DIRECT OBSERVATION OF VORTEX PINNING BY DISLOCATIONS IN BI2SR2CACU2O8 SINGLE-CRYSTALS

A technique allowing simultaneous observation of vortex images and crystal defects in superconductors by transmission electron microscopy (TEM) has been developed. The vortices are visualized by the Bitter decoration method followed by preparation of the samples for TEM. The technique was used to study the effect of crystal dislocations on the flux line lattice (FLL) in single crystals of Bi2Sr2CaCu2O8. Individual dislocations having a local density comparable with that of the vortices are found to cause local disorder in the otherwise well-ordered FLL, acting as attractive pinning centres. We show that the main contribution to pinning in our experiments comes from strain fields around dislocations. Relevant pinning mechanisms are discussed such as those arising from local changes of superconducting parameters caused by the dislocation strain fields or from vortex-dislocation interaction of an elastic nature.