The First Principles Equation for Coherent Phonons: Dynamics and Polaron distortions

TL;DR

This paper develops a first-principles framework for understanding coherent phonons and polaron distortions, revealing nonadiabatic effects, a revised electron-phonon coupling, and a new polaron theory based on Ehrenfest dynamics.

Contribution

It reformulates the Ehrenfest equation using Born-Oppenheimer phonons, incorporating nonadiabatic effects and developing a first-principles polaron theory for doping-induced distortions.

Findings

Nonadiabatic effects renormalize phonon frequencies and lifetimes.

Electron-phonon coupling is smaller than the bare coupling even in the adiabatic limit.

A new first-principles polaron theory for lattice distortions is proposed.

Abstract

This paper addresses the first principles description of coherent phonons in systems subjected to optical excitations and/or doping. We reformulate the first-principles Ehrenfest equation (fpEE) [Phys. Rev. X {\bf 13}, 031026 (2023)] in terms of Born-Oppenheimer phonon frequencies and dynamical Born effective charges. We demonstrate that nonadiabatic effects renormalize the Born-Oppenheimer frequencies and introduce a damping term responsible for the finite lifetime of coherent phonons. Notably, both the frequency renormalization and the lifetime are identical to those of quantum phonons. Furthermore, we show that electrons exert a force driven by an unconventional dynamically screened electron-phonon coupling. This coupling is smaller than the bare one even in the adiabatic limit, highlighting the need to revise current models. The fpEE is also used to develop a first-principles polaron theory that describes lattice distortions induced by doping.