Figure 2. (A) Manipulated atom image of Co over Cu(111) surface. Tunnel current, 50 nA; sample bias, −5 mV; T = 4.3 K. The labels A, B, and C denote fcc, hcp, and top sites, respectively. (B) Tunnel current recorded during manipulated atom image going through the hcp and fcc sites as indicated by the horizontal line in (A). The arrow shows the increased noise in the tunnel current corresponding to the position of the hcp site.

Figure 3. (A) Top view of the Cu(111) surface with the Co adatom shown in its natural fcc binding site. (B) Schematic potential well for the Co atom in fcc and hcp sites: blue curve, native potential well, no tip- Co interaction; green curve, tip-induced potential well; red curve, native potential with added tip-induced potential. The potential at the hcp site increases in depth because of the increase in tip-Co interaction as the tip-Co distance decreases. The tip-induced potential well over the hcp site causes the Co atom to switch between the fcc and hcp sites, producing discrete changes in the tunnel current. (C to E) Schematic of manipulated atom tip height trace. Initially, with the tip over the fcc site, the force on the Co atom is vertical and the tip images the Co atom. As the tip moves down the side of the Co atom, a lateral force develops (D). When the tip reaches the hcp site, the lateral force is large enough to induce the Co atom to hop to the hcp site (E). The green curve is the measured tip height trace from the manipulated atom image in Fig. 2A.

Figure 4. (A) Co manipulated atom image on Cu(111). Tunnel current, 100 nA; sample bias, 11.0 mV; junction resistance, 110 kilohms; T = 2.3 K. (B and D) Tunnel current versus time measurements recorded at the positions indicated by the corresponding red spots in (A) near the hcp site. Sample bias, 3.3 mV. (C and E) The corresponding histograms of the current distributions. (F) Tunnel current versus \time measurement showing two-state random telegraph noise near the hcp site for Co on Cu(111) measurement at junction resistance of 120 kilohms. Sample bias, 8.4 mV; T = 2.3 K. (G) Corresponding histogram of the current distribution.

Figure 5. Transfer rate versus sample bias at constant tip height, obtained by measuring the distribution of residence time in the hcp and fcc states from two-state random telegraph noise in the tunnel current. Junction resistance 0 150 kilohms; T = 2.3 K. R_hcp, red circles; R_fcc, black squares. Solid red line shows a power-law fit to the initial threshold region; blue horizontal line shows the average transfer rate for the low-bias region for the hcp transfer rate.

Figure 6. Current distribution from two-state telegraph noise obtained near the hcp site during a Co manipulated atom measurement as a function of tip-sample distance. Zero tip-sample distance corresponds to the initial set point at junction resistance of 90 kilohms; Z = 0.35 Å corresponds to 180 kilohms. Sample bias, −5 mV; T = 2.3 K.