Ultrafast relaxation dynamics and nonequilibrium processes in graphite

We present a detailed investigation of ultrafast carrier dynamics in single-crystal graphite using time- and angle-resolved photoemission spectroscopy (Tr-ARPES). By systematically analyzing the electronic temperature ${T}_{e}$, chemical potential $\ensuremath{\mu}$, and nonthermal carrier dynamics across various pump fluence and energy regimes, we provide insights into the multistep relaxation processes that govern the system's evolution from nonequilibrium to quasi-equilibrium states. Our results reveal that at low fluences ($\mathit{F}<\phantom{\rule{0.16em}{0ex}}0.50\phantom{\rule{4pt}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$), the electronic system behaves predominantly as a quasithermalized ensemble, aligning well with the Fermi-Dirac distribution. However, at higher fluences ($\mathit{F}\ensuremath{\geqslant}\phantom{\rule{4pt}{0ex}}0.50\phantom{\rule{4pt}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$), nonthermal carriers emerge, driving significant deviations from thermal equilibrium. These carriers exhibit ultrafast lifetimes of approximately $284\ifmmode\pm\else\textpm\fi{}38$ fs, primarily dictated by electron-electron scattering and strong coupling to optical phonons. Through fluence-dependence analysis, we demonstrate the distinct relaxation dynamics of high-energy carriers ($>0.2$ eV) dominated by ultrafast scattering, compared with low-energy carriers ($<0.2$ eV) where the phonon bottleneck effect prolongs decay times to up to 32 ps at low fluences. In addition, we demonstrate a linkage between dynamic high-temperature quasi-equilibrium states from pump excitation and static ARPES measurements at 1000 K, highlighting the correspondence between dynamic quasi-equilibrium and thermal equilibrium states. Energy region dependent investigations reveal the critical role of free-carrier absorption in the low-energy regime and nonlinear interactions at higher energies, emphasizing the intricate balance between carrier-carrier and carrier-phonon interactions in shaping the relaxation pathways. This study provides a unified framework for understanding carrier dynamics in graphite, bridging static and dynamic perspectives.