Which type two s-wave superconductor has the smallest London penetration depth? What is l_lamda ?
In element superconductors like Pb, Tl, or In the penetration depth is of the order of several µm. In the Cuprate superconductors about 100 nm scitation.aip.org. I am not sure if the new iron arsenide superconductors have a smaller penetration depth but probably not much smaller than the cuprates.
The penetration depth can be nicely visualized with magnetic force microscopy. The vortices of a type 2 superconductor in field decay over the London penetration depth.