Figure 5: Comparison of calculated harmonic interatomic force constants at equilibrium and under 0.5% photoexcitation.

(a) Nonzero harmonic interatomic force constants (IFCs) calculated for both equilibrium (blue dots) and 0.5% photoexcited PbTe (red crosses) on 216-atom supercells. The IFCs for the first eleven nearest neighbouring atoms are displayed. Particularly large IFCs are observed between first, fourth and eighth nearest neighbours, which correspond to interactions along the <100> bonding direction responsible for PbTe’s incipient ferroelectricity. See Fig. 6 for a real-space depiction of these neighbours. The absolute values of these large IFCs notably decrease with photoexcitation, driving PbTe further away from the ferroelectric phase. (b) Differences between the absolute values of the equilibrium and photoexcited IFCs in a.