Observation of single-electron-tunneling oscillations

The time correlation of tunneling events has been predicted to occur in ultrasmall tunnel junctions: a periodic tunneling with a frequency determined by the current f=I/e. To test the predictions, we have fabricated one-dimensional arrays of Al/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{O}}_{\mathit{y}}$/Al tunnel junctions with areas down to 0.006 μ${\mathrm{m}}^2$ corresponding to capacitances of ∼0.2 fF. Each array contained between 15 and 53 junctions. Current-voltage characteristics were measured at temperatures between 0.05 and 4.2 K. We observed phase locking of single-electron-tunneling oscillations to external microwaves in the frequency range 0.7–5 GHz. The phase locking manifested itself as peaks in the differential resistance at bias currents I=nef (n=±1,±2), corresponding to multiples of the microwave frequency. Numerical simulations show good agreement with our experimental results.