Josephson plasma resonance in Bi2Sr2CaCu2O8+delta in vortex liquid and solid states

Both vortex liquid and solid states of high-temperature superconductor Bi2S r2CaCu2O8+delta have been studied by means of Josephson plasma resonance (J PR). It is found that although the JPR frequency changes at the first-order vortex lattice melting transition (VLMT), T-M, by 20-30% at low temperatur es, it crosses T-M almost continuously at high temperatures near T-c. Such a nearly continuous change from liquid to solid state strongly implies that the phase transition is so weak that the disappearance of the superconduct ing phase correlations is nearly continuous. This situation may be realized by considering the strong 2D superconducting fluctuations, resulting in th e strong softening of the proper plasma mode in the solid phase. (C) 2000 E lsevier Science B.V. All rights reserved.