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

We present the resistively determined upper critical field $H_{c2\mathrm{}}^{\rho }\mathrm{}\left(T\right)\mathrm{}$ and the irreversibility lines $H_{\mathrm{irr}}^{\rho }\mathrm{}\left(T\right)\mathrm{}$ 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 $H_{c2\mathrm{}}^{\rho }\mathrm{}\left(T\right)\mathrm{}$ and $H_{\mathrm{irr}}^{\rho }\mathrm{}\left(T\right)\mathrm{}$ depends monotonically on the anisotropy of the material and none of the samples show saturation of $H^{\rho }\mathrm{}\left(T\right)\mathrm{}$ at low temperatures. The anomalous positive curvature $d^2{H}^{\rho }{/dT}^2>0\mathrm{}$ 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.