Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

Significance Elucidating the role of different degrees of freedom in a phase transition is crucial in the comprehension of complex materials. A phase transformation that attracts significant interest is the insulator-to-metal transition of Mott insulators, in which the electrons are thought to play the dominant role. Here, we use ultrafast laser spectroscopy and theoretical calculations to unveil that the correlated insulator La2CuO4, precursor to high-temperature superconductivity, is unstable toward metallization when its crystal structure is displaced along the coordinates of specific vibrational modes. This, in turn, supports the involvement of the lattice in this phase transition. Our results pave the way toward the geometrical design of metallic states in Mott insulators, with technological potential for ultrafast switching devices at room temperature. 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 buildup of a three-dimensional (3D) 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.