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 temperatu res close to T-c, the Josephson plasma resonance field (H-p) shows a c haracteristic temperature dependence of decoupled pancake vortices sim ilar to the pristine Bi2Sr2CaCu2O8+y. The drastic increase in H-p occu rs at a frequency independent characteristic field even in the vortex liquid phase. We interpret this change as a manifestation of recouplin g of decoupled vortices by the columnar defects. The increase in H-p b ecomes larger with increasing the density of columnar defects. The tem perature dependence of H-p at lower temperatures but above the irrever sibility line can be understood by considering two kinds of vortices, some trapped in the columnar defects and others located between them g iving a decoupled liquid-like response. (C) 1997 Elsevier Science B.V.