Penetration Depth and Related Properties of Al Films with Enhanced Superconductivity

Using combined resistivity, susceptibility, critical field, and NMR measurements it was found that Al films with enhanced superconductivity can be divided into two separate categories depending on their residual resistivities rho(0). Films with resistivities smaller than similar or equal to 10(-3) Omega.cm are metallic (rho((300) (K))/rho(0) is larger than but very close to unity); the experimentally measured penetration depth, the upper critical field H-perpendicular to, and the penetration field H-p are in good agreement with theoretically calculated values using an experimental value of the mean free path l(eff). The density of states for such films as inferred from NMR measurements is very close to that of bulk. On the other hand, films with rho(0) larger than similar or equal to 10(-3) Omega.cm have a semiconducting behavior (rho(300 K)/rho(0) <= 1) and are grossly inhomogeneous. The experimental values of H-perpendicular to no longer agree with the theoretical estimates using l(eff). In such films rho(0) is no longer a meaningful parameter that can be related to l(eff), for it is to a large extent a tunneling resistivity as shown by the fact that films not superconducting down to 0.95 K, when measured resistively, are superconducting at 1.2 K, as shown by a susceptibility measurement. These experiments seem to indicate that in a homogeneous film l(eff) has a lower limit of approximately 1 angstrom.