Observation of Vortex Clustering in Nano-Size Superconducting Pb Island Structures by Low-Temperature Scanning Tunneling Microscopy/Spectroscopy. - Tominaga, T.; Sakamoto, T.; Nishio, T.; An, T.; Eguchi, T.; Yoshida, Y.; Hasegawa, Y. (Journal of Superconductivity & Novel Magnetism, 2012)