Anisotropy of the upper critical field of the spin-ladder Sr14−Ca Cu24O41

We report on the resistive upper critical field, H c2 (T) for the hole-doped two-leg ladder Sr 14-x Ca x Cu 24 O 41 (x = 12) single crystal, which becomes superconducting with T c ∼ 5 K in a pressure above ∼ 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 20T along the a-axis (⊥ ladder) and up to 7T both along the b-axis (⊥ ladder-plane) and along the c-axis (|| ladder). A clear difference in H c2 (T) is observed among these three directions, indicating that this system has an anisotropic superconducting ground state. Also, it is found that H h c2 (T) shows non-linearity near T c at zero field and no saturation down to T/T c ∼0.03, exhibiting an upward curvature. Furthermore. H a c2 is found to exceed the Pauli limit by a factor of more than 2. This fact suggests a possibility of triplet superconductivity.