Andreev Reflections in Micrometer-Scale Normal Metal-Insulator-Superconductor Tunnel Junctions

Understanding the subgap behavior of Normal metal-Insulator-Superconductor (NIS) tunnel junctions is important in order to be able to accurately model the thermal properties of the junctions. Hekking and Nazarov (Phys. Rev. B 49:6847, 1994) developed a theory in which NIS subgap current in thin-film structures can be modeled by multiple Andreev reflections. In their theory, the current due to Andreev reflections depends on the junction area and the junction resistance area product. We have measured the current due to Andreev reflections in NIS tunnel junctions for various junction sizes and junction resistance area products and found that the multiple reflection theory is in agreement with our data.