Measurement of magnetic penetration depth and superconducting energy gap in very thin epitaxial NbN films

We report the evolution of the magnetic penetration depth ( λ ) and superconducting energy gap ( Δ ) in epitaxial NbN films with thickness ( d ) varying between 51-3 nm. With decrease in film thickness T c and Δ ( 0 ) monotonically decreases, whereas λ ( 0 ) monotonically increases. Our results show that while the values of Δ ( 0 ) and λ ( 0 ) are well described by Bardeen-Cooper-Schrieffer theory, at elevated temperatures, films with d ≤ 6.5 nm show sudden drop in superfluid density associated with the Kosterlitz-Thouless-Berezinski transition. We discuss the implication of these results on the time response of superconducting bolometers made out of ultrathin NbN films.