Fig. 5: Photoinduced domain formation in SnSe.
From: Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe
After photoexcitation, the electrons are ‘heated’ into excited states. During electron excitation and relaxation processes, via energy flowing from electrons to phonons, in the harmonic energy potential, the atom vibration near the equilibrium position is enhanced, which increases the instability of the lattice periodicity. Additionally, the transverse optical (TO) Ag phonon modes are excited through electron–phonon coupling, atoms deviate from the original positions along c axis (Δdc). There are four Ag phonon modes in the b–c plane in SnSe. At 90 K, the direction of atomic displacements dominates at different delay times. The incompatibility reduces the correlation length \(\left( {\xi _{t1}^c} \right)\) generates domains driven by the shear strain between layers. The strain release drives the atoms move to the opposite direction, which gradually increases the correlation length \(\xi _{t2}^c\).
