Highly sensitive and broadband carbon nanotube radio-frequency single-electron transistor

By studying the Coulomb diamond pattern around the optimum sensitivity, we deduce that $EC∼ΔEN$ and $ΔEN∼3kBT$⁠. This suggests that we are in the quantum Coulomb blockade (QCB) regime, where $kBT⪡ΔEN<2EC$ and only few levels are involved in transport. However, comparing the conductance peak line shapes around optimum sensitivity with theory (Ref. 30), we find deviations between the experimental data and QCB predictions. From the experimental data we get $GN=G/G∞∼0.45$ at peak maximum and $dGN/dVG∼10V−1$ as optimum transconductance. These values should be compared to $GN∼0.75$ and $dGN/dVG∼40V−1$ as predicted by theory. Also the predictions from the metallic Coulomb blockade regime with $GN∼0.5$ and $dGN/dVG∼20V−1$ are deviating, but the peak maximum is close to the measured. This discrepancy is explained by the fact that we are in the strong tunneling regime with $RQ/RΣ∼2.5$⁠, which broadens the energy levels. Since we still have strong Coulomb blockade, we are not quite in the Breit–Wigner limit but rather in an intermediate regime $kBT<hΓ∼ΔEN<2EC$⁠, which behaves closest to the strong tunneling description.