Structural Transition in Atomic Chains Driven by Transient Doping

A reversible structural transition is observed on Si(553)-Au by scanning tunneling microscopy, triggered by electrons injected from the tip into the surface. The periodicity of atomic chains near the step edges changes from the $1\times 3$ ground state to a $1\times 2$ excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first-principles density-functional calculations it is shown that the $1\times 2$ phase is created by temporary doping of the atom chains. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing lifetimes in the millisecond range.

Figure 1

STM topography [$(6.0\times 3.3){\mathrm{nm}}^2$] at different temperatures and current set points. For low currents a $1\times 3$ structure is observed (left column). The periodicity changes to $1\times 2$ at high currents (right column). With decreasing temperature the transition between the two structures occurs at progressively lower currents, falling below our experimental limit at 7 K (c). Near the transition current a mixture of the $1\times 2$ and $1\times 3$ structures is visible, resulting in an apparent $1\times 6$ periodicity [inset to (b), $(3.2\times 6)\mathrm{n}{\mathrm{m}}^2$].

Figure 2

(a) Time dependence of the tunneling current (black) at a fixed tip position above a bright protrusion of the $1\times 3$ structure at 63 K. Two distinct levels are evident, corresponding to the $1\times 3$ and $1\times 2$ phases of Si(553)-Au. The red curve represents the result of a threshold analysis. (b) Histograms of the transient tunneling current for different average currents $⟨I⟩$. The lifetimes $⟨{\tau }_0⟩/⟨{\tau }_1⟩$ are determined from cumulative histograms of the residence times for the ground and excited state as illustrated for $⟨I⟩=43\mathrm{pA}$ in (c) and plotted versus the average current $⟨I⟩$ in (d). At $⟨I⟩\approx 35\mathrm{pA}$ the lifetime of the ground state ($⟨{\tau }_0⟩$, green) falls below that of the excited state ($⟨{\tau }_1⟩$, black). While $⟨{\tau }_0⟩$ decreases with $⟨I⟩$, $⟨{\tau }_1⟩$ remains constant, reflecting the spontaneous decay of the excited $1\times 2$ state into the $1\times 3$ ground state.

Figure 3

Relative total energy of the $1\times 3$ (red) and $1\times 2$ (blue) phase as a function of electron doping. Above a doping level of $\sim 2\times {10}^{13}{\mathrm{cm}}^{-2}$ the $1\times 2$ phase becomes energetically preferred. The arrows indicate the changes of the doping level that cause switching between the two phases.