Direct Observation of Vortex Shells and Magic Numbers in Mesoscopic   Superconducting Disks
  We have visualised and studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to result in the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we were able to trace the evolution of vortex states and identify stable and metastable configurations. Furthermore, quantitative analysis of shell filling with increasing L allowed us to identify 'magic' numbers corresponding to the appearance of consecutive new shells. 
I.V. Grigorieva, L.Y. Vinnikov, W. Escoffier, J. Richardson, S.V.   Dubonos, V. Oboznov, A.K. Geim