RESISTIVE UPPER CRITICAL FIELD OF SUPERCONDUCTING SPIN-LADDER Sr14-xCaxCu24O41

We report the resistive upper critical field Hc2(T) of the hole-doped two-leg ladder Sr14-xCaxCu24O41(x=12) single crystal, which becomes superconducting with Tc~5 K in a pressure range above ~3.0 GPa . Using a newly developed compact high-pressure cell, the magneto-transport measurements have been performed at temperatures down to ~140 mK under high pressure above 3.5 GPa. It is found that Hc2 (perpendicular to the ladder Cu2O3 plane) increases anomalously as the temperature is decreased having an upward curvature and shows no saturation down to T/Tc~ 0.03. Also, its temperature dependence is found to resemble that of the high-Tc superconducting oxide Tl2Ba2CuO6. This fact suggests that the anomalous behavior of Hc2 has a common origin between the ladders and the two-dimensional high-Tc superconducting oxides.