Optical and transport properties of NbN thin films revisited

Highly disordered NbN thin films exhibit promising superconducting and optical properties. Despite extensive study, discrepancies in its basic electronic properties persist. Analysis of the optical conductivity of disordered ultrathin NbN films, obtained from spectroscopic ellipsometry by the standard Drude-Lorentz model, provides inconsistent parameters. We argue that this discrepancy arises from neglecting the presence of quantum corrections to conductivity in the IR range. To resolve this matter, we suggest a modification to the Drude-Lorentz model, incorporating quantum corrections. The parameters obtained from the modified model are consistent with transport and superconducting measurements. The revisited values describing conduction electrons, which differ significantly from commonly adopted ones, are the electron relaxation rate $\mathrm{\ensuremath{\Gamma}}\ensuremath{\approx}1.8\phantom{\rule{4pt}{0ex}}\text{eV}/\ensuremath{\hbar}$, the Fermi velocity ${v}_{F}\ensuremath{\approx}0.7\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{4pt}{0ex}}{\text{ms}}^{\ensuremath{-}1}$, and the electron density of states $N({E}_{F})=2\phantom{\rule{0.16em}{0ex}}$ states of both spins/eV/${V}_{\text{f.u.}}$.