Dimensionality Transition of the Vortex State in Bi2Sr2CaCu2O8+δ

The second peak effect in the magnetisation of Bi2Sr2CaCu2O8+δ has been investigated for both under-, near optimally-, and over-doped single crystals. The size and position of the peak depend strongly on the doping, and hence anisotropy, showing a large enhancement in the overdoped sample. For all states, though, the peak persists to high temperatures, with no evidence for an intersection of the curve describing peak position in the H-T plane with the irreversibility line. The temperature dependence can be described with recent theories of a 3D-2D transition in the vortex system, allowing derivation of values of the anisotropy γ. This interpretation is supported by μSR experiments on the same samples which indicate a reduction of vortex dimensionality at crossover fields matching the peak fields.