INTERLAYER PHASE COHERENCE IN THE VORTEX STATE OF BI2SR2CACU2O8+DELTAPROVED BY JOSEPHSON PLASMA RESONANCE

The interlayer phase coherence in the vortex state of Bi2Sr2CaCu2O8+de lta is studied by Josephson plasma resonance in a wide range of temper atures and magnetic fields. The plasmon frequency changes drastically at the first order vortex liquid-solid melting transition line at low field. The vortex lines are shown to be almost decoupled just above th e melting line. The effect of columnar defects on the plasma frequency is also investigated in a heavy ion irradiated Bi2Sr2CaCu2O8+delta. W e show the direct evidence that the strongly correlated vortex-line-li quid is formed by the columnar defects.