Can the collective pinning approach be applied to YBa2Cu3O7-delta superconductors?

The widely used collective pinning approach is applied to describe the crit ical current density J(c) of YBa2Cu3O7-delta single crystals. The field dep endence of J(c) at low temperatures, before and after reactor neutron irrad iation and annealing, points to strong pinning. Collective effects are expe cted at high densities of pinning centers, but the description is not possi ble in terms of the standard weak pinning model. A comparison of the data o f untreated and neutron irradiated crystals indicates that strong pinning a lso prevails in the as-grown material. Further results on related single cr ystal samples (RE-123 and Y-124) support the findings.