Stanford University

We employ femtosecond time-resolved photoelectron spectroscopy to directly uncover the time-domain signatures of electron-phonon interactions in a cuprate superconductor. We discover an abrupt decrease of the electron lifetime above a characteristic energy of 55 ± 8 meV. Remarkably, the lifetime at this energy is independent of excitation density. By fundamental considerations of electron-phonon interactions, these behaviors are explained by coupling with a 55 meV mode which scatters nodal electrons to the nodal region in momentum space. Our experiment complements equilibrium photoemission by providing sensitivity to sub-dominant mode coupling, and suggests a pathway towards the full description of bosonic mode coupling in cuprates.