D- andsp-like surface states on fcc Co(001) with distinct sensitivity to surface roughness

Abstract

The sensitivity of electronic surface states to modifications of the surface topography has been investigated in regard to the orbital character of the respective states. The surface of face-centered-cubic Co(001) shows two characteristic electronic states: an occupied $d$-like minority surface state around the center of the surface Brillouin zone $\overline{\ensuremath{\Gamma}}$ and an unoccupied $sp$-like surface state with an exchange splitting of 0.56 eV around the zone boundary $\overline{\text{X}}$. These states were studied experimentally on a 15-monolayer Co film on Cu(001) by spin-resolved direct and inverse photoemission. In addition, the Co(001) surface was theoretically described by calculations of the Bloch spectral function and of the direct and inverse photoemission intensities within the one-step-model approach. Different surface topographies from atomically smooth to very rough, as proved by scanning tunneling microscopy, were achieved by choosing varied growth and annealing temperatures for the preparation of the Co films. Both types of surface states are highly sensitive to the film roughness, yet in a distinctly different way. The less localized $sp$-like wave function causes a much higher sensitivity of the unoccupied surface state to lateral inhomogeneities than the more localized $d$-type wave function of the occupied surface state.