Self-Alignment of Co Adatoms on In Atomic Wires by Quasi-One-Dimensional Electron-Gas-Meditated Interactions

Low-density Co atoms are found to self-align on the $\mathrm{Si}(111)\mathrm{\text{−}}(4\times 1)$-In surface in the direction of In atomic wires at incommensurate adsorption sites. Indirect interaction between a pair of Co adatoms is investigated through a site distribution function of adatoms determined with scanning tunneling microscopy. In the direction of self-alignment, the potential of the mean force between two Co adatoms is long-range and oscillatory with multiple frequencies, which correlate strongly to the electronic scattering vectors of the surface-state bands at the Fermi level. We thus attribute the Co-Co interaction to that mediated by a quasi-one-dimensional electron gas confined within the In atomic wires.

Figure 1

(a) Bird’s view of an STM image of the In atomic wire array covered with 0.02 ML of Co atoms. $V_s=-0.3\mathrm{V}$ and $I=0.1\mathrm{nA}$. (b) A typical STM image ($60\mathrm{nm}\times 19\mathrm{nm}$) at the Co coverage of 0.04 ML. $V_s=-0.5\mathrm{V}$ and $I=0.1\mathrm{nA}$. Co adatoms (round protrusions) are self-aligned along the In atomic wires in array. (c) Fourier transform of the Co occupation site function $\xi (r)$ defined in the In wire direction.

Figure 2

Enlarged STM image showing the round protrusions of Co adatoms and $\times 2$ modulation in the Co-free sections of In atomic wires. $V_s=-0.3\mathrm{V}$ and $I=0.1\mathrm{nA}$. A model of the $\mathrm{Si}(111)\mathrm{\text{−}}(4\times 1)$-In surface is superimposed to determine the registry of bright particles relative to the In wires.

Figure 3

(a) Pair correlation function $g(d)$ and (b) potential of mean force between Co adatoms $W(d)$; (c) Fourier transform of $W(d)$; (d) schematic diagram showing the Fermi surface of $\mathrm{Si}(111)\mathrm{\text{−}}(4\times 1)$-In together with the scattering vectors that have correspondent frequencies in (c).

Figure 4

Fourier transforms of mean force potentials shown in the respective inset. $W_{\mathrm{adj}}(d)$ in (a) is defined between two Co adatoms on adjacent In atomic wires, while $W_{2\mathrm{D}}(d)$ in (b) between arbitrary two Co adatoms in the STM images [15].