Shape and motion of vortex cores in Bi2Sr2CaCu2O8+delta

We present a detailed study on the behavior of vortex cores in Bi2Sr2CaCu2O 8+delta using scanning tunneling spectroscopy. The very irregular distribut ion and shape of the vortex cores imply a strong pinning of the vortices by defects and inhomogeneities. The observed vortex cores seem to consist of two or more randomly distributed smaller elements. Even more striking is th e observation of vortex motion where the vortex cores are divided between t wo positions before totally moving from one position to the other. Both eff ects can be explained by quantum tunneling of vortices between different pi nning centers.