Persistence of small polarons into the superconducting phase of Ba${}_{1-x}$K${}_x$BiO${}_3$

Abstract

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba${}_{1-x}$K${}_x$BiO${}_3$ (BKBO); however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine resonant inelastic x-ray and neutron total scattering techniques with advanced modelling to study the local lattice distortions, electronic structure, and electron-phonon coupling ($e$-ph) in BKBO as a function of doping. Data for the parent compound ($x=0$) indicates that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable ($e$-ph) coupling persist into the superconducting regime ($x=0.4$). Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that BKBO's metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability

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Cited by in corpus (1)

• The influence of phonon symmetry and electronic structure on the electron-phonon coupling momentum dependence in cuprates