Suppressed Superconductivity in Ultrathin Mo₂N Films due to Pair-Breaking at the Interface

A strong disorder characterized by a small product of the Fermi vector k_F and the electron mean free l drives superconductors towards insulating state. Such disorder can be introduced by making the films very thin. Here, we present 3-nm Mo₂N film with k_F*l ~ 2 with a resistive superconducting transition temperature T_c = 2 K heavily suppressed in comparison with the bulk T_c. Superconducting density of states (DOS) with smeared gap-like peaks and in-gap states, so called Dynes DOS, is observed by the low-temperature tunneling spectroscopy despite a sharp resistive transition. By scanning tunneling microscope, the spectral maps are obtained and related to the surface topography. The maps show a spatial variation of the superconducting energy gap on the order of 20% which is not accidental but well correlates with the surface corrugation: protrusions reveal larger gap, smaller spectral smearing, and smaller in-gap states. In agreement with our previous measurements on ultrathin MoC films, we suggest that the film-substrate interface introducing the local pair-breaking is responsible for the observed effects and generally for the suppression of the superconductivity in these ultrathin films.