Abstract
We have investigated the evolution of the electronic state of hole-doped two-leg ladder compounds, Sr14 − xCaxCu24O41 with Ca content x (x = 0–12) and hydrostatic pressure P (P ⩽ 8 GPa) by measuring anisotropic resistivity on single crystals. An insulator-to-metal transition occurs by applying P 6.5 GPa for Sr14Cu24O41 which is activation-type insulating at low P. The compounds with x ⩽ 8 show an insulating behavior at all P, though the doped hole density is fairly high (0.07–0.17 holes/Cu). On the other hand, the compounds with x ⩾ 10 become a superconductor by applying pressure P = 3–5 GPa. From these results, we constructed an x-P phase diagram.