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 SrMnBi$_{\mathrm{2}}$ by pump-and-probe photoemission spectroscopy [1]. At \textless 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 \textasciitilde 100 ps, indicating that spatial heat diffusion is still ill defined at \textasciitilde 100 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. [1] Phys. Rev. B 93, 100302(R) (2016).