Influence of the environment on the Coulomb blockade in submicrometer normal-metal tunnel junctions

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Abstract
Submicrometer normal-metal tunnel junctions were fabricated with thin-film leads of either about 2 kΩ/μm or about 30 kΩ/μm. The current-voltage (I-V) characteristics at millikelvin temperatures displayed a much sharper Coulomb blockade for the high-resistance leads than for the low-resistance leads. The zero-bias differential resistance increased as the temperature was lowered, flattening off at the lowest temperatures. A heuristic model based on the quantum Langevin equation is developed, which explains these effects qualitatively in terms of the Nyquist noise generated in the leads; in this model, the flattening of the zero-bias resistance arises from zero-point fluctuations. The data are also compared with a more accurate phase-correlation model that treats the junction and the circuit coupled to it as a single quantum circuit. This model accounts for the observed I-V characteristics quite accurately except near zero bias where it overestimates the dynamic resistance by roughly 50% at the lowest temperatures. This model, however, does not account for the flattening of the zero-bias resistance at the lowest temperatures. It is suggested that the addition of quantum fluctuations in the junction to the phase-correlation theory may account for this discrepancy.