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

Abstract

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-ofthe-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. RESULTS Crystal Structure and Equilibrium Optical PropertiesAs a model material system we study La 2 CuO 4 (LCO), one of the simplest insulating cuprates exhibiting metallicity upon hole doping. In this solid, the two-dimensional network of corner-sharing CuO 4 units is accompanied by two apical O atoms below and above each CuO 4 plaquette. As a result, the main building blocks of LCO are CuO 6 octahedra ( Fig. 1 A) that are elongated along the c-axis due to the Jahn-Teller distortion. The unit cell of LCO is tetragonal above and orthorhombic below 560 K. A simplified scheme of the electronic density of states is shown in Fig. 1 B (left panel). An energy gap (∆ CT ∼2 eV) opens between the filled O-2p band and the unoccupied Cu-3d upper Hubbard band (UHB), thus being of the CT type. In contrast, the occupied Cu-3d lower Hubbard band (LHB) lies at lower energy. First, we present the optical properties of LCO in equilibrium. Figure 1 C shows the absorptive part of the optical conductivity (σ 1 ), measured via ellipsometry. The in-plane response (σ 1a , solid violet curve) is dominated by the optical CT gap at 2.20 eV [17, 18].This transition is a non-local resonant exciton that extends at least over two CuO 4 units.The strong coupling to the lattice degrees of freedom causes its broadened shape [18][19][20]. As such, this optical feature can be modeled as involving the formation of an electron-polaron and a hole-polaron, coupled to each other by a short-range interaction [18,21]. At higher energy (2.50−3.50 eV), the in-plane spectrum results from charge excitations that couple the O-2p states to both the Cu-3d states in the UHB and the La-5d /4f states. In contrast, the out-of-plane optical conductivity (σ ...

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