Circuit effect on the current-voltage characteristics of ultrasmall tunneljunctions

We have used the method of generating functional in imaginary time to deriv e the current-voltage characteristics of a tunnel junction with arbitrary t unneling conductance, connected in series with an external impedance and a voltage source. We have shown that via the renormalized charging energy and the renormalized environment conductance, our nonperturbative expressions of the total action can be mapped onto the corresponding perturbative formu las. This provides a straightforward way togo beyond the perturbation theor y. For the impedance being a pure resistance, we have calculated the conduc tance for various voltages and temperatures, and the results agree very wel l with experiments. [S0163-1829(99)04819-5].