  The role of the crystal lattice for the electronic properties of cuprates and
other high-temperature superconductors remains controversial despite decades of
theoretical and experimental efforts. While the paradigm of strong electronic
correlations suggests a purely electronic mechanism behind the
insulator-to-metal transition, recently the mutual enhancement of the
electron-electron and the electron-phonon interaction and its relevance to the
formation of the ordered phases have also been emphasized. Here, we combine
polarization-resolved ultrafast optical spectroscopy and state-of-the-art
dynamical mean-field theory to show the importance of the crystal lattice in
the breakdown of the correlated insulating state in an archetypal undoped
cuprate. We identify signatures of electron-phonon coupling to specific
fully-symmetric optical modes during the build-up of a three-dimensional
metallic state that follows charge photodoping. Calculations for coherently
displaced crystal structures along the relevant phonon coordinates indicate
that the insulating state is remarkably unstable toward metallization despite
the seemingly large charge-transfer energy scale. This hitherto unobserved
insulator-to-metal transition mediated by fully-symmetric lattice modes can
find extensive application in a plethora of correlated solids.
