Coulomb blockade and incoherent tunneling of Cooper pairs in ultrasmall junctions affected by strong quantum fluctuations

We have fabricated and investigated submicron-size (0.01 and 0.04 {μ}m²) Pb-alloy single Josephson junctions attached to high-resistance thin-film NiCr or Cr leads which provide the necessary conditions for a strong charging effect. When the Josephson coupling energy is less than the charging energy, we find a novel shape of the current-voltage curves, with a Coulomb gap at small voltages, and a current peak at larger voltages. A theory based on the concept of incoherent Cooper-pair tunneling has explained the data well.