Single-Atom Transistors – Quantum Electronics on the Atomic Scale

Controlling the electronic conductivity at the quantum level is of great interest both from the point of view of fundamental physics and for potential applications. Atomic-scale quantum transistors, which allow to open and close an electronic circuit by the controlled and reproducible repositioning of one single atom, were developed in our group within the CFN. They open intriguing perspectives for the emerging fields of quantum electronics and logics on the atomic scale.

Within this subproject, we developed a method which allows the controlled and reproducible switching of an atomic-scale quantum point contact between a quantized, electrically conducting “on-state” exhibiting a conductance of G0 = 2e2/h (≈ 1/12.9 kΩ) or preselectable integer multiples of this value and an electrically insulating “off-state” by means of an independent electrochemical gate electrode. The switch operates at room temperature and can be applied as a basic functional unit for quantum electronics, representing an atomic relay. In contrast to previous approaches, no movement of macroscopic electrodes or leads is necessary to induce the switching process: The only movable parts of the switch are the contacting atoms.

Very recently, we have demonstrated a multilevel atomic quantum transistor that allows gate-controlled switching between different quantized conducting states [1]. Multilevel logics and storage devices on the atomic scale are of great interest as they allow more efficient data storage and processing with a smaller number of logical gates. Our experiments are combined with computer simulations in close cooperation with theory groups within the CFN that provide a detailed understanding of the switching process. The results provide a basis for the future development of ultra-small devices for multilevel logics on the atomic scale. Besides atomic-sized transistors made of silver, we also successfully fabricated atomic quantum transistors with other metals including lead and ferromagnetic metals.

1. Multilevel Logics and Storage Devices on the Atomic Scale

• Multilevel atomic quantum transistors were demonstrated to allow gate controlled switching between different quantized conducting states.
• Detailed computer simulations provide an understanding of the multilevel switching process. 
• Electronic transport properties

• First successful switching experiments with atomic-scale Pb contacts were performed in a newly-developed inert-atmosphere setup [2].
• Charge transport calculations based on density functional theory (DFT) for various ideal Pb contact geometries are in good agreement with the experimental results.
• Depending on the local atomic configuration of the contacting atom, single-atom contacts of the same metal yield very different conductance. Utilizing the dependence of conductance on the atomic configuration, single-atom transistors of lead can switch between 0 and different conductance values such as 0.7, 1.0, 1.4, 2.0, and 2.8 G0.