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-temperat ure superconductor Bi2.15Sr1.85CaCu2O8+delta for a range of oxygen sto ichiometries. The low-temperature in-plane penetration depth lambda(ab ) was determined as a function of oxygen content. In addition, for eac h sample the crossover field B-cr, separating the regime of three-dime nsional 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 p redominantly by electromagnetic interactions. The results are contrast ed with other materials, such as HgBa2Ca3Cu4O10+delta, where Josephson coupling plays a more significant role.