Fig. 2: Selective strong coupling of magnons and phonons in FePSe3.

a Schematic diagrams of magnon polaron (MP) bands under different types of coupling and their phenomenological coupling matrixes. A pair of degenerate AFM magnons (Mα and Mβ) and a pair of degenerate phonons with identical frequencies are considered (ωmag = ωph). The magnon Zeeman energy is H. Left panel: no interaction between magnons and phonons. Middle panel: non-selective strong coupling, where all magnon and phonon modes couple identically. Right panel: selective strong coupling, where each phonon mode only couples with one magnon mode. b Normalized far-infrared transmission spectra of bulk FePSe3 up to 17 T, four MP branches are identified (MP1 to MP4). c Peak positions of MP branches (blue circles) fitted with selective strong coupling model (orange lines). The deviation of the MP4 fitting line from the experimental peak positions is due to a series of additional magnon-phonon avoided crossings. Above 20 T, they are in good agreement (see Supplementary Fig. S3 for spectra up to 35 T).