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 $k_{F}l\ge 5.6$, 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 $k_{F}l$ 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 $T_C$ 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) ($T_C\approx 0.4T_C^{\mathrm{bulk}}$), 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.