Enhancement of Interlayer Phase Coherence in the Vortex Liquid State of Bi2Sr2CaCu2O8+y with Columnar Defects

We studied the phase coherence in the vortex liquid state of Bi2Sr2CaCu2O8+ywith columnar defects (CDs) using Josephson plasma resonance. The phase coherence as a function of magnetic field is nonmonotonic in samples with higher density of CDs, and we observe multiple resonance peaks in a limited temperature range. The characteristic field H*for the enhancement of the phase coherence normalized by the matching field (BΦ) is constant between 1/5 and 1/3 for samples with larger BΦ, while H*is constant in samples with smaller BΦ. In samples with CDs introduced at angles from the c-axis, H*is determined only by the density of CDs in the CuO2layers. In a sample with CDs at 80° from the c-axis, a large enhancement of the phase coherence was observed when the field is applied parallel to the direction of CDs and clear triple resonance peaks are observed.