IntrinsicΛ1-symmetry surface state on Co(0001)

An intrinsic surface state on Co(0001) has been found using angle-resolved photoemission with synchrotron radiation which has an energy of $E_i=-0.3\mathrm{}$ eV at $\overline{\Gamma }$ (zone center) relative to the Fermi energy $E_F$. Its symmetry has been determined to be of ${\Lambda }_1$ character using measurements with $s$- and $p$-polarized radiation. Its momentum dispersion ($E$ vs ${\overrightarrow{\mathrm{k}}}_{\parallel }$) and $h\nu$-dependent photoionization cross section have also been determined. At the zone center $\overline{\Gamma }$, the surface state lies in an absolute energy gap just above the uppermost ${\Lambda }_{3\uparrow }$ $d$-like ($E_i=-0.35\mathrm{}$ eV) bulk state, well above the uppermost $\mathrm{sp}$-like ${\Lambda }_1$ state ($E_i=-1.15\mathrm{}$ eV), and below occupied ${\Lambda }_{3\downarrow }$ $d$-like bulk states near $\Gamma$ (energies between $E_i=-0.05\mathrm{} \mathrm{eV}\mathrm{and} 0$). As previously reported for Ni, this surface state is extremely sensitive to absorbates and significantly affects field emission, spin-polarized photoemission, chemisorption, etc. for Co(0001).