Shuttle Transport of Charge in Normal and Superconducting Nanostructures

Doktorsavhandling, 2002

In nanoscale systems, accumulation of charge is associated with large charging energies. If the forces arising due to the Coulomb interaction are comparable to the rigidity of the structures mechanical deformations can couple to the electronic degrees of freedom. Using analytical and numerical methods, the interplay between charge transfer and mechanical motion has been studied in both normal and superconducting double junction geometries. In the first part of this thesis, the dynamics of a Coulomb blockade double junction where the central island may execute one dimensional center-of-mass motion in a harmonic potential are considered. It is shown that, for sufficiently large bias voltages, a dynamical instability occurs. The instability leads to a new shuttle mechanism for charge transfer where the island executes periodic motion thereby transferring charge mechanically between the leads. This is reflected in the zero temperature I - V characteristics as steps of quantized current in units of the grain vibration frequency and the electron charge. It is also shown that the onset of the mechanical instability can be associated with hysteresis in the I - V curves. Further, by considering a similar system, a mechanically flexible three terminal device in the high temperature limit, an example of electromechanical coupling leading to chaotic behavior is demonstrated. Finally, coherent charge transport in a superconducting shuttle junction is considered. This junction, a movable single-Cooper-pair box, consists of an externally driven grain oscillating between two remote superconducting leads. Close to each lead gates ensure degeneracy between nearby charge states. The degeneracy allows for a coherent superposition of zero and one extra Cooper pair on the grain and, hence, mechanically assisted coherent charge transport between the leads. Two different limiting cases are treated here; strongly and weakly coupled leads. In the first case it is shown that the shuttle junction can support a nonzero super current between two phase biased leads. In the second case, it is demonstrated that shuttle junction operation can establish phase coherence between two initially disconnected leads.