Ge/Si nanowire mesoscopic Josephson junctions

Superconductor-normal conductor-superconductor (S-N-S) Josephson junctions have displayed rich macroscopic quantum phenomena. A novel mesoscopic regime emerges when the width of the normal conductor shrinks to become comparable to carrier Fermi wavelength and its normal conductance becomes quantized in multiples of 2e2/h due to quantum confinement. We have previously demonstrated transport through individual 1D subbands in the hole gas formed in Ge/Si core/shell nanowire (NW) heterostructures. Here we present a study of the interplay between quasi-1D transport and proximity-induced superconductivity using Ge/Si NWs contacted by superconducting leads. Transport measurements on S-NW-S devices reveal high order resonant multiple Andreev reflections, indicating that the NW channel is smooth and that transport is highly coherent. By using a top gate to modulate carrier density in the NW, the critical supercurrent Ic can be tuned from zero to $>$ 100 nA. Significantly, we found that Ic exhibits step-wise increases as a function of gate voltage, corresponding to transport through discrete 1D subbands due to radial carrier confinement. The implications of these results and possible applications of S-NW-S devices will be discussed.