Granularity-induced gapless superconductivity in NbN films: evidence of   thermal phase fluctuations
  Using a single coil mutual inductance technique, we measure the low temperature dependence of the magnetic penetration depth in superconducting NbN films prepared with similar critical temperatures around 16 K but with different microstructures. Only (100) epitaxial and weakly granular (100) textured films display the characteristic exponential dependence of conventional BCS s-wave superconductors. More granular (111) textured films exhibit a linear dependence, indicating a gapless state in spite of the s-wave gap. This result is quantitatively explained by a model of thermal phase fluctuations favored by the granular structure. 
G. Lamura, J. C. Villegier, A. Gauzzi, J. Le Cochec, J.Y. Laval, B.   Placais, N. Hadacek, and J. Bok