First-Order Decoupling Transition in the Vortex State of Bi_2Sr_2CaCu_2O_(8+y)

摘要

The interlayer phase coherence in a wide field range of Bi_2Sr_2CaCu_2O_(8+y) was studied by the Josephson plasma resonance (JPR) at several microwave frequencies. At the first-order phase transition line, which was determined by the local magnetization measurements, we found a novel frequency-independent JPR, making a clear contrast to the anticyclotronic frequency dependence both above and below the transition line. The resonance width becomes sharp when the resonance field coincides with the transition field. These results indicate that the interlayer phase coherence changes drastically at the transition line, implying the decoupling nature of the first-order phase transition.