Observation of single-electron-tunneling oscillations

Abstract

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}}$/Al tunnel junctions with areas down to 0.006 \ensuremath{\mu}${\mathrm{m}}^{2}$ corresponding to capacitances of \ensuremath{\sim}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=\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2), corresponding to multiples of the microwave frequency. Numerical simulations show good agreement with our experimental results.