Recoupling of Decoupled Vortex Liquid by Columnar Defects in ${{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_{2}O}_{8+\mathit{y}}$

We report on the effect of columnar defects on the interlayer phase coherence in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ using Josephson plasma resonance as a probe. In the vortex liquid phase close to $T_c$, the resonance field ( $H_p$) shows a characteristic exponential temperature dependence similar to that of the irreversibility line. A drastic increase in $H_p$ occurs at a frequency-dependent temperature in the vortex liquid phase. We interpret this change as a manifestation of field-induced recoupling of decoupled vortex liquid by columnar defects. The double resonance peaks observed in this temperature range indicate the coexistence of coupled and decoupled vortex regions.