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Persistence of small polarons into the superconducting doping range of Ba1−xKxBiO3

Muntaser Naamneh1, Eric C. O’Quinn2, Eugenio Paris3, Daniel McNally3, Yi Tseng3, Wojciech R. Pudełko3, Dariusz J. Gawryluk4, Jacob Shamblin2, Benjamin Cohen-Stead5,6 et al.

Ming Shi3,7, Milan Radovic3, Maik K. Lang2, Thorsten Schmitt3, Steven Johnston5,6,*, and Nicholas C. Plumb3,†

  • *Contact author: sjohn145@utk.edu
  • †Contact author: nicholas.plumb@psi.ch
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Phys. Rev. Research 7, 043082 – Published 21 October, 2025

DOI: https://doi.org/10.1103/s3p1-cy1s

Abstract

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba1−xKxBiO3 (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 modeling to study the local lattice distortions, electronic structure, and electron-phonon (e−ph) coupling in BKBO as a function of doping. Data for the parent compound (x=0) indicate 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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Phys. Rev. Research 7, 043082– Published 21 October, 2025
  • Received 6 April 2025
  • Accepted 16 September 2025
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