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We report the observation of H 2 adsorption on the H͞Si͑100͒ surface using scanning tunneling microscopy. Predosing the surface by atomic H leads to the efficient adsorption of H 2 in an interdimer configuration of adjacent singly occupied dimers. This strong and local promotion of dissociative adsorption is explained by the noninteracting character of the relevant dangling bonds. By way of contrast, H 2 sticking is strongly inhibited on the clean Si͑100͒-͑2 3 1͒ surface where the dangling bonds are rendered less reactive by their mutual interaction.

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We report bcc-like crystal structures in 2-4 ML Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The non-pseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.Both academic interest in novel nanomagnetic phenomena as well as their technological importance for magneto-electronics and high density magnetic storage devices make the study of ultrathin ferromagnetic films particularly worthwhile. These extremely thin films, typically less than 10 monolayers (ML) thick, exhibit significantly different magnetic properties in contrast to the bulk material, e.g., different magnetization directions, enhanced magnetic moments, and lower Curie temperatures. Fe films epitaxially grown on Cu(100) are distinguished by a particular complex behavior since they are variable both with respect to magnetic ordering (ferroor antiferromagnetic) and crystal structure (fcc or bcc). Although the epitaxial system Fe/Cu(100) is under intense scrutiny for more than a decade and its magnetic properties have been mapped very precisely, no conclusive overall picture of the relation between structure and magnetic states has emerged yet. Regarding the model for films deposited at room temperature presently discussed in the literature, there is clear evidence for an antiferromagnetic, pseudomorphic fcc phase between 5 and 10 ML film thickness. The character and origin of the ferromagnetic phase between 2 and 4 ML, however, remains unclear. Low energy electron diffraction (LEED) [1], surface extended X-ray absorption fine-structure (SEXAFS) [2], and medium energy ion scattering studies (MEIS) [3] indicate a distinct distortion of the fcc lattice in 2-4 ML films. This reconstruction, which is considered to comprise the entire film thickness [1], is accompanied by a substantial increase of the film volume (interlayer distance) by about 5% [1,4]. In the past, these results led to the notion of a second, ferromagnetic fcc-like phase with an expanded film volume, i.e., a face centered tetragonal (fct) phase. While ab-initio calculations of bulk Fe do support the possibility of a ferromagnetic fcc phase with expanded volume under substantial tensile strain [5], the respective calculations of ultrathin films on Cu(100) did not provide an unambiguous confirmation of the ferromagnetic fct model [6,7]. This is an important question since the hypothetical existence of a ferromagnetic fcc-like phase is relevant also for the solid state physics of Fe. The two-γ-state model introduced by Kaufman, Clougherty, and Weiss [8], assuming two fcc states of bulk Fe either ferro-or antiferromagnetic, is still under discussion.In this Letter, we resolve this issue by char...

Scanning tunneling spectroscopy permits real-space observation of one-dimensional electronic states on a Fe(100) surface alloyed with Si. These states are localized along chains of Fe atoms in domain boundaries of the Fe(100) c͑2 3 2͒Si surface alloy, as confirmed by first-principles spinpolarized calculations. The calculated charge densities illustrate the d-like orbital character of the one-dimensional state and show its relationship to a two-dimensional state existing on the pure Fe(100) surface. [S0031-9007(96)00335-3]