Momentum-Transfer-Resolved Electron Energy Loss Spectroscopy ofBaBiO3: Anisotropic Dispersion of Threshold Excitation and Optically Forbidden Transition

Momentum-transfer-resolved electron energy loss spectroscopy of the valence band transitions in ${\mathrm{BaBiO}}_3$ has revealed for the first time that dispersion of the excitation at the optical gap $\left(\sim 2\mathrm{eV}\right)$ and an optically forbidden transition at 4.5 eV are all anisotropic along [100] and [110]. The anisotropic dispersion of the threshold excitation cannot be described by a simple charge density wave picture but can be explained by a small exciton model proposed in this paper. The optically forbidden transition is found to agree well with a proposed molecular orbital model, where the transition is assigned as the excitation from the $O2p\sigma$ nonbonding states to the empty Bi $6s$ state.