Stabilization of the Si(553) surface by Au adsorption results in two different atomically defined chain types, one of Au atoms and one of Si. At low temperature these chains develop two-and threefold periodicity, respectively, previously attributed to Peierls instabilities. Here we report evidence from scanning tunneling microscopy that rules out this interpretation. The ×3 superstructure of the Si chains vanishes for low tunneling bias, i.e., close the Fermi level. In addition, the Au chains remain metallic despite their period doubling. Both observations are inconsistent with a Peierls mechanism. On the contrary, our results are in excellent, detailed agreement with the Si(553)-Au ground state predicted by density-functional theory, where the ×2 periodicity of the Au chain is an inherent structural feature and every third Si atom is spin-polarized.Atoms can form chain-like architectures by selfassembly on various semiconductor surfaces. Such chains have been widely studied because they may offer physical realizations of various one-dimensional (1D) electronic ground states -Peierls instabilities (i.e., charge density waves, CDW) [1] or Tomonaga-Luttinger liquids [2] -in which Coulomb interactions are dominant. An equally interesting scenario arises if the electron's spin degree of freedom is important or even dominant. For example, a proposal was made [3] to use atomic chains as a spin shift register, where spin encodes the information. Recent research has focused on spin alignment in 2D atom lattices on semiconductor substrates [4,5]. The fate of spin ordering in 1D chains on surfaces, however, has remained less explored experimentally.The variability offered by chains formed on different high index Si surfaces allows us to investigate this interplay of charge, spin, and lattice in a family of related structures. Specific representatives include the chain structures stabilized by Au on Si(557)-Au and Si(553)-Au. These systems carry Au-induced metallic electron bands as seen in photoemission [6]. In Si(553)-Au -the focus of the present work -the situation is particularly complex. The structure as derived from x-ray diffraction [7] and density-functional theory (DFT) [8,9] exhibits a dimerized double-strand Au chain, in contrast to the single Au row in Si(557)-Au. As an additional key characteristic of both variants, there is a second type of chain located at the terrace edge, formed by Si atoms which are arranged in a graphene-like honeycomb chain [8,9].In two seminal papers, changes in the periodicity of both types of chains upon cooling were observed by scanning tunneling microscopy (STM) [10,11], leading to two-and threefold patterns for the Au and the Si chain, respectively. Moreover, from photoemission data [11] a temperature-dependent gap opening was inferred. These observations for Si(553)-Au were interpreted as Peierls instabilities, driven by nesting in the metallic bands, and resulting in energy gaps and periodic lattice distortions at low temperature.However, several pieces of evidence do not suppor...

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High-index surfaces of silicon with adsorbed gold can reconstruct to form highly ordered linear step arrays. These steps take the form of a narrow strip of graphitic silicon. In some cases-specifically, for Si(553)-Au and Si(557)-Au-a large fraction of the silicon atoms at the exposed edge of this strip are known to be spin-polarized and charge-ordered along the edge. The periodicity of this charge ordering is always commensurate with the structural periodicity along the step edge and hence leads to highly ordered arrays of local magnetic moments that can be regarded as "spin chains." Here, we demonstrate theoretically as well as experimentally that the closely related Si(775)-Au surface hasdespite its very similar overall structure-zero spin polarization at its step edge. Using a combination of density-functional theory and scanning tunneling microscopy, we propose an electron-counting model that accounts for these differences. The model also predicts that unintentional defects and intentional dopants can create local spin moments at Si(hhk)-Au step edges. We analyze in detail one of these predictions and verify it experimentally. This finding opens the door to using techniques of surface chemistry and atom manipulation to create and control silicon spin chains.

We propose a quantitative and reversible method for tuning the charge localization of Au-stabilized stepped Si surfaces by site-specific hydrogenation. This is demonstrated for Si(553)-Au as a model system by combining density functional theory simulations and reflectance anisotropy spectroscopy experiments. We find that controlled H passivation is a two-step process: step-edge adsorption drives excess charge into the conducting metal chain "reservoir" and renders it insulating, while surplus H recovers metallic behavior. Our approach illustrates a route towards microscopic manipulation of the local surface charge distribution and establishes a reversible switch of site-specific chemical reactivity and magnetic properties on vicinal surfaces.

We report on the electronic structure of the elemental topological semimetal α-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional p-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.