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

We present the resistively determined upper critical field H-c2(rho)(T) and the irreversibility lines H-irr(rho)(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-c2(rho)(T) and H-irr(rho)(T) depends monotonically on the anisotropy or the material and none of the samples show saturation of H-rho(T) at low temperatures. The anomalous positive curvature d(2)H(rho)/dT(2) >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. [S0163-1829(99)10341-2].