Fig. 2: Phonon spectral function along high symmetry lines in GeTe at the phase transition.

The spectral function is calculated using Eq. (2). Black lines represent the temperature-dependent effective potential (TDEP) frequencies calculated at 631 K (close to the critical temperature 634 K) and dashed cyan lines are the TDEP phonon band structure at 300 K. The yellow colour corresponds to the lower, while the purple colour corresponds to the higher value of the phonon spectral function.