Interlayer Phase Coherence in the Vortex Matter Phases of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\mathit{y}}$

The interlayer phase coherence in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ was quantitatively determined in a wide field range by using the Josephson plasma resonance (JPR). At the first-order transition (FOT), a novel frequency-independent JPR with sharp resonance width is found, indicating a jump in the interlayer phase coherence. Above the FOT, sample-moving magnetization measurements show an anomaly consistent with the report by Fuchs et al. [Phys. Rev. Lett. 80, 4971 (1998)], where the interlayer phase coherence shows no anomaly. These results suggest the decoupling nature of the FOT, and can rule out the existence of the vortex line liquid state.