  The recent observation of high-temperature superconductivity in one unit cell
thick FeSe on SrTiO3 (1UC FeSe/STO)1 initiated a new pathway to realizing even
higher transition temperature (Tc) superconductors and established a promising
different paradigm to understand the mechanism of high temperature
superconductivity2-10. However, independent confirmation on the high Tc value,
the role of capping layer in reducing Tc, and the underlying mechanism of the
novel superconductivity remain open questions11. Here we report a time-resolved
study of the excited state ultrafast dynamics of 1UC FeSe/STO protected by 2UC
FeTe capping layer, and identify the superconducting Tc to be 68 (-5/+2) K. We
found a coherent acoustic phonon mode in the capping layer, which provides an
additional decay channel to the gluing bosons and explains the reduced Tc
value. Our investigation supports the phonon pairing scenario.
