Giant vortex state in perforated aluminum microsquares

Abstract

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 $\ensuremath{\Phi}/{\ensuremath{\Phi}}_{0}$ (where $\ensuremath{\Phi}$ is the applied magnetic flux threading the total square area and ${\ensuremath{\Phi}}_{0}$ 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.