MOMENTUM-TRANSFER-RESOLVED ELECTRON-ENERGY-LOSS SPECTROSCOPY OF BABIO3 - ANISOTROPIC DISPERSION OF THRESHOLD EXCITATION AND OPTICALLY FORBIDDEN TRANSITION

Momentum-transfer-resolved electron energy loss spectroscopy of the va lence band transitions in BaBiO3 has revealed for the first time that dispersion of the excitation at the optical gap (similar to 2 eV) and an optically forbidden transition at 4.5 eV are all anisotropic along [100] and [110]. The anisotropic dispersion of the threshold excitatio n cannot be described by a simple charge density wave picture but can be explained by a small exciton model proposed in this paper. The opti cally forbidden transition is found to agree well with a proposed mole cular orbital model, where the transition is assigned as the excitatio n from the O 2p sigma nonbonding states to the empty Bi 6s state.