VISUALIZATION OF COLUMNAR DEFECTS IN SUPERCONDUCTORS

Columnar defects in single crystals of superconductors were investigat ed using scanning probe microscopy. We show that the observable topogr aphy strongly depends on the crystal structure as well as on the type of the interaction with the probe. In scanning tunneling microscopy st udies, the low conductance of the amorphous tracks leads to tip-surfac e contact. Owing to this contact, the defects are imaged as hollows ha ving a depth that primarily reflects the tunneling distance. For the h igh transition temperature materials, atomic force microscopy images t he real defect structure as hillocks growing out of the surface. This outgrowth of amorphous material is time dependent and produced by the relaxation of irradiation-induced stress. The dynamic outgrowth of the columnar defects is discussed in terms of a so-called ''tooth paste'' model.