IRREVERSIBILITY BOUNDARIES OF BI1.95SR1.65LA0.4CUO6, BI2SR2CACU2O8, AND OXYGEN DEPLETED YBA2CU3O7-DELTA

We measured the irreversibility boundaries B-irr(T) of single crystall ine Bi1.95Sr1.65La0.4CuO6, Bi2Sr2CaCu2O8, and YBa2Cu3O7-delta, for mag netic fields H parallel to the c axis of the crystals. The results dem onstrate that the temperature dependence of B-irr is not universal for all cuprate superconductors. The Bi-based compounds exhibit a charact eristic upturn in logB(irr)(T) with decreasing temperature T, which we ascribe to a dimensional crossover of the vortex ensemble. The interp retation of B-irr(T) near T-c in these compounds is complicated by geo metry effects and by the restricted validity of the nonlocal approxima tion for describing the vortex state.