Resistive upper critical field of superconducting spin-ladder Sr14-xCaxCu24O41

We report the resistive upper critical field H-c2(T) of the hole-doped two- leg ladder Sr14-xCaxCu24O41 (x = 12) single crystal, which becomes supercon ducting with T-c similar to 5 K in a pressure range above similar to 3.0 GP a. Using a newly developed compact high-pressure cell, the magneto-transpor t measurements have been performed at temperatures down to similar to 140 m K under high pressure above 3.5 GPa. It is found that H-c2 (perpendicular t o the ladder Cu2O3 plane) increases anomalously as the temperature is decre ased having an upward curvature and shows no saturation down to T/T-c simil ar to 0.03. Also, its temperature dependence is found to resemble that of t he high-T-c superconducting oxide Tl2Ba2CuO6. This fact suggests that the a nomalous behavior of H-c2 has a common origin between the ladders and the t wo-dimensional high-T-c superconducting oxides.