Non adiabatic dynamics of the ferroelectric soft mode

TL;DR

This paper investigates non-adiabatic effects in ferroelectric dynamics of SnTe, revealing how electronic and lattice degrees of freedom can evolve separately, leading to nonlinear polarization behavior beyond the adiabatic approximation.

Contribution

It provides a physical framework for understanding non-adiabatic ferroelectric dynamics and the mixed displacive/order-disorder nature of SnTe based on scale separation.

Findings

Transient suppression of the ferroelectric double-well barrier causes nonlinear polarization dynamics.

Coherent phonon dynamics remain harmonic despite electronic polarization decoupling.

A unifying description of non-adiabatic effects in ferroelectric soft modes is proposed.

Abstract

Most microscopic descriptions of structural dynamics assume the Born-Oppenheimer separation, where electrons adjust adiabatically to ionic motion. When this separation breaks down, electronic and lattice degrees of freedom can evolve on different timescales, giving rise to new physical phenomena beyond the adiabatic limit. Here we use time-resolved, phase-sensitive second-harmonic generation and pump-probe reflectivity to reshape the ferroelectric free-energy landscape of SnTe while separately tracking polar order and coherent lattice motion. When photoexcitation transiently suppresses the double-well barrier, polarization dynamics become strongly nonlinear, while the coherent phonon dynamics remain harmonic. This decoupling cannot be described by a single adiabatic coordinate for the electronic polarization and ionic positions. We provide a unifying physical description for the non adiabatic dynamics of the ferroelectric mode and the mixed displacive/order-disorder nature of SnTe based on a separation of scales for the renormalization of the ferroelectric stiffness.