Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

There is still no general consensus on how one can describe the out-of-equilibrium phenomena in matter induced by an ultrashort light pulse. We investigate the pulse-induced dynamics in a layered Dirac semimetal ${\mathrm{SrMnBi}}_{2}$ by pump-and-probe photoemission spectroscopy. At $\ensuremath{\lesssim}1$ ps, the electronic recovery slowed upon increasing the pump power. Such a bottleneck-type slowing is expected in a two-temperature model (TTM) scheme, although opposite trends have been observed to date in graphite and in cuprates. Subsequently, an unconventional power-law cooling took place at $\ensuremath{\sim}100$ ps, indicating that spatial heat diffusion is still ill defined at $\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$. We identify that the successive dynamics before the emergence of heat diffusion is a canonical realization of a TTM scheme. Criteria for the applicability of the scheme is also provided.