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

We present the resistively determined upper critical field ${H}_{c2}^{\ensuremath{\rho}}(T)$ and the irreversibility lines ${H}_{\mathrm{irr}}^{\ensuremath{\rho}}(T)$ 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 ${H}_{c2}^{\ensuremath{\rho}}(T)$ and ${H}_{\mathrm{irr}}^{\ensuremath{\rho}}(T)$ depends monotonically on the anisotropy of the material and none of the samples show saturation of ${H}^{\ensuremath{\rho}}(T)$ at low temperatures. The anomalous positive curvature ${d}^{2}{H}^{\ensuremath{\rho}}{/dT}^{2}g0$ 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.