Due to the rapid technological advances in micro and nanofabrication techniques, the mature field of microelectromechanical systems (MEMS) has now reached the nanoscale, and consequently nanoelectromechanical systems (NEMS) have become an active field of research.
In 1998 an interesting example of a NEMS was proposed by researchers at Chalmers University in Sweden. The system consists of a movable, nanosize metallic grain spatially confined in a harmonic potential between two leads (source and drain) – see figure. In the strong Coulomb blockade regime only a single excess electron at a time is allowed to occupy the grain. When a sufficiently high bias is applied between the leads, a single electron may tunnel onto the grain, and due to the electrostatic field between the leads, the charged grain is driven towards the drain, where the electron tunnels off. Due to the harmonic potential the uncharged grain is forced back towards the source, and the process is repeated. In this manner, the grain is driven into a self-sustained steady motion, where single electrons are “shuttled” from the source to the drain. Consequently, the device is known as a charge shuttle.
In the Theoretical Nanotechnology group we have in recent years been asking the question, what happens if the vibrations of the grain are quantized? In which aspects would such a device differ from its classical counterpart? In particular, we have studied in detail the statistics of the charge being transported through the system, the so-called full counting statistics (FCS). We have developed a systematic analytic theory for the moments (or cumulants) of the charge transport through NEMS, and with a numerical evaluation of the second cumulant (the zero-frequency current noise) we have shown that shuttle transport in the quantum regime is characterized by a very low noise-to-current ratio just as in the classical case.
Furthermore, based on a numerical evalution of the first three cumulants (average current, zero-frequency current noise, and skewness, respectively), we have shown that the device in a certain parameter regime effectively behaves as a bistable system, switching slowly between two current channels. Besides studying the quantum shuttle as described above, we have studied a related system (a vibrating quantum dot array), where the physics compared to the quantum shuttle is significantly enriched by the inclusion of additional coherently coupled states. Also in this case, the FCS reveals clear signatures of certain transport mechanisms as well as bistabilities.
To learn more, please contact Christian Flindt or Antti-Pekka Jauho.