Extended Data Fig. 3: Field strength dependencies of other low energy modes.

a, b, c, and d are THz field strength dependencies of the 2.64 THz phonon, 3.69 THz magnon, 4.80 THz phonon, and 7.51 THz phonon mode amplitudes, respectively. Dashed lines in a and b are linear fits, whereas in c and d the fits are quadratic. The lower panels are the modulation patterns of the nearest-neighbour exchange interactions under the corresponding phonon displacements. The Q₁ (2.64 THz) and Q₄ (7.51 THz) phonons induce so called columnar modulations (see Supplementary Note 5). Since the columnar phase is far from the zig-zag phase, the contributions of these two phonons to the Ginzburg-Landau free energy can be neglected. The Q₃ (4.80 THz) phonon has a “zig-zag” type modulations, and it couples individually to M² and L². Unless there is a mechanism that breaks the degeneracy between opposite signs of magnetization, the Q₃ is also not expected to generate a net magnetization. These findings are further supported by spin Monte Carlo simulations (see Supplementary Note 6), and therefore, we neglect their contributions in the main text.