Tracking the Ultrafast Non-Equilibrium Energy Flow between Electronic and Lattice Degrees of Freedom in Crystalline Nickel

TL;DR

This study combines ab initio calculations with ultrafast diffuse electron scattering to elucidate the detailed non-equilibrium energy transfer dynamics between electrons and phonons in laser-excited nickel, revealing mode-specific occupation patterns and energy backflow.

Contribution

It provides a comprehensive, wavevector-resolved understanding of electron-phonon energy transfer in nickel, integrating experimental and theoretical insights into non-equilibrium phonon populations.

Findings

Zone-boundary phonons become occupied first.

Energy backflow from lattice to electrons occurs when phonon energy exceeds electron energy.

System thermalizes on the picosecond timescale with non-thermal phonon occupations.

Abstract

Femtosecond laser excitation of solid-state systems creates non-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering we gain a detailed understanding of the complex non-equilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wavevector resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond timescale. We determine the evolving non-equilibrium phonon occupations which we find to deviate markedly from thermal occupations.