First principles theory of artificial metal chains on NiAl(110) surface

Using first-principle calculations we have studied the stable phases of metal-atom chains on the (110) surface of NiAl. Our investigation is mainly focused on the technologically relevant case of Au chains, but the results will be analyzed in the broader framework of a family of metallic systems. We demonstrate that the nature of the adatom (Au, Mn, Ni, Cu, Al, H, C, Na, K, and Ca) is responsible for different levels of interaction with the substrate and gives rise to a variety of electronic behaviors. With some transition metals (such as Au, Mn, Ni, and Cu) the NiAl surface acts as a simple structural template for the formation of the artificial one-dimensional system and does not affect the electronic properties of the chains. With other atomic species (H, C, Na, K, and Ca) we observe substantially different couplings and stronger interactions. We demonstrate that the different electronic properties of the various adatoms are responsible for different couplings with the substrate and compare our findings with the existing experimental results. Finally, in the case of Au chains we have investigated the role of adatom-adatom interactions in the formation of such one-dimensional structures.

Figure 1

(Color online) Schematic top (left) and side (right) views of NiAl(110) surface. Gray, dark blue, and large yellow circles describe Al, Ni, and Au atoms, respectively. The Au adsorption site is indicated (Ad), similar configurations have been adopted for other $\mathrm{Ad}∕\mathrm{Ni}\mathrm{Al}\left(110\right)$ systems. Top view model: The $1\times 2$ cell has been periodically repeated to show the gold chains along the [001] direction. The $1\times 1$ unit cell is indicated by the shaded area. Small diamonds represent the second plane positions. $a_0$ is the bulk lattice parameter.

Figure 2

(Color online) Clean NiAl(110) surface. (a) Total (thin black line) and projected density of states; thick blue and dashed red lines indicate the Ni and Al contribution, respectively. (b) Calculated STM image for clean NiAl(110) surface, obtained at constant current and bias voltage $-400\mathrm{meV}$. The area corresponding to a $3\times 2$ cell is shown. A schematic atomic structure (as defined in Fig. 1) is superimposed.

Figure 3

(Color online) Calculated STM image for Au/NiAl(110) (a) $-\times 2$ and (b) $-\times 3$ surfaces. $1\times 2$ cell has been repeated along [001] direction for clarity. A schematic atomic structure (as defined in Fig. 1) is superimposed.

Figure 4

(Color online) $\mathrm{Au}∕\mathrm{Ni}\mathrm{Al}\left(110\right)$ surface. (a) Total DOS (black area) and projected DOS on Au atoms (yellow area) from $\mathrm{Au}∕\mathrm{Ni}\mathrm{Al}\left(110\right)$ interface; total DOS of clean surface (gray vertical lines). (b) Contour plot of the total charge density in a plane perpendicular to the surfaces containing gold chain and the corresponding Ni row along the [001] direction. Atoms are label as in Fig. 1. The $1\times 2$ cell has been repeated along [001] direction for clarity. (c) Surface band structure along $\overline{\Gamma }\text{−}\overline{Y}$ direction: Projection of even NiAl (gray area) and Au (black lines) states from $\mathrm{Au}∕\mathrm{Ni}\mathrm{Al}\left(110\right)$ interface, and band structure of isolated Au-chain (triangles). (d) Charge density of a selected state corresponding to the arrow in the band structure.

Figure 5

(Color online) Top panel: Spin polarized STM images and density of states for $\mathrm{Mn}∕\mathrm{Ni}\mathrm{Al}$. Total (gray area), and Mn-projected DOS for spin up (squared blue area) and spin down (dashed red area) components. Bottom panel: STM images for selected (H, C, Na) $\mathrm{Ad}∕\mathrm{Ni}\mathrm{Al}\left(110\right)$ structures. $1\times 2$ cell has been repeated along [001] direction for clarity.

Figure 6

(Color online) Finite chain structures: (a) Top view of different length (1-2-7) gold structures on $\mathrm{Ni}\mathrm{Al}\left(110\right)\text{−}(8\times 2)$ surface. Atoms are labeled as in Fig. 1, green (circled) atoms indicate those employed in the dimer (single adatom) simulations. Calculated STM images for (b) single Au adatom, (c) Au dimer, (d) seven-atom-long Au chain.

Figure 7

(Color online) $\sigma$-like Wannier function for (a) isolated Au chain and (b) LDOS contour plot obatined from calculated $dI∕dV$ STM image for Au dimer on NiAl surface.