  Since the very first experimental realization of Josephson flux-flow
oscillator (FFO), its theoretical description has been limited by the
phenomenological per- turbed sine-Gordon equation (PSGE). While PSGE can
qualitatively describe the topological excitations in Josephson junctions that
are sine-Gordon solitons or flux- ons, it is unable to capture essential
physical phenomena of a realistic system such as the coupling between tunnel
currents and electromagnetic radiation. Furthermore, PSGE neglects any
dependence on energy gaps of superconductors and makes no distinction between
symmetric and asymmetric junctions: those made of two iden- tical or two
different superconducting materials. It was not until recently when it became
possible to calculate properties of FFO by taking into account information
about energy gaps of superconductors [D. R. Gulevich et al., Phys. Rev. B 96,
024215 (2017)]. Such approach is based on the microscopic tunneling theory and
has been shown to describe essential features of symmetric Nb-AlOx-Nb
junctions. Here we extend this approach to asymmetric Nb-AlN-NbN junctions and
compare the calculated current-voltage characteristics to our experimental
results.
