Inverse photoemission was used to probe the unoccupied electronic levels within 25 eV of $E_F$ for the high-temperature superconductor Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$. Our results show that the density of states is low from $E_F$ to ≈3.7 eV above $E_F$. Large emission features appearing at 6.6 and 8.6 eV are associated with Ba $5d$- and Y $4d$-derived bands, respectively, while a structure at 13.4 eV above $E_F$ is due to localized Ba $4f$ states. Energy-dependent studies reveal a dispersive band between ≈4 and ≈6 eV above $E_F$. An oxygen-induced feature was observed at 1.8 eV, but there is no evidence that high-energy electron-stimulated desorption removes these chemisorption states.

• Received 22 June 1987

DOI:https://doi.org/10.1103/PhysRevB.36.3899