Gigantic surface pinning created by columnar defects

Columnar tracks created by high energy heavy ions are considered to be the most efficient defects since they pin the Vortices over their whole length. However, our magneto-optical observations and local Halt-probe measurement s of the magnetization of YBa2Cu3O7-delta single crystals with columnar def ects reveal that at moderate fields the shielding current due to pinning by the columns is not proportional to the sample thickness, as it should be i n the case of bulk pinning. We conclude that, in contradiction to what is c ommonly believed, the vortices are depinned from the columnar defects only at the surface. Experiments supporting this idea are reported and the pecul iarities of this gigantic surface pinning are discussed.