QUANTUM FLUCTUATIONS AND CHARGING EFFECTS IN SMALL TUNNEL-JUNCTIONS

The finite voltage conductance of an ultrasmall single tunnel junction of nominal resistance R0 is calculated within a model that includes quantum voltage fluctuations. Fluctuations are induced by charge transfer across the junction as well as by coupling to its electromagnetic environment via a transmission line of arbitrary impedance Z(omega). Both types of quantum fluctuations suppress the Coulomb blockade. Within a harmonic approximation we show that for a general frequency-dependent transmission-line impedance the impedances simply add in parallel. the low-frequency fluctuations are given by an effective coupling constant gamma-tot = R(H)/2Z(tot), where Z(tot)-1 = Z(O)-1 + R(O)-1 and R(H) = h/e2 is the quantum of resistance.