Importance of phase fluctuations for the magnetic penetration depth of conventional and cuprate superconductors

In order to unveil the origin of the gapless properties of cuprate supercon ductors, we study the low temperature dependence of the magnetic penetratio n depth lambda in: a) conventional NbN superconducting films with granular and epitaxial microstructure; b) single crystals of La2-xSrxCuO4, Bi2Sr2CaC u2O8 (BSCCO) and YBa2Cu3O6+x at various doping levels ranging from the unde r- to the overdoped regimes. The measurements are performed by using a nove l single-coil technique achieving 10 pm resolution. Our experimental result s are as follows. a) The granular NbN films exhibit a linear low-temperatur e dependence of lambda similar to the dependence reported earlier in cuprat es. In the epitaxial NbN films, we find the characteristic exponential beha viour of conventional s-wave superconductors. b) In ail single crystals of cuprates, the data confirm the linear dependence of lambda (ab) reported ea rlier. In BSCCO, we find that the slope d lambda (ab)/ddT is rapidly increa sing as doping decreases. As to a), our results give evidence for a gapless behaviour in conventional s-wave superconductors induced by a granular mic rostructure. As to b), our analysis indicates that a simple d-wave model ac counts for the experimental slope values only in the optimally and over-dop ed samples. In the underdoped samples. the d-wave model underestimates the large slope values observed experimentally. Such large values are rather co nsistent with a simple model of thermodynamic phase fluctuations suited for metals with reduced dimensionality and low carrier density, as in cuprates . We argue that this fluctuation model also accounts for the linear depende nce of lambda found in our granular NbN films. We discuss possible mechanis ms associated with the existence of weak links across the grains, which wou ld favour thermally activated phase fluctuations of the gap in this convent ional superconductor Finally, we put forward the hypothesis that a similar phenomenon may occur in cuprates because of the intrinsic granularity assoc iated with phase-separated metallic and insulating regions in the supercond ucting ab-plane.