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 Bi 2 Sr 2 CaCu 2 O 8+y with 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 (B Φ ) is constant between 1/5 and 1/3 for samples with larger B Φ , while H * is constant in samples with smaller B Φ . In samples with CDs introduced at angles from the c-axis, H * is determined only by the density of CDs in the CuO 2 layers. 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.