Atomically resolved elevated-temperature scanning tunneling microscope (STM) images of (001) cobalt and nickel monoxide surfaces obtained under similar conditions show an order of magnitude difference in the atomic corrugation heights. Surface-electronic structure calculations taking into account the Hubbard U term show that on the CoO (001) surface the lowest unoccupied state has $d_{\mathrm{xy}}$ character as compared with the predominantly $d_{{3z}^2-r^2}$ lowest empty state on NiO (001). The difference in the symmetry of unoccupied d orbitals on CoO (001) and NiO (001) surfaces is responsible for the much lower level of atomic contrast observed in the STM images of cobalt oxide.

• Received 23 November 1998

DOI:https://doi.org/10.1103/PhysRevB.59.7342