Interlayer Phase Coherence in the Vortex State ofBi2Sr2CaCu2O8+δProbed by Josephson Plasma Resonance

Interlayer phase coherence in the vortex state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ is studied by Josephson plasma resonance over a wide field range. In almost the entire region of the vortex liquid phase, the plasmon frequency is well described by the high temperature expansion theory that assumes almost decoupled layers. The plasmon frequency changes drastically below the melting transition line at low field. The rapid decrease of the interlayer coherence with decreasing $T$ below the irreversibility line reported previously is shown to be due to the unusual critical state of this material.