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 Tc(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 Tc(H), expected from the specific sample geometry, and reminiscent of the network behavior. In high magnetic fields, the Tc(H) boundaries of the perforated and a reference nonperforated microsquare reveal cusps at the same values of Φ/Φ₀ (where Φ is the applied magnetic flux threading the total square area and Φ₀ is the superconducting flux quantum), while the background on Tc(H) becomes quasilinear, indicating that a giant vortex state is established. The influence of the actual geometries on Tc(H) is analyzed in the framework of the linearized Ginzburg-Landau theory.