Shape and motion of vortex cores in Bi₂Sr₂CaCu₂O_8+δ

We present a detailed study on the behavior of vortex cores in Bi2Sr2CaCu2O8+δ using scanning tunneling spectroscopy. The very irregular distribution 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 the observation of vortex motion where the vortex cores are divided between two positions before totally moving from one position to the other. Both effects can be explained by quantum tunneling of vortices between different pinning centers