Imaging of vortex states in mesoscopic superconductors

Enhanced vortex pinning in nanoscale-engineered superconductors increases the superconducting critical currents by orders of magnitude. Spatial imaging of vortices in these systems at high magnetic fields would provide further insight into the pinning mechanisms and enable development of high-pinning-strength materials. We have developed a novel method of fabricating atomically flat superconductor surfaces containing periodic array of normal metal pinning centers. Using scanning tunneling microscopy and spectroscopy, we map the local density of states in this heterostructure showing the vortex distribution at different applied magnetic fields. By increasing the applied magnetic field, the normal metal pinning centers accommodate several vortices per center until reaching the saturation point, beyond which new vortices get accommodated in the interstitial superconducting regions. The arrangement and pinning of the interstitial vortices is determined by the periodic pinning potential, and repulsive vortex-vortex interaction. (C) 2005 American Institute of Physics.