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.
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