Nonadiabatic dynamics of the ferroelectric soft mode.

Photoexcitation in SnTe transiently suppresses the double-well barrier, making polarization dynamics strongly nonlinear while coherent phonon dynamics stay harmonic, revealing nonadiabatic decoupling.

• Why it matters: The work addresses how breakdown of Born-Oppenheimer separation alters coupled electronic and lattice evolution, where a single adiabatic coordinate may fail to describe ferroelectric dynamics.
• What they did: Using time-resolved, phase-sensitive second-harmonic generation and pump-probe reflectivity, the study separately tracks polar order and coherent lattice motion in SnTe after excitation.
• The result: The observed nonlinear polarization alongside harmonic coherent phonons cannot be captured by one adiabatic description, enabling a unified account of nonadiabatic ferroelectric-mode dynamics and SnTe’s mixed displacive/order-disorder character.

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