Proximity semiconducting nanowire junctions from Josephson to quantum dot regimes

Experimental low-temperature transport results are presented on proximity-effect Josephson junctions made from low bandgap III-V semiconductor nanowires contacted with Nb. Two regimes are explored in terms of the Nb/nanowire interface transparency $t$. (i) High $t$ allows a supercurrent to flow across the junction with magnitude $I_c$, which can be modulated using the voltage $V_g$ on a global back gate or a local gate. Relatively high values are obtained for the figure-of-merit parameter $I_c R_N/(e \Delta) \sim 0.5$, and $t \sim 0.75$, where $R_N$ is the normal state resistance and $\Delta$ the superconducting gap of the Nb leads. With the application of an axial magnetic field, $I_c$ decays but exhibits oscillations before being fully suppressed. The period and amplitude of the oscillations depend on $V_g$. Possible explanations for this behaviour are presented, including Josephson interference of the orbital subbands in the nanowire. (ii) Lower transparency correlates with a spontaneous quantum dot (QD) formed in the nanowire channel. Pairs of Andreev Bound States (ABS) appear at energies $|E| < \Delta$, with one pair unexpectedly pinned at $E=0$ for a wide range of $V_g$. A description of the QD-ABS system beyond the Anderson model is presented to explain the latter results.