# Ultrafast relaxation of symmetry-breaking photo-induced atomic forces

## TL;DR

This paper introduces a first-principles method to analyze how symmetry-breaking atomic forces in photoexcited materials relax over time, emphasizing electron-phonon interactions and their dependence on photon energy.

## Contribution

It provides a novel computational approach to quantify the temperature-dependent decay of photoinduced forces and links the decay rate to the electron self-energy in group-V semimetals.

## Key findings

- Force decay lifetimes of ~10 fs for Bi and Sb match experiments.
- Larger symmetry-breaking motion can be induced by optimizing pump photon energy.
- Imaginary part of electron self-energy estimates force decay rates.

## Abstract

We present a first-principles method for the calculation of the temperature-dependent relaxation of symmetry-breaking atomic driving forces in photoexcited systems. We calculate the phonon-assisted decay of the photoexcited force on the low-symmetry $E_g$ mode following absorption of an ultrafast pulse in the prototypical group-V semimetals, Bi, Sb and As. The force decay lifetimes for Bi and Sb are of the order of $10$ fs and in good agreement with recent experiments, demonstrating that electron-phonon scattering is the dominant mechanism relaxing the symmetry-breaking forces. Calculations for a range of absorbed photon energies suggest that larger amplitude, symmetry-breaking atomic motion may be induced by choosing a pump photon energy which maximises the product of the initial $E_g$ force and its lifetime. We also find that the high-symmetry $A_{1g}$ force undergoes a partial decay to a non-zero constant on similar timescales, which has not yet been measured in experiments. We observe that the imaginary part of the electron self-energy, averaged over the photoexcited carrier distribution, provides a reasonable estimate for the decay rate of symmetry-breaking forces.