Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals

TL;DR

This study uses femtosecond pump-probe spectroscopy to investigate ultrafast electron and phonon dynamics in transition metals Zn and Cd, revealing temperature-dependent behaviors and decay mechanisms of coherent optical phonons and electrons.

Contribution

It provides a detailed analysis of electron-phonon thermalization and phonon damping in transition metals, introducing a model linking amplitude changes to quasiparticle density.

Findings

Electron-phonon thermalization time increases with temperature.

Damping of coherent phonons increases with temperature.

Amplitude of phonon and electronic responses correlates with quasiparticle density.

Abstract

The femtosecond optical pump-probe technique was used to study dynamics of photoexcited electrons and coherent optical phonons in transition metals Zn and Cd as a function of temperature and excitation level. The optical response in time domain is well fitted by linear combination of a damped harmonic oscillation because of excitation of coherent $E_{2g}$ phonon and a subpicosecond transient response due to electron-phonon thermalization. The electron-phonon thermalization time monotonically increases with temperature, consistent with the thermomodulation scenario, where at high temperatures the system can be well explained by the two-temperature model, while below $\approx$ 50 K the nonthermal electron model needs to be applied. As the lattice temperature increases, the damping of the coherent $E_{2g}$ phonon increases, while the amplitudes of both fast electronic response and the coherent $E_{2g}$ phonon decrease. The temperature dependence of the damping of the $E_{2g}$ phonon indicates that population decay of the coherent optical phonon due to anharmonic phonon-phonon coupling dominates the decay process. We present a model that accounts for the observed temperature dependence of the amplitude assuming the photoinduced absorption mechanism, where the signal amplitude is proportional to the photoinduced change in the quasiparticle density. The result that the amplitude of the $E_{2g}$ phonon follows the temperature dependence of the amplitude of the fast electronic transient indicates that under the resonant condition both electronic and phononic responses are proportional to the change in the dielectric function.