2E PERIODIC MODULATION OF THE IV CURVE OF A CURRENT-BIASED SUPERCONDUCTING TRANSISTOR

We have made an experimental study of the tunnelling through a current -biased superconducting single electron transistor at temperatures dow n to 40 mK. The current-biased transistor consisted of two ultrasmall tunnel junctions connected in series, biased through two high-Ohmic re sistors (length 10 mum) located very close to the tunnel junctions. Th e two-probe measurement of the current-voltage characteristic has show n a periodic modulation with a gate voltage, having two separate funda mental periods in gate charge, DELTAQ1=e and DELTAQ2=2e, as well as hi gher harmonics of these two. We have found that the amplitude of the 2 e-component was enhanced as superconductivity of the electrodes was su ppressed with an external magnetic field. These results are discussed within the context of a parity model.