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 $(-5/+2)\mathrm{K}$ and a SC gap of $\mathrm{\Delta }(0)=20.2\pm 1.5\mathrm{meV}$. The obtained electron-phonon coupling strength $\lambda$ 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.

• Received 9 June 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.107001