Tracking emergence of superconductivity in perovskite BaPb_1-xBi_xO₃

BaPb_1-xBi_xO₃ (BPBO) is a superconducting perovskite with high transition temperature (T_c) reaching 13 K for x ~ 0.25. Even though its phase diagram exhibits no magnetism and the demonstration of an isotope effect clearly indicate phonon-mediated mechanism of Cooper pairs formation, the T_c value is much higher than one would expect from non-transition metal perovskite with such low density of states at the Fermi level. In a purely ionic picture, BaBiO₃ (BBO, x = 1) would be a half-filled metal, and BaPbO₃ (BPO, x = 0) would be an insulator. In reality, these electronic phases are exactly reversed, which highlights the role of negative charge transfer from the strongly hybridized oxygen states.

By preparing thin films with an in situ pulsed laser deposition system, we obtain the first ARPES data from BPBO for the parent metallic (x = 0) and optimally doped (x = 0.25) samples. We compare the results with DFT calculations. A near-Lifshitz transition predicted by some models, which might have defined the normal-superconductor transition with respect to x, appears to be avoided. In BPBO, we are able to resolve k-dependent quasiparticle lifetimes, as signatures of scattering along the zone diagonal. This is taken as evidence that (π, π, π)-directed phonon interactions, which are responsible for the insulating behavior in BBO, persist into the superconducting phase of BPBO.