Correlated quantization of supercurrent and conductance in a superconducting quantum point contact

We have measured the supercurrent and conductance of a superconducting quantum point contact in a superconductor two-dimensional electron gas-superconductor Josephson junction. We observe that the supercurrent and conductance change stepwise in a correlated manner as a function of the gate voltage. This was achieved by simultaneous measurement of the supercurrent and conductance at high bias from the same current voltage characteristic.

Figure 1

(a) Cross sectional view of the superconducting quantum point contact (SQPC). The carrier concentration $n_S$ in the 2DEG can be varied by applying a voltage to the gate electrode. (b) Top view of the SQPC. The width of the sample is $W=10\mu \mathrm{m}$, the length $L=400\mathrm{nm}$, and the width between the two gate electrodes is $W_g=100\mathrm{nm}$.

Figure 2

Measured current voltage characteristics (IVC) at $T=25\mathrm{mK}$ for different gate voltages. In the inset the arrows indicate the onset of the positive $I_C^{+}$ and negative $I_C^{-}$ critical current at $V_{\mathit{\text{gate}}}=-1.978\mathrm{V}$ which is carried by the first open transport mode.

Figure 3

(a) Critical current $I_C$ (open symbols) and conductance $G_{\mathit{IVC}}$ (full symbols) extracted from the current-voltage characteristics as a function of the gate voltage $V_g$ at $T=25\mathrm{mK}$. The solid lines are fits (for details see the text). The mode number is indicated by an $n$. The upper curve is the differential zero bias conductance $G_{\mathit{ac}}$ measured at 200 mT which has been shifted by $2e^2∕h$ for clarity. (b) A closeup of the critical current $I_C$ (open symbols) and conductance $G_{\mathit{IVC}}$ (full symbols) extracted from the current-voltage characteristics. Solid lines are fits (for details see the text).