Stability of vortex states in mesoscopic superconducting squares with artificial pinning

We experimentally study the stability of the vortex states in mesoscopic superconducting squares with zero, three and four submicron artificial pinning centers (antidots) by measuring the width of the magnetic field region over which a particular superconducting state with vorticity L   is stabilized. The transitions between different vortex states are detected by the voltage change across current-biased small tunnel junctions attached to the square. The vortex states (L≠0L≠0) are significantly stabilized by the existence of antidots. We discuss the effect of the number of antidots on the stability of mesoscopic vortex states.