Phase coherence along the c-axis in Bi2Sr2CaCu2O8+y with columnar defects probed by Josephson plasma resonance

We studied the c-axis phase coherence in Bi2Sr2CaCu2O8+y with columnar defects using the Josephson plasma resonance as a probe. At temperatures close to T-c, the Josephson plasma resonance field (H-p) shows a characteristic temperature dependence of decoupled pancake vortices similar to the pristine Bi2Sr2CaCu2O8+y. The drastic increase in H-p occurs at a frequency independent characteristic field even in the vortex liquid phase. We interpret this change as a manifestation of recoupling of decoupled vortices by the columnar defects. The increase in H-p becomes larger with increasing the density of columnar defects. The temperature dependence of H-p at lower temperatures but above the irreversibility line can be understood by considering two kinds of vortices, some trapped in the columnar defects and others located between them giving a decoupled liquid-like response. (C) 1997 Elsevier Science B.V.