Figure 4: Characteristics of the Pb₃ switch.

(a) Tip–sample distance dependences of the current (green) and frequency-shift (red) traces during the voltage pulse of +1.8 V. z₀ indicates the feedback tip–sample distance at I_t=20 pA and V_s=+0.5 V. (b) Current dependences of the forward-switch rate at V_s=+1.8 V (solid circle) and V_s=−1.0 V (open circle). Solid lines are linear fits of the data. Vertical error bars represent the s.e.m. for the measurements repeated up to 20 times per data point, and horizontal ones, s.e.m. for the measured current in the L state. (c) Current dependences of the tip–sample electrostatic force at V_s=+1.8 V (solid circle) and V_s=−1.0 V (open circle). Dashed lines are guides to the eye. Vertical error bars are s.e.m. derived from the systematic uncertainty in the measurements of the cantilever oscillation amplitude and cantilever stiffness. Horizontal error bars are identical with those in b. (d) Yields of the forward (red circle) and backward (blue circle) switches as a function of pulse bias voltage. Red and blue dashed lines are guides to the eye. Red (blue) cross in the STM images shows an active site for forward (backward) switch. Black cross indicates a centre Si adatom site without Pb occupation. Error bars were estimated by an error-propagation analysis from the errors in the R_f(b) and I_t. (e) Local density of states acquired at red (ρ_Pbsub), blue (ρ_Sisub) and black (ρ_Si) crossed sites in d. Fermi level (E_F) is referenced to energy zero. Typical peaks or onsets of the spectra are labelled a–f. (f) STM topographies of a Pb₃ (I_t=40 pA and V_s=+0.5 V) imaged at the intervals of 10-s voltage pulse applications (V_s=−1.4 V). Red cross in each image indicates a fixed tip position during the voltage pulse. Solid line in each image indicates a chiral axis.