Unoccupied electronic states in CaB6 studied by density functional theory and EELS measurements

The electronic structure of CaB6 has recently attracted great interest because of the discovery that this compound is unexpectedly ferromagnetic and exhibits a surprisingly large thermoelectric effect. We have concentrated on studying the unoccupied electronic states by investigating the boron 1s core-level spectroscopy. Comparison of our data from electron energy-loss spectroscopy with that from x-ray absorption spectroscopy using the total electron yield mode reveals evidence for sample inhomogeneity. The ab initio band structural method has been successfully applied to simulate the overall features found in the boron 1s absorption spectrum. Correlation with the results of a ground state energy-band calculation allows the interpretation of the spectral features in terms of the density of states of the unoccupied conduction band, in particular the identification of a feature associated with hybridization of Ca d orbitals with B p orbitals. This interpretation is confirmed by comparison with similar calculations for related boride systems. However, simulation spectral data cannot be reconciled with high-resolution experimental data at the absorption threshold. The possibility of either the failure of the theoretical method employed to account for many-body effects and/or the need for improved experimental measurements is discussed.