# Electron-ion coupling in semiconductors beyond Fermi's golden rule

## TL;DR

This paper presents a theoretical study of electron-ion coupling in semiconductors, revealing limitations of Fermi's golden rule on femtosecond timescales and proposing a dynamical coupling model that aligns better with experimental data.

## Contribution

It introduces a dynamical electron-ion coupling model that surpasses Fermi's golden rule in accuracy for ultrafast semiconductor excitation.

## Key findings

- Fermi's golden rule can break down on femtosecond timescales.
- The proposed dynamical coupling model agrees well with experimental data.
- Transient optical coefficient changes reflect band gap shrinkage.

## Abstract

In the present work, a theoretical study of electron-phonon (electron-ion) coupling rates in semiconductors driven out of equilibrium is performed. Transient change of optical coefficients reflects the band gap shrinkage in covalently bonded materials, and thus, the heating of atomic lattice. Utilizing this dependence, we test various models of electron-ion coupling. The simulation technique is based on tight-binding molecular dynamics. Our simulations with the dedicated hybrid approach (XTANT) indicate that the widely used Fermi's golden rule can break down describing material excitation on femtosecond time scales. In contrast, dynamical coupling proposed in this work yields a reasonably good agreement of simulation results with available experimental data.