Interlayer phase coherence in the vortex matter phases of Bi₂Sr₂CaCu₂O_8+y

The interlayer phase coherence in Bi2Sr2CaCu2O8+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. © 1999 The American Physical Society.