Anisotropy of the upper critical field of the spin-ladder Sr14-xCaxCu24O41

We report on the resistive upper critical field, H-c2(T) for the hole-doped two-leg ladder Sr14-xCaxCu24O41(x = 12) single crystal, which becomes supe rconducting with T-c similar to 5 K in a pressure above similar to 3.0 GPa. From the results of magneto-transport measurements on a single sample at a pressure of 3.5 GPa, H-c2(T) has been determined up to 20 T along the a-ax is (perpendicular to ladder) and up to 7 T both along the h-axis (perpendic ular to ladder-plane) and along the c-axis (parallel to ladder). A clear di fference in H-c2(T) is observed among these three directions, indicating th at this system has an anisotropic superconducting ground state. Also, it is found that H-c2(b)(T) shows non-linearity near T-c at zero field and no sa turation down to T/T(c)similar to0.03, exhibiting an upward curvature. Furt hermore, H-c2(a) is found to exceed the Pauli limit by a factor of more tha n 2. This fact suggests a possibility of triplet superconductivity. (C) 200 1 Elsevier Science B.V. All rights reserved.