Sulfur-induced occupation of Delta -bar Fe minority surface states in c(2 x 2) S/Fe(001).

Abstract

The poisoning of iron's catalytic value in the synthesis of hydrocarbons through contamination by small amounts of sulfur is a well-known, though poorly understood, phenomenon. Using results from our recent first-principles, full-potential linearized augmented-plane-wave study of $c(2\ifmmode\times\else\texttimes\fi{}2)$ sulfur chemisorbed on magnetic Fe(001), we review a number of important adsorption-induced modifications of electronic and magnetic structure, which probably have bearing on this poisoning phenomenon. These include a significant reduction in the density of states (DOS) at ${E}_{F}$, and a narrowing and sharpening of various surface resonance states above ${E}_{F}$. The most prominent of these features involves a striking level repulsion of minority surface states (SS's), all along the $\overline{\ensuremath{\Delta}}$ line of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface Brillouin zone, which is largely responsible for the decline in the DOS at ${E}_{F}$. At $\overline{X}$, this level repulsion leads to a large (\ensuremath{\sim} 1 eV) upward shift of one of the two degenerate $d(xz,yz)$ SS's, located 0.2 eV above ${E}_{F}$, accompanied by a downward shift of the remaining SS, which becomes occupied and remains highly localized all along $\overline{\ensuremath{\Delta}}$. We examine the origins of this effect and suggest various spin-polarized photoemission and inverse-photoemission experiments, which could provide independent confirmation of our prediction of this dramatic effect.