Effects of Columnar Defects on the Josephson Plasma Resonance inBi2Sr2CaCu2Oy

Effects of columnar defects on the Josephson plasma resonance (JPR) were investigated in heavy-ion irradiated ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_y$. A microwave magnetoabsorption peak caused by the JPR shifted to higher fields with increasing defect densities. The amount of the shift indicated that the interlayer coupling was enhanced even above the matching field of the columnar defects. Another absorption peak was found around the matching field, which we attribute to a novel JPR caused by the nonmonotonic field dependence of the interlayer critical current density. These results indicated that the feature of the vortex system was drastically altered even by small number of columnar defects.