Environmental Effect on Coulomb Blockade in Ultrasmall Single Tunnel Junctions

We measured the lead-resistance dependence of the Coulomb blockade in ultrasmall Al single tunnel junctions. The leads consisted of Al junctions connected in various ways, in order to achieve lead resistances R L in a wide range 1–970 kΩ. As R L decreased below the resistance quantum R K = h / e 2 =25.8 kΩ, the normalized zero-bias resistance R / R n , where R is the zero-bias resistance of the single junction and R n is the tunnel resistance in the absence of charging effect, gradually decreased, indicating the weakening of the Coulomb blockade. On the other hand, R / R n exhibited little dependence on R L when R L increased above R K . These features are consistent with theoretical predictions. The dependence of the Coulomb blockade on the lead resistance is very similar to that on the tunnel resistance. A simple model circuit of a single junction coupled with a lead impedance accounts for this similarity.