Pinning by an antidot lattice: The problem of the optimum antidot size

Abstract

Critical current densities ${(j}_{c})$ and pinning forces ${(f}_{p})$ in superconducting Pb/Ge multilayers and single WGe films are strongly enhanced by introducing regular arrays of submicron holes (``antidot lattices'') acting as artificial pinning centers. Comparative measurements of ${j}_{c}$ and ${f}_{p}$ for several well-defined antidot diameters $D$ have shown that pinning centers with a size considerably larger than the temperature-dependent coherence length $\ensuremath{\xi}(T)$ are much more efficient than those with a size close to $\ensuremath{\xi}(T).$ Moreover, the antidot size realizing the optimum pinning is field-dependent: we need smaller antidots to optimize pinning in lower fields and larger antidots for optimum pinning in higher fields. Crossover between different pinning regimes is controlled by the saturation number ${n}_{s}$ that defines the largest possible number of flux lines trapped by an antidot. In dependence upon the ${n}_{s}$ value, we have observed various composite flux lattices with vortices at antidots and interstices ${(n}_{s}\ensuremath{\approx}1),$ multiquanta vortex lattices ${(n}_{s}g1),$ and finally we have reached the limit of superconducting networks at ${n}_{s}\ensuremath{\gg}1.$