Photon-assisted multiple-Andreev reflections in high-Jc NbN/AlN/NbN tunnel junctions

Interplay between microwave field and Andreev current in epitaxial NbN/AlN/NbN tunnel junctions have been investigated. The subharmonic-gap structures (dV/dI dip-structures) were clearly observed for almost all junctions without microwave field, and the voltage positions of dV/dI dips were well explained by Octavio, Tinkham, Blonder and Klapwijk (OTBK) theory based on multiple-Andreev reflection (MAR). In the presence of microwave field, the dV/dI-V curve only in a high-Jc (5 kA/cm2) junction has a rich subgap structure due to photon-assisted MAR. The observed microwave-induced dV/dI structure was qualitatively explained by the OTBK theory with an effective time-dependent voltage model