will discuss the development of techniques for probing collective magnon dynamics in nanoscale CrCl3 samples down to the atomically-thin limit, utilizing the quantum coherence of nearby Nitrogen-Vacancy (NV) centers in diamond. This technique has allowed us to find evidence for viscous damping of a magnon sound mode in the ferromagnetic phase of atomically-thin CrCl3. By integrating NV detection with microwave drive to the material, we establish direct spectroscopic evidence for these propagating magnon sound waves. I will discuss the extent to which the phenomena observed here could generalize to other magnetic materials.

Speaker: Nikola Maksimovic, Harvard University