New vortex-matter size effect observed in Bi2Sr2CaCu2O8+delta

The vortex-matter 3D to 2D phase transition is studied in micron-sized Bi2S r2CaCu2O8+delta single crystals using local Hall magnetization measurements . At a given temperature, the second magnetization peak, the signature of a possible 3D-2D vortex phase transition, disappears for samples smaller tha n a critical length. We suggest that this critical length should be equated with the 2D vortex lattice ah-plane correlation length R-c(2D). The magnit ude and temperature dependence of R-c(2D) agree well with Larkin-Ovchinniko v collective pinning theory.