  Based on first-principles calculations, we predict a half-metallic surface
state in layered bulk insulator NaCoO2, with tunable surface hole
concentration. The half-metallic surface has a single Fermi surface with a
helical spin texture, similar to the surface state of topological insulators,
but with the key difference of time-reversal symmetry breaking in the present
case. We propose the realization of topological superconductivity and Majorana
fermions when the half-metallic surface states are in proximity contact with a
conventional superconductor.
