Upper critical field of the spin ladder system Sr2.5Ca11.5Cu24O41

We present transport measurements under pressure of the anisotropic upper critical field of the superconducting spin ladder compound Sr2.5Ca11.5Cu24O41. When the field is applied perpendicular to the ladder plane, the temperature dependence of Hc2 is anomalous and shows a strong positive curvature. Hc2 for fields applied in the ladder plane is only weakly anisotropic between the a and c directions, however the initial slope is a factor of 6 larger than for H∥b. We compare these measurements of Hc2(T) with results in the high-Tc cuprates and in the quasi-1D organic superconductors.