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

We present transport measurements under pressure of the anisotropic upper c ritical field of the superconducting spin ladder compound Sr2.5Ca11.5CU24O4 1. When the field is applied perpendicular to the ladder plane, the tempera ture dependence of H-c2 is anomalous and shows a strong positive curvature. H-c2 for fields applied in the ladder plane is only weakly anisotropic bet ween the a and c directions, however the initial slope is a factor of 6 lar ger than for H \\ b. We compare these measurements of H-c2( T) with results in the high-T-c cuprates and in the quasi-1D organic superconductors. (C) 2000 Elsevier Science Ltd. All rights reserved.