Dimensional crossover in the magnetic phase diagram of Bi2.15Sr1.85CaCu2O8+delta crystals with different oxygen stoichiometry

We have performed muon-spin-rotation measurements on the high-temperature superconductor Bi2.15Sr1.85CaCu2O8+delta for a range of oxygen stoichiometries. The low-temperature in-plane penetration depth lambda(ab) was determined as a function of oxygen content. In addition, for each sample the crossover field B-cr, separating the regime of three-dimensional and quasi-two-dimensional vortex behavior, was measured. It is found that B-cr depends only on the penetration depth as B-cr = Phi(0)/lambda(ab)(2) rather than on the superconducting ab anisotropy gamma. This may be understood in terms of a system of vortices controlled predominantly by electromagnetic interactions. The results are contrasted with other materials, such as HgBa2Ca3Cu4O10+delta, where Josephson coupling plays a more significant role.