Ultrafast Dynamics Evidence of High Temperature Superconductivity in Single Unit Cell FeSe on SrTiO₃

We report the time-resolved excited state ultrafast dynamics of single unit cell (1 UC) thick FeSe films on SrTiO₃ (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 Tc of 68(-5/+2) K and a SC gap of Δ(0) = 20.2±1.5 meV. The obtained electron-phonon coupling strength λ 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.If time permits, I will also address our recent progress in other superconductors.