Atom-By-Atom Quantum State Control in Adatom Chains on a Semiconductor

The vertical manipulation of native adatoms on a III–V semiconductor surface was achieved in a scanning tunneling microscope at 5 K. Reversible repositioning of individual In atoms on InAs(111)A allows us to construct one-atom-wide In chains. Tunneling spectroscopy reveals that these chains host quantum states deriving from an adatom-induced electronic state and substantial substrate-mediated coupling. Our results show that the combined approach of atom manipulation and local spectroscopy is capable to explore atomic-scale quantum structures on semiconductor platform.

Figure 1

(a) Constant-current STM image ($54\AA \times 54\AA$, 1 nA, 0.5 V) with two native In adatoms on InAs(111)A. (b) Enhanced gray scale image ($54\AA \times 54\AA$, 2 nA, 0.1 V) indicating the adatom position relative to the ($2\times 2$) In-vacancy reconstruction. (c) Stick-and-ball model of the first two atomic layers; the rhombic surface unit cell is indicated in (b) and (c). (d) $dI/dV/(I/V)$ spectra yielding the LDOS measured with the tip over the surface (blue) and over the adatom (red). The red spectrum reveals an adatom-derived state at 0.68 eV above $E_F$; resonant $dI/dV$ mapping [$27\AA \times 27\AA$ inset, set point 0.1 nA, 0.68 V, surface In atom positions marked by white dots] shows that the adatom state density is centered at the atomic position. All $dI/dV$ measurements at constant tip height, lock-in modulation and frequency: $20{\mathrm{mV}}_{\mathrm{pp}}$, 670 Hz.

Figure 2

(a),(b) STM images ($105\AA \times 43\AA$, 0.1 nA, 0.5 V) documenting single-atom surface-to-tip transfer (a) before and (b) after tip approach toward the marked adatom at 1.25 V sample bias. (c) Current versus tip displacement $\Delta z$ during approach (red) and retraction (blue) indicating the pick-up event ($P$) at $\Delta z=-3.0\AA$ (initial set point here and in the following: 0.1 nA, 0.5 V). (d),(e) STM images showing tip-to-surface back transfer (d) before and (e) after moving the tip at $-1.0\mathrm{V}$ toward the marked vacancy site; corresponding current-vs-$\Delta z$ curves (f) indicate a jump to contact ($J$) during approach. (g) Current traces measured with the tip over an adatom at fixed junction settings of $\Delta z=-3\AA$ and positive biases as indicated showing different delay times $\tau$ prior to transfer. (h) Probability distribution of the normalized delay time $\tau /⟨\tau ⟩$ obtained at various settings with $\Delta z$ $\in [-2.73\AA ,-3.05\AA ]$ and biases $\in [1.03\mathrm{V},1.35\mathrm{V}]$; the exponential decay indicates a statistical surface-to-tip transfer process.

Figure 3

(a) STM images ($33\AA \times 84\AA$, 0.1 nA, 0.1 V) showing the successive assembly of a ${\mathrm{In}}_6$ chain by vertical atom manipulation. (b) $dI/dV/(I/V)$ spectra for ${\mathrm{In}}_N$ chains ($N=1$ to $N=5$) with the tip centered over the atoms indicated in (a). (c) Constant-height $dI/dV$ map (set point 0.2 nA, 0.25 V) for ${\mathrm{In}}_5$ with the bias tuned to the resonance energy. (d) Corresponding bias dependency of the along-chain $dI/dV$ modulation indicating envelope functions with a single lobe between 0.2 and 0.3 V (ground state) and with two lobes between 0.4 and 0.5 V (first excited state). (e) Resonant along-chain $dI/dV$ profiles for $N=3$ to 5 measured at identical constant tip height; for clarity, the profiles are offset along $y$. The on-atom-centered $dI/dV$ maxima agree well with the corresponding squared LCAO coefficients $c_i^2$ (indicated by circles and plotted along the right-hand side $y$ axis).