All-electron local-density-functional theory of alkali-metal adsorption on transition-metal surfaces: Cs on Mo(001)

The electronic structure of a clean Mo(001) surface and the bonding between a dense [c(2×2)] Cs overlayer with the Mo(001) substrate are studied using all-electron local-density-functional theory and the full-potential linearized augmented-plane-wave (FLAPW) method for thin films. We find that Cs(s)-Mo(d) interactions lead to a shift of the high-lying surface state at Γ¯ from 0.1 to 0.9 eV below the Fermi level and to a Cs(s)-Mo(d) band with an upward dispersion away from Γ¯. Furthermore, Cs(d)-Mo(d) interactions reduce the high Mo-surface-projected density of states at $E_F$ by shifting some of the Mo(d) bands [notably those midway between Γ¯ and M of the Mo(001) surface Brillouin zone] to larger binding energies. In addition, Cs is found to induce unoccupied adsorbate-surface states of Cs p and d character, located 0.8 eV above the Fermi level. Similar to the case of tungsten metal, the lowering of the work function of Mo(001) due to Cs adsorption is explained by the formation of multiple surface dipoles involving a polarization of the Cs 6s–derived states towards the transition metal and a counterpolarization of the Cs 5p states.