Visualizing How Electrons Delocalize and Couple to Bosons in a Doped Correlated Insulator*

Most ABO₃ transition metal perovskite oxides are insulating due to the strong Coulomb repulsion between electrons at the transition metal (B) and at the oxygen ions. As aliovalent A ions are introduced, the material usually transitions to a metallic state. However, the critical dopant concentration is often large and accompanied by structural changes, obscuring the study of band formation in these materials. In CaMnO₃, minute doping induces a sharp transition to a metallic state. This compound thus appears as an ideal platform to explore the interplay between doping, correlations and disorder in perovskite oxides. Here, we use angle-resolved photoemission spectroscopy to determine the electronic structure of doped CaMnO₃ thin films. We show that doping promotes the formation of several Fermi pockets with dispersive and non-dispersive bands, coexisting with polaronic signatures and quasiparticle mass renormalization signaling coupling to plasmonic excitations. These data shed light on the complex transport response of Ce-doped CaMnO₃ and suggest strategies to engineer two-dimensional quantum matter from correlated oxides.