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| Main Authors: | , |
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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2401.16621 |
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Table of Contents:
- Using the Boltzmann transport equation (BTE), we study the evolution of nonequilibrium carrier distributions in simple ($sp$) metals, assumed to have been instantaneously excited by an ultrafast laser pulse with photon energy $h ν$. The mathematical structure of the BTE scattering integrals reveals that $h ν$ is a natural energy scale for describing the dynamics. Normalizing all energy quantities by $h ν$ leads to a set of three unitless parameters -- $β/ δ$, $γ$, and $α$ -- that control the relaxation dynamics: $β/ δ$ is the normalized ratio of electron-phonon to electron-electron scattering strengths, $γ$ is the normalized phonon (lattice) temperature, and $α$ is the normalized absorbed energy density. Using this theory, we systematically investigate relaxation times for the high-energy part of the distribution ($τ_H$), energy transfer to the phonon subsystem ($τ_E$), and intracarrier thermalization ($τ_{th}$). In the linear region of response (valid when $α$ is sufficiently small), we offer heuristic descriptions of each of these relaxation times as functions of $β/ δ$ and $γ$. Our results as a function of excitation level $α$ show that many ultrafast experimental investigations lie in a transition region between low excitation (where the relaxation times are independent of $α$) and high excitation (where the two-temperature model of carrier dynamics is valid). Approximate boundaries that separate these three regions are described by simple expressions involving the normalized parameters of our model.