Dependence of Coulomb Blockade in Ultrasmall Single Tunnel Junctions on Tunnel Resistance

We studied small-capacitance single tunnel junctions with various tunnel resistances. In order to reduce the environmental effect and to observe the Coulomb blockade clearly, we have adopted high-impedance leads which consist of multiple junctions to isolate the single junction. We observed the gradual weakening and disappearance of the Coulomb blockade as the tunnel resistance in the absence of charging effects is lowered below the resistance quantum. As the tunnel resistance increases above the resistance quantum, the zero-bias resistance divided by the tunnel resistance tends to flatten off. The observed dependence of the zero-bias resistance on the tunnel resistance is in good agreement with the theory of Brown and Šimánek.