Cascaded superconducting junction refrigerators: Optimization and performance limits

We demonstrate highly transparent silicon-vanadium and silicon-aluminum tunnel junctions with relatively low sub-gap leakage current discuss how a trade-off typically encountered between transparency affects their refrigeration performance. theoretically investigate cascaded superconducting junction refrigerators two or more stages. In particular, we develop an approximate method that takes into account self-heating effects but still allows us to optimize the cascade single stage at time. design consisting of energy-efficient stages, which makes cooling of, e.g., quantum devices from above 1 K below 100 mK realistic experimental target.