First-principles study of the electronic and magnetic structure of c(2×2) sulfur chemisorbed above Fe(001)

We have performed the first, first-principles study of the adsorption of sulfur above a magnetic, Fe surface. Our results, derived from the all-electron, film full-potential linearized augmented-plane-wave method applied to a seven-layer Fe film with and without c(2×2) layers of S positioned next to the two surface Fe [Fe(S)] layers, include determinations of the equilibrium sulfur height ($H_{\mathrm{eq}}$) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 Å) of $H_{\mathrm{eq}}$ with the earlier result [1.09(5) Å] derived by Legg, Jona, Jepsen, and Marcus from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below $E_F$ which play an important role both in reducing the magnetic moment of the Fe(S) atom (by ∼20%) and in the rather small calculated increase (0.85 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved, spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields and in the angle-resolved, spin-polarized surface state spectra, and relate our findings to questions associated with the sulfur-induced poisoning of an iron catalyst.