An atomic-scale quantum conductance switch is demonstrated that allows us to open and close an electrical circuit by the controlled and reproducible reconfiguration of silver atoms within an atomic-scale junction. The only movable parts of the switch are the contacting atoms. The switch is entirely controlled by an external electrochemical voltage applied to an independent third gate electrode. Controlled switching was performed between a quantized, electrically conducting "on state" exhibiting a conductance of G(0)=2e(2)/h ( approximately 1/12.9 kOmega) or preselectable multiples of this value and an insulating "off state."

The controlled fabrication of actively switchable atomic-scale devices, in particular transistors, has remained elusive to date. Here, we explain the operation of an atomic-scale three-terminal device by a novel switching mechanism of bistable, self-stabilizing reconstruction of the electrode contacts at the atomic level: While the device is manufactured by electrochemical deposition, it operates entirely on the basis of mechanical effects of the solid-liquid interface. We analyze mechanically and thermally stable metallic junctions with a predefined quantized conductance of 1-5 G0 in experiment and atomistic simulation. Atomistic modeling of structural and conductance properties elucidates bistable electrode reconstruction as the underlying mechanism of the device. Independent room temperature operation of two transistors at low voltage demonstrates intriguing perspectives for quantum electronics and logics on the atomic scale.

The controlled fabrication of well-ordered atomic-scale metallic contacts is of great interest: it is expected that the experimentally observed high percentage of point contacts with a conductance at noninteger multiples of the conductance quantum G0=2e2∕h in simple metals is correlated to defects resulting from the fabrication process. Here we demonstrate a combined electrochemical deposition and annealing method that allows the controlled fabrication of point contacts with preselectable integer quantum conductance. The resulting conductance measurements on silver point contacts are compared with tight-binding-like conductance calculations of modeled idealized junction geometries between two silver crystals with a predefined number of contact atoms.

Atomic-sized lead ͑Pb͒ contacts are deposited and dissolved in an electrochemical environment, and their transport properties are measured. Due to the electrochemical fabrication process, deformation-induced mechanical strain is largely avoided, and we obtain conductance histograms with sharply resolved, individual peaks. Charge transport calculations based on density-functional theory for various ideal Pb contact geometries are in good agreement with the experimental results. Depending on the atomic configuration, single-atom-wide contacts of one and the same metal yield very different conductance values.

Atomic-scale transistors [1][2][3] based on metallic quantum point contacts were demonstrated recently. They allow controlled binary switching of an electrical current between a conducting ''on-state'' and a non-conducting ''off-state'' by means of an independent gate electrode. The devices that operate reproducibly at room temperature open fascinating perspectives towards quantum electronics and logics on the atomic scale. Even an integrated circuit consisting of atomic-scale transistors [3] as well as a nanoelectromechanical atomic switch [4] were shown. Here, we demonstrate a multilevel atomic quantum transistor that allows gate-controlled switching between different quantized conducting states. Multilevel logic and storage devices on the atomic scale are of great interest as they will allow more efficient data storage and processing with a smaller number of logical gates. Our experiments are combined with detailed computer simulations that provide a detailed understanding of the multilevel switching process. The results provide a basis for the future development of ultra-small devices for multilevel logics on the atomic scale.