Giant vortex state in perforated aluminum microsquares

We investigate the nucleation of superconductivity in a uniform perpendicular magnetic field H in aluminum microsquares containing a few (two and four) submicron holes (antidots). The normal/superconducting phase boundary T-c(H) of these structures shows a quite different behavior in low and high fields. In the low magnetic-field regime fluxoid quantization around each antidot leads to oscillations in T-c(H), expected from the specific sample geometry, and reminiscent of the network behavior. In high magnetic fields, the T-c(H) boundaries of the perforated and a reference nonperforated microsquare reveal cusps at the same values of Phi/Phi(0) (where Phi is the applied magnetic flux threading the rural square area and Phi(0) is the superconducting flux quantum), while the background on T-c(H) becomes quasilinear, indicating that a giant vortex state is established. The influence of the actual geometries on T-c(H) is analyzed in the framework of the linearized Ginzburg-Landau theory. [S0163-1829(99)09129-8].