Dynamic crossover from pinning controlled to geometrical barrier controlled flux penetration in Bi2Sr2CaCu2O delta high-temperature superconductor

Penetration of AC field in BSCCO-2212 single crystals was studied using the magneto-optic technique. Profiles of the field amplitude were measured across the sample. The profile shape was found to change essentially with the field frequency. At lower frequencies the profiles have a dome-like shape characteristic of the geometrical barrier, whereas at higher frequencies the profiles acquire a typical Bean-like shape manifesting the flux pinning. Measurements at higher frequencies have shown that pinning is effective in the whole temperature range up to T-C. This shows that the position of the depinning line, which is commonly supposed to lie well below T-C even in zero field, has to be corrected to take into account the giant flux creep in BSCCO. (C) 2002 Elsevier Science B.V. All rights reserved.