Reconfigurable Quantum-Dot Molecules Created by Atom Manipulation

Yi Pan, Jianshu Yang, Steven C. Erwin, Kiyoshi Kanisawa, and Stefan Fölsch

Phys. Rev. Lett. 115, 076803 – Published 14 August 2015

Abstract

Quantum-dot molecules were constructed on a semiconductor surface using atom manipulation by scanning tunneling microscopy (STM) at 5 K. The molecules consist of several coupled quantum dots, each of which comprises a chain of charged adatoms that electrostatically confines intrinsic surface-state electrons. The coupling takes place across tunnel barriers created reversibly using the STM tip. These barriers have an invariant, reproducible atomic structure and can be positioned—and repeatedly repositioned—to create a series of reconfigurable quantum-dot molecules with atomic precision.

Received 29 May 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.076803

Authors & Affiliations

Yi Pan¹, Jianshu Yang¹, Steven C. Erwin², Kiyoshi Kanisawa³, and Stefan Fölsch¹

¹Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
²Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375, USA
³NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

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Issue

Vol. 115, Iss. 7 — 14 August 2015

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Images

Figure 1
(a) Model of the reconstructed $InAs (111) A − (2 \times 2)$ surface with indium atoms (green) in the surface layer and arsenic atoms (orange) in the second layer. The $2 \times 2$ In-vacancy reconstruction has a hexagonal unit cell (gray) with lattice vector $a^{'} = a_{0} \sqrt{2} = 8.57 Å$ ( $a_{0} = 6.06 Å$ : cubic InAs lattice constant). (b) STM topography image (5 nA, 75 mV) of six In adatoms ( ${In}_{ad}$ ) arranged as three dimers. Within each dimer, the ${In}_{ad}$ are located at neighboring In-vacancy sites, as indicated by black circles in (a). The faint increase in apparent height within the dimer structure is due to the local electrostatic potential induced by the positively charged adatoms. (c) STM image after creating a defect in the middle dimer by switching the surface In atom in the center of the structure [the circled green atom in (a)] to its metastable popped-up position. The complex consisting of the popped-up surface atom and the two adjacent ${In}_{ad}$ appears as a uniform oblong protrusion.
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Figure 2
(a) Top panel: STM topography image (0.1 nA, $- 0.3 V$ ) of a 14-atom ${In}_{ad}$ chain with a popped-up In surface atom in the center. Center and lower panels: Corresponding spatial DOS maps revealing the formation of a bonding ( $σ$ , lower panel) and an antibonding state ( $σ^{*}$ , center panel). (b) STM image of two six-atom chains separated by a gap of two empty vacancy sites, showing that the bonding and antibonding states are qualitatively very similar to the states in (a). (c) Differential conductance ( $d I / d V$ ) spectra (red and blue curves) recorded at the positions marked by the red and blue crosses in (a) and (b), establishing the resonant character of the $σ$ and $σ^{*}$ states. The $σ - σ^{*}$ splitting $Δ$ is essentially the same for the structures in (a) and (b). For comparison, the green spectrum shows the ground-state resonance ( $n = 1$ ) of a single ${In}_{6}$ chain.
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Figure 3
(a) STM topography images (0.1 nA, $- 0.3 V$ ) of a 20-atom ${In}_{ad}$ chain with one popped-up In surface atom at different positions within the chain. The defect divides the ${In}_{20}$ chain into two dots denoted $A$ and $B$ . (b) Bias and position-dependent DOS maps recorded along the $x$ axis defined by the dashed line in (a). $N_{A}$ and $N_{B}$ denote the number of ${In}_{ad}$ atoms within dots $A$ and $B$ , respectively. Starting from the symmetric case with $(N_{A}, N_{B}) = (9, 9)$ , the $σ - σ^{*}$ splitting gradually increases as the double dot is changed stepwise to $(N_{A}, N_{B}) = (12, 6)$ . (c) Experimental splitting (red points) for different ( $N_{A}, N_{B}$ ) plotted versus the energy difference $E_{A} - E_{B}$ , where $E_{A}$ and $E_{B}$ are the ground-state energies of single chains with $N_{A}$ and $N_{B}$ atoms, respectively. A simple LCAO model for the avoided crossing in a two-level system is shown in blue for comparison.
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Figure 4
(a) STM topography image (0.1 nA, $- 0.3 V$ ) of a 22-atom ${In}_{ad}$ chain with two popped-up In surface atoms dividing the chain into three six-atom dots $A$ , $B$ , and $C$ . (b) $d I / d V$ spectra taken at the positions marked by crosses in (a), revealing a triplet state with a splitting $Δ = 48 meV$ . (c) Spatial DOS maps recorded at the respective bias voltages where $d I / d V$ resonances are detected in (b). The observed DOS distributions are consistent with the LCAO molecular orbitals of a linear triatomic molecule, namely, $| σ ⟩ = (+ 1, + \sqrt{2}, + 1)$ for the bonding orbital (lower panel) and $| σ_{1}^{*} ⟩ = (+ 1, 0, + 1)$ and $| σ_{2}^{*} ⟩ = (+ 1, - \sqrt{2}, + 1)$ for the antibonding orbitals (center and upper panel, respectively).
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