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We show that excessive Pb substitution of Bi in oxygen-overdoped BSCCO-2212 single crystal further reduces the anisotropy ( c / a ratio͒ of the system in addition to enhancing disorder in its vortex system. These effects have led to the suppression of first-order vortex melting as well as a peculiar second-peak effect exhibiting a sensitive temperature dependence over an unusually broad temperature range. Our experimental data further reveal, in a Bi-based system, the presence of a disentangled vortex liquid phase separating the solid vortex from the entangled vortex liquid phase in the H-T diagram constructed from the transport and magnetization data. INTRODUCTIONThe unusually rich variety of vortex phases in the H-T phase diagram is one of the most remarkable features distinguishing the new copper oxide superconductors from the conventional type-II superconductors. The appearance of a vortex liquid phase over a large area in the H-T diagram initiated intensive research on the structure and characteristics of the liquid phase as well as the true nature of the related thermodynamical transition across its boundary with the solid phase. It has now been established theoretically 1-5 and confirmed experimentally 6-12 that vortex melting in a clean sample is a first-order transition, while the presence of disorder is likely to change its character into a more gradual or second-order process.It is also well known that the traditional picture of the solid vortex phase has been greatly modified by the presence of disorder and anisotropy. It has been found necessary to include, in addition to the ordered vortex lattice state at low H and T, the less ordered or disordered glassy state at the higher H-T regime. This is further complicated by the occurrence of dimensional crossover at a certain boundary within the solid phase. 2,8,13-15 Studies on second-peak effects in BSCCO-2212 superconductors also suggest further complications of the solid vortex phase. 16,17 No less complicated is the vortex liquid phase where a multiphase structure emerges as a result of the intricate interplay among the thermal fluctuation effect, the pinning, and Lorentz forces as well as the plastic response of the vortex lines in highly anisotropic superconductors. The BSCCO-2212 superconductor, among others, offers a particularly rich and novel phenomenology of the vortex system due to its high T c and strongly layered structure. It has thus become one of the most intensely studied systems to date. It is theoretically predicted that apart from the metal-like viscous liq-

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