Fig. 2: Analysis of the oscillatory response.

a Temperature dependence of the Fast Fourier Transform spectra, FFT, demonstrating the presence of several modes at low temperatures. The extracted mode frequencies, ν_i in the low-temperature limit are denoted by red arrows (see also Supplementary Note 3). The top axis presents the energy scale in wavenumbers, ω (cm⁻¹). Insert presents the FFT of the data recorded at 13 K, with white arrows pointing at the modes. The temperature dependence of the parameters of the four strongest low-frequency modes, obtained by fitting the oscillatory response with the sum of four damped oscillators: b central frequencies ν_i, c linewidths Γ_i, and d spectral weights S_i. The triclinic phase transition temperature T_S is denoted by vertical dashed lines. The dashed red line in c presents the expected T-dependence of the linewidth of 1.45 THz mode for the case, when damping is governed by the anharmonic phonon decay³³. e–g present the zoom-in of the b–d, emphasizing the evolution of the parameters across the triclinic transition at T_S = 138 K. The error bars are obtained from the standard deviation of the least-squared fit.