Multiple Andreev Reflections in MgB$_{2}$/I/Pb Heterojunctions Cooled below 1 Kelvin

We have measured the I-V and dI/dV-V characteristics of two MgB$_{2}$/I/Pb heterojunctions below 1 Kelvin. In both junctions which were grown on c-axis substrates, we observed the characteristic pi peak corresponding to c-axis tunneling, consistent with theoretical predictions. Furthermore, we have observed sub-harmonic peaks in dI/dV that are consistent with Multiple Andreev Reflections (MAR) usually associated with high-transparency junctions. We also describe the temperature dependence of other sub-gap peaks observed. By systematically reducing noise in our setup and using sub-Kelvin temperatures, we were able to observe relatively sharp MAR peaks which are unusual for junctions with a large R$_{\mathrm{subgap}}$/R$_{\mathrm{n}}$ ratio.