Figure 4. Ratios of step widths Q₁₂ and Q₃₄ versus frequency f.

For each length L of the JJ, we calculate the ratios of step amplitudes Q₁₂=w₁/w₂ (a) and Q₃₄=w₃/w₄ (b) and plot them as a function of the rf frequency. Q₁₂ shows a very clear decrease as frequency f is lowered. A minimum around 0.05 is obtained for the 150-nm junction, but we observe that this minimum tends to increase with the length L of the junction. In contrast, even if the measurements show some scattering, the ratio of higher order steps Q₃₄ does not show significant variation. For comparisons, we evaluated Q₁₂ and Q₃₄ from a conventional RSJ model, and show the results as a grey area.