Charge Transport in Hybrid Tunnel Superconductor—Quantum Dot—Superconductor Junctions

We have studied thin-film superconductor - semiconductor (with quantum dots)-superconductor MoRe-Si(W)-MoRe junctions, where electrons are tunneling through a single or several quantum dots within the Si(W) barrier. Current-voltage characteristics (CVCs) of the samples have been measured in a wide voltage range from -900 to 900 mV at temperatures from 4.2 to 8 K. At relatively high tungsten content in the barrier, we have observed emergence of the Josephson effect. Characteristic voltages I_cR_N, the product of the critical Josephson supercurrent I_c and the normal-state resistance R_N, of the samples were unusually high. Simultaneously, we have observed large excess quasiparticle currents Iexc in the dissipative part of CVCs, which is a strong evidence of intensive electron-to-hole Andreev reflections in the junctions studied. When the W content in the barrier was decreased, the Josephson current disappeared, and we have observed resonant current peaks in the CVCs at bias voltages from 40 to 300 mV, which were symmetrical for positive and negative voltages. In the studied heterostructures, metal clusters inside the barrier behave as quasi-one-dimensional quantum dots; hence, the charge transport can be adequately described by scattering matrices within the quantum model of one-dimensional charge transport.