Ultrafast Dynamics Evidence of High Temperature Superconductivity in Single Unit Cell FeSe onSrTiO3

We report the time-resolved excited state ultrafast dynamics of single unit cell (1 UC) thick FeSe films on ${\mathrm{SrTiO}}_{3}$ (STO), with FeTe capping layers. By measuring the photoexcited quasiparticles' density and lifetime, we unambiguously identify a superconducting (SC) phase transition, with a transition temperature ${T}_{c}$ of 68 $(\ensuremath{-}5/+2)\text{ }\text{ }\mathrm{K}$ and a SC gap of $\mathrm{\ensuremath{\Delta}}(0)=20.2\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }\mathrm{meV}$. The obtained electron-phonon coupling strength $\ensuremath{\lambda}$ is as large as 0.48, demonstrating the likely crucial role of electron-phonon coupling in the high temperature superconductivity of the 1 UC FeSe on STO systems. We further find a 0.05 THz coherent acoustic phonon branch in the capping layer, which provides an additional decay channel to the gluing bosons.