Josephson coupling in the vortex-liquid state of Bi2Sr2CaCu2O8+delta with columnar defects

The interlayer Josephson coupling in the vortex-liquid phase of Bi2Sr2CaCu2 O8+delta with columnar defects is studied by means of the Josephson plasma resonance. At high temperatures well above the Bose-glass melting line, dou ble resonance peaks appear as a function of the magnetic field in a narrow temperature range deep within the vortex-liquid state. This indicates that two types of vortex liquid, the well-coupled and the decoupled pancake vort ex liquids, appear in the irradiated crystals. The pancake vortices tend to align and couple with increasing magnetic field when it approaches approxi mately one-third of the matching field. From the angular and frequency depe ndences of the resonance, we estimate the c-axis correlation length of the pancake vortices quantitatively. [S0163-1829(99)06613-8].