Electron-phonon coupling and polaronic effects in the high-Tc superconductor Ba1-xKxBiO3

Ba1-xKxBiO3 (BKBO) is a phonon-mediated superconductor (max Tc ~ 32 K) exhibiting many hallmarks of “unconventional” superconductivity. It is therefore an ideal system for exploring how electron-phonon interactions may give rise to unexpectedly high Tc’s and other anomalous behaviors. In order to probe collective excitations in BKBO, we have performed resonant inelastic x-ray scattering (RIXS) at the O-K edge on thin films of BKBO in the doping range from x = 0 (insulator) to x = 0.4 (superconductor near optimal doping). The data have been analyzed by a cluster exact diagonalization approach, adapting a model successfully employed on 3d transition metal oxides to the s-p orbital system of BKBO. For all dopings, the extracted coupling constant exceeds a critical strength, beyond which bipolarons tend to become stabilized. The results help to quantify the polaronic nature of BKBO and imply that these compounds are susceptible to intrinsic disorder due to polaronic distortions across the entire investigated doping range. Implications for superconductivity will be discussed.