Highly spin-polarized, nearly free-electron states in front of Co(101¯0)

We report on the first observation of highly spin-polarized nearly free-electron image-potential states. At the surface Brillouin zone boundary Y{} of Co(101{}0) spin-polarized inverse photoemission reveals large exchange splittings of 125 meV and 96 meV for two n=1 image states, separated by 0.6 eV due to the surface corrugation potential. The exchange splittings are nearly as large as the intrinsic linewidths so that electrons trapped in these two-dimensional states are highly spin polarized. By their large exchange splitting these image states serve as a test case for a possible spin dependence of the surface-barrier potential.