Structural and stoichiometric modifications in ultrathin epitaxial BaBiO3 films

Abstract

${\mathrm{BaBiO}}_{3}$ (BBO) is well known as the parent material for the high-${T}_{c}$ superconducting compounds ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ and ${\mathrm{BaPb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{O}}_{3}$. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the ${\mathrm{BiO}}_{6}$ octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on ${\mathrm{SrTiO}}_{3}$ (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on ${\mathrm{SrTiO}}_{3}$(001).