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 Hall-probe measurements of the magnetization of YBa₂Cu₃O_7-δ single crystals with columnar defects 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 in the case of bulk pinning. We conclude that, in contradiction to what is commonly believed, the vortices are depinned from the columnar defects only at the surface. Experiments supporting this idea are reported and the peculiarities of this gigantic surface pinning are discussed.