Upper critical field of spin-ladder Sr2Ca12Cu24O41

We report the upper critical magnetic field, H-c2(T), of the spin-ladder Sr 14-xCaxCU24O41 (X = 12) Single crystal, which becomes superconducting above similar to 3.0GPa. From the measurements of the temperature dependence of AC susceptibility at 4.0 GPa in magnetic fields applied along the a-axis (p erpendicular to ladder), the b-axis (perpendicular to ladder-plane) and the c-axis (parallel to ladder), we have determined H-c2(T) for these three di rections down to about 1.6 K. A clear difference is observed among these th ree curves, indicating that this system has an anisotropic superconducting ground state. (C) 2000 Elsevier Science B.V. All rights reserved.