PINNING BY AN ANTIDOT LATTICE - THE PROBLEM OF THE OPTIMUM ANTIDOT SIZE

Critical current densities (j(c)) and pinning forces (f(p)) in superco nducting 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 th at pinning centers with a size considerably larger than the temperatur e-dependent coherence length xi(T) are much more efficient than those with a size close to xi(T). Moreover, the antidot size realizing the o ptimum pinning is field-dependent: we need smaller antidots to optimiz e pinning in lower fields and larger antidots for optimum pinning in h igher fields. Crossover between different pinning regimes is controlle d by the saturation number n(s) that defines the largest possible numb er of flux lines trapped by an antidot. In dependence upon the n(s) va lue, we have observed various composite flux lattices with vortices at antidots and interstices (n(s) approximate to 1), multiquanta vortex lattices (n(s)>1), and finally we have reached the limit of supercondu cting networks at n(s) much greater than 1.