Evaluation of a Tunnel Barrier in Superconducting NbN Junctions With Subharmonic Gap Structures

We have investigated the current density dependence of subharmonic gap structures in NbN tunnel junctions. Using multiple Andreev reflection theory for a homogeneous barrier, we evaluated barrier transparency for junctions with various current densities ranging from 4 to 20 kA/cm² by observing the current step height at a voltage of 2Delta/ne for n = 1, 2, and 3. The results suggest that the tunnel barrier in NbN junctions is inhomogeneous due to a small number of "thinner" regions with high transparency. This transparency is still in the low transmission regime and is independent of the current density. This evaluation method should be useful for understanding the nature of tunnel barriers.