Unconventional superconductivity in ultrathin superconducting NbN films studied by scanning tunneling spectroscopy

Using scanning tunneling spectroscopy, we address the problem of the superconductor-insulator phase transition in homogeneously disordered ultrathin (2--15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter kFl≥5.6, manifest a conventional superconductivity: a spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a disordered vortex lattice in magnetic field. Upon thickness reduction, however, while kFl lowers, the STS reveals striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak height and small spatial inhomogeneities. In addition, the gap below TC develops on a spectral background, which becomes more and more ``V-shaped'' approaching the localization. The thinnest film (2.16 nm), while not being exactly at the superconductor-insulator transition (SIT) (TC≈0.4TCᵇᵘˡᵏ), showed unconventional signatures such as the vanishing of the coherence peaks and the absence of vortices. This behavior suggests a weakening of long-range phase coherence, when approaching the SIT in this quasi-2D limit.