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

摘要

We report the evolution of the magnetic penetration depth (λ) and superconducting energy gap (△) in epitaxial NbN films with thickness (d) varying between 51-3 nm. With decrease in film thickness T_c and △(0) monotonically decreases, whereas λ(0) monotonically increases. Our results show that while the values of △(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.