Suppression of Bi disproportionation in minimally electron doped BaBiO3 epitaxial thin films

BaBiO₃ is a semiconductor due to three dimension charge density wave (CDW), and it can become a superconductor, with a maximum T_c of 30K via holes doping. We have studied the three-dimensional charge-density-wave in high quality BaBiO₃ and electron doped BaBiO₃ single crystal thin films by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Raman shift and ellipsometry technique.The electron doping is achieved by very small amount of oxygen vacancies purposely induced by vacuum annealing. Our results have found that the electron doping can remove bismuth disproportionation (2Bi⁴⁺→Bi³⁺ + Bi⁵⁺) but has less effect to the insulating gap. Different from the hole-doping side, the amount of electron needed to suppress the bismuth disproportionation is extremely small. Our study provides new insights to the intricate correlation between CDW and the hybridization between oxygen 2p and Bi 6s orbitals.