Pinning of Adsorbed Cobalt Atoms by Defects Embedded in the Copper (111) Surface

Using a low-temperature scanning tunneling microscope (STM), we observe that Co adatoms are unusually strongly bound to a particular type of pinning centers on the Cu(111) surface. Using density-functional-theory calculations, the pinning centers are identified as Ag substitutional atoms embedded in the topmost atomic layer of the surface. These impurities are hardly detectable in the STM images as they have low topographic height and produce no standing-wave patterns. They do not affect the exchange coupling of the Co adsorbate with the substrate electrons; thus, the Kondo resonances measured on pinned and free Co adatoms show no detectable differences. Whereas free Co adatoms undergo significant surface diffusion already above 8 K, Ag-stabilized Co adatoms remain pinned up to 12.7 K.

Figure 1

(a),(b) Lateral manipulation of a Co adatom reveals an embedded defect (marked by *) at the pinning center. Above 8 K, the position of a depinned Co adatom is unstable and the atom appears blurred. The distance between the two discernible adsorbate positions in (b) is $0.26\pm 0.04\mathrm{nm}$, which corresponds to the distance between two fcc binding sites. (c)–(f) The adatoms can be reversibly manipulated between three embedded defects. ($I=0.55\mathrm{nA}$, $U=30\mathrm{mV}$, $T=9\mathrm{K}$).

Figure 2

(a) Topographic LT-STM image ($35\times 26{\mathrm{nm}}^2$) of typical defects on Cu(111) surface ($T=7\mathrm{K}$, $I=1\mathrm{nA}$, $U=100\mathrm{mV}$). The embedded defects (white spots) show no standing-wave patterns. (b) Line profile across the embedded defect recorded at gap voltage of $+100\mathrm{mV}$ (1 nA tunneling current) showing an apparent height of only $\sim 8\mathrm{pm}$.

Figure 3

(a) White dots are the experimentally determined binding sites of Co adatoms relative to the defect position. (b) Top view of the model Cu(111) surface with an embedded defect. The adatom always occupies one of the six numbered fcc and hcp sites on the surface.

Figure 4

Surface diffusion of a “free” Co adatom on Cu(111) (blue filled circles) and the stability of a pinned Co adatom (red squares) at different temperatures. The step size is the distance between the positions of the particle in two consecutive images. The time interval between two images (data points) is 120 sec. Note the discrete values of the displacements. The inset shows the recorded trajectory of the Co adatom during surface diffusion. The random motion of the Co adatom is not influenced by the STM tip, as the scanning direction is always from left to right and from top to bottom, whereas the displacements show no systematic drift. $U=10\mathrm{mV}$, $I=0.88\mathrm{nA}$.