Topological nature and the multiple Dirac cones hidden in Bismuth high-Tc superconductors

Recent theoretical studies employing density-functional theory have predicted BaBiO₃ (when doped with electrons) and YBiO₃ to become a topological insulator (TI) with a large topological gap (∼0.7 eV). This, together with the natural stability against surface oxidation, makes the Bismuth-Oxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the hole-doped Bismuth Oxides, i.e. Ba_1-xK_xBiO₃ and BaPb_1-xBi_xO₃, which are "high-Tc" bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy E_F a topological gap with a Dirac-type of topological surface state. Our electronic structure calculations predict the K-doped family to emerge as a TI, with a topological gap above E_F. Thus, these compounds can become superconductors with hole-doping and potential TIs with additional electron doping. Furthermore, we predict the Bismuth-Oxide family to contain an additional Dirac cone below E_F for further hole doping, which manifests these systems to be candidates for both electron- and hole-doped topological insulators.