Resistive upper critical fields and irreversibility lines of optimally doped high-T_c cuprates

We present the resistively determined upper critical field H^ρ_c2(T) and the irreversibility lines H^ρ_irr(T) of various high-T_c cuprates, deduced from measurements in 61-T pulsed magnetic fields applied parallel to the c axis. The shape of both H^ρ_c2(T) and H^ρ_irr(T) depends monotonically on the anisotropy of the material and none of the samples show saturation of H^ρ(T) at low temperatures. The anomalous positive curvature d²H^ρ/dT² > 0 is the strongest in materials with the largest normal state anisotropy, regardless of whether anisotropy is varied by changing the carrier concentration or by comparing a variety of optimally doped compounds.