We have fabricated small tunnel junctions with a variety of ratios of charging energy $E_c$ to Josephson coupling energy $E_{\mathrm{J}}$ and measured their $I-V$ characteristics. For $E_c\sim E_{\mathrm{J}}$, the $I-V$ curve is resistive at all currents and the critical current is greatly reduced, in agreement with our theoretical estimates. As $E_{\mathrm{J}}$ is further reduced by application of a magnetic field, we find a novel regime in which aspects of the Coulomb blockade of tunneling coexist with features typical of Josephson tunneling. At still higher magnetic fields, the junction becomes normal and shows the simple Coulomb blockade.

• Received 17 February 1988

DOI:https://doi.org/10.1103/PhysRevLett.60.2414