Observation of Ultrastrong Magnon-Magnon Coupling in YFeO₃ in High Magnetic Fields

There is currently much interest in exploring resonant light-matter interaction in the ultrastrong-coupling (USC) regime where light and matter mix to an extreme degree and a variety of new phenomena emerge [1]. Recently, we have extended the concept of USC to a magnetic context in the form of matter-matter interaction [2]. Specifically, the exchange interaction of paramagnetic Er³⁺ spins with an Fe³⁺ magnon field in ErFeO₃ exhibited a large vacuum Rabi splitting. Here, we report another type of matter-matter USC between two magnon modes in YFeO₃. In rare-earth orthoferrite materials, there are ferromagnetic (FM) and antiferromagnetic (AFM) modes of magnons in the terahertz (THz) frequency range. By performing single-shot THz time-domain spectroscopy with a table-top pulsed magnet [3], we have investigated the magnetic field dependent magnon frequency and discovered anticrossing behavior for the FM and AFM magnon modes in high magnetic fields. The magnitude of the vacuum Rabi splitting suggests USC strength between the magnon modes when applying high magnetic fields at certain finite angles with respect to the crystallographic axes of bulk YFeO₃.
[1] P. Forn-Díaz et al., arXiv:1804.09275.
[2] X. Li et al., Science 361, 794 (2018).
[3] G. T. Noe et al., Opt. Exp. 24, 30328 (2016).