Interaction between columnar defects and pancake vortices in Bi2Sr2CaCu2O8+y studied by Josephson plasma resonance

Interlayer pancake vortex arrangement is studied by Josephson plasma resona nce in Bi2Sr2CaCu2O8+y with columnar defects in a wide range of matching fi eld B-Phi (5 G similar to 40 kG). The c-axis coherence is found to be drast ically enhanced above a characteristic field H* in the vortex liquid state, which we interpreted as recoupling of pancake Vortices by columnar defects . The recoupling field H* scales as (0.2-0.3) B-Phi at high doses (B-Phi > 500 G), but at lower doses it is almost constant (H* similar to 60 Oe), sug gesting that the recoupling occurs when vortex-vortex interactions are sign ificant at low doses. At high doses we found that (1) H* does not depend on oxygen content of the crystal and (2) when columnar defects are introduced at an angle theta from the c axis, H* is scaled as (0.2-0.3) B-Phi cos the ta. These results indicate that the recoupling at high doses is controlled only by the density of defects in each CuO2 layers.