Observation of Ultrastrong Magnon-Magnon Coupling in YFeO3 using Terahertz Magnetospectroscopy

We studied magnon-magnon ultrastrong coupling in YFeO
₃
using terahertz magnetospectroscopy in magnetic fields up to 30 T, which led to an extreme breakdown of the rotating-wave approximation where the counter-rotating term dominates the co-rotating term.