BLOCH OSCILLATIONS AND COULOMB BLOCKADE OF COOPER PAIR TUNNELING IN ULTRASMALL JOSEPHSON-JUNCTIONS

We have studied charging effects in ultrasmall aluminum or lead alloy Josephson junctions (S almost-equal-to 0.01-0.02 mum2) protected from the shunting impedance of the external circuit by high resistance leads with R almost-equal-to 100-300 kOMEGA. Measurements of the single junctions have shown two new effects predicted by theory: the Coulomb blockade of Cooper pair tunneling and the time correlation of individual Cooper pair tunneling events (so-called Bloch oscillations). The former effect was observed for a wide range of Josephson coupling energies less than and grater than the charging energy. Bloch oscillations were indicated by phase locking to an external signal of frequency 0.4 to 10 GHz for Josephson energies close to charging energy. The shape of the current-voltage characteristics and the bandwidth of Bloch oscillations as measures from the derivative dV/dI, were analyzed within the context of three different theoretical models.