ROLE OF EDGE SUPERCONDUCTING STATES IN TRAPPING OF MULTI-QUANTA VORTICES BY MICROHOLES - APPLICATION OF THE BITTER DECORATION TECHNIQUE

The Bitter decoration technique is used to study the trapping of singl e and multiple quanta vortices by a lattice of circular microholes, By keeping a thin superconducting layer (the bottom) inside each hole we are able to visualise the trapped vortices. From this We determine, f or the first time, the filling factor FF, i.e. the number of vortices captured inside a hole. In all cases the sample is cooled at a constan t field before making the decoration. Two qualitatively different stat es of the vortex crystal are observed. (i) Ir? case when the interhole distance is much larger than the coherence length, the filling factor averaged over many identical holes ([FF]) is a stepwise function of t he magnetic flux (of the external field) through the hole, because eac h hole captures the same number of vortices, The density of fluxoids i nside rite openings is higher than in the uniform film, but much lower than it should be in the state of equilibrium. We claim that the numb er of trapped vortices is determined by the edge superconducting state s which appear around each hole at the modified third critical field H -c3 > H-c2. Below H-c2 such states produce a surface barrier of a new type. This barrier for the vortex entrance and exit is due to the str ong increase of the order parameter near the hole edge, If keeps const ant the number of captured vortices during the cooling at a fixed fiel d. (ii) An increase of the hole density or of the hole radius initiate s a sharp redistribution of fluxoids: all of them drop inside holes. T his first order transition leads to a localization of all vortices and consequently to a qualitative change of the transport properties (TAF F in our case). In the resulting new state the filling factor is not a ny more the same for neighbouring holes and its averaged value is equa l to the frustration of the hole network.