New Perspectives from Spectroscopy on the Bismuth Oxide Superconductors

Despite being known for decades, the perovskite bismuth oxide superconductors (max T_c > 30 K) were never experimentally probed to nearly the extent of more famous high-T_c materials, such as cuprates and iron-based superconductors. This is a pity, because their phenomenology and underlying physics connect with a wide array of contemporary interests: not only unconventional/high-T_c superconductivity, but also metal-insulator and insulator-superconductor transitions, (bi)polarons, CDWs/charge-order, disordered systems, and so on. Recently we have studied Ba_1-xK_xBiO₃ films using angle-resolved photoemission and resonant inelastic x-ray scattering. These experiments establish the unusual "bond disproportionated" nature of the parent compound and track its evolution into the superconducting doping region. The under- to optimally-doped region of the phase diagram is particularly fascinating. There spectra show a highly dispersive conduction band forming a well-defined Fermi surface, despite a total absence of sharp peaks that would typically accompany weakly interacting quasiparticles. We observe, moreover, two types of pseudogap-like spectral behaviors: The first extends over a broad energy scale and persists above room temperature; the other is set in a narrow region around E_F and opens in a well-defined temperature range above T_c. I will discuss how these psuedogaps appear to be linked - namely that they represent the precipitation of ordered bipolaronic insulating regions out of a disordered polaronic liquid.