Anomalous static and dynamic behaviors of the vortex liquid phase in Bi2Sr2CaCu2O8+delta

We have studied static and dynamic properties of Bi2Sr2CaCu2O8+delta single crystals in the vortex liquid regime by means of local ac permeability and static magnetization measurements in magnetic fields parallel to the c-axi s. A weak pinning phenomenon was observed even in the vortex liquid state b elow a characteristic field, H-F(T), above which there is no pinning, resul ting in a reversible magnetization behavior. The local permeability is rapi dly reduced around H-F(T) as temperature is decreased and turns negative be fore becoming zero at the vortex glass transition line. These results stron gly suggest that the vortex liquid state below H-F(T) may be distinguished as a new vortex phase obviously different from that above H-F(T). (C) 2001 Elsevier Science B.V. All rights reserved.