Pinning efficiency of splayed columnar defects in Bi-2212 single crystal: Evidence of a cage pinning effect

A three-directional configuration of columnar defects has been induced in a Bi₂Sr₂CaCu₂O₈ single crystal by irradiation with heavy ions of high energy. Persistent current densities have been extracted, using the Bean model, from hysteresis loops recorded in the orientation H||c. We have shown that improvements in pinning properties are larger in this three-directional splayed configuration than in the one obtained with columnar defects parallel to the c-axis. This effect exists only for H larger than H_Φ, where H_Φ is the matching field, and disappears as temperature is increased and vortices become less stiff. This is the first time that such a beneficial effect is reported for a compound of such a high electronic anisotropy.