Figure 7: Effect of transverse optical (TO) phonon displacements on the electronic band structure.

(a) A TO phonon displacement at Γ induces a notable separation of the lowest conduction and highest valence bands (dashed) in comparison with the equilibrium configuration (solid). The colour shading of the bands indicates the change of occupation of electron and hole states following photoexcitation. The band shifts occur most dramatically within a region of reciprocal space limited along the L–W line. The widening of the gap is consistent with a Jones-Peierls distortion mechanism, demonstrating an inclination towards the lower-symmetry rhombohedral ferroelectric structure. Photoexcitation reduces the effectiveness of this mechanism, resulting in a hardening of the TO frequency near zone centre. (b) Separately, a TO phonon displacement at X results in a notable closing of the gap between the bottommost conduction and topmost valence bands (dashed) relative to the ground state configuration (solid). Once again, the activity is along the L–W line. In contrast with zone centre, photoexcitation results in a softening of the TO frequency here.