Electron-Phonon-Driven Three-Dimensional Metallicity in an Insulating Cuprate

Proceedings Of The National Academy Of Sciences Of The United States Of America 117 (12), 6409 - 6416 (2020)

Electron-Phonon-Driven Three-Dimensional Metallicity in an Insulating Cuprate

Edoardo Baldini, Michael A. Sentef, Swagata Acharya, Thomas Brumme, Evgeniia Sheveleva, Fryderyk Lyzwa, Ekaterina Pomjakushina, Christian Bernhard, Mark van Schilfgaarde, Fabrizio Carbone, Angel Rubio, Cedric Weber

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.

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We are grateful to Anthony J. Leggett, Antoine Georges, Jose Lorenzana, and Ferdi Aryasetiawan for insightful discussions. E.B. and F.C. acknowledge support from the NCCR MUST. T.B. and A.R. were supported by the European Research Council (ERC-2015- AdG694097), European Union H2020 program under GA no.676580 (NOMAD), and Grupos Consolidados (IT578-13). M.A.S. acknowledges support by the DFG through the Emmy Noether programme (SE 2558/2-1). This work was supported by EPSRC (EP/R02992X/1, EP/N02396X/1, EP/M011631/1), and the Simons Many-Electron Collaboration. E.S. and C.B. acknowledge funding from the SNSF by Grant No.200020-172611. For computational resources, S.A., M.v.S., and C.W. were supported by the ARCHER UK National Supercomputing Service and the UK Materials and Molecular Modelling Hub for computational resources (EPSRC Grant No. EP/ P020194/1); T.B. was supported by the MPCDF Garching.

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