Unexpected differences in the surface electronic structure of NiO and CoO observed by STM and explained by first-principles theory

Atomically resolved elevated-temperature scanning tunneling microscope (STM ) images of (001) cobalt and nickel monoxide surfaces obtained under simila r conditions show an order of magnitude difference in the atomic corrugatio n heights. Surface-electronic structure calculations taking into account th e Hubbard U term show that on the CoO (001) surface the lowest unoccupied s tate has d(xy) character as compared with the predominantly d(3z)(-r)(2)(2) lowest empty state on NiO (001). The difference in the symmetry of unoccup ied cl 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 ox ide. [S0163-1829(99)04111-9].