Electronic phase diagram of the two-leg ladder compound Sr14-xCaxCu24O41 as investigated by the transport property measurements

We study the charge transport properties of the two-legged ladder compound, Sr14-xCaxCu24O41, with various Ca concentrations. For 0 less than or equal to x less than or equal to 3, the charge dynamics of this compound is char acterized by the strong suppression in the low-energy optical conductivity (sigma(omega)) with decreasing temperature, which is not observed for highe r x. The decrease in sigma(omega) is responsible for the insulating behavio r in these regions and may be associated with the ordering of hole carriers within the ladders. This possible charge ordering is suppressed by applyin g high pressure and the system becomes metallic for x similar to 0, althoug h the metallic state is distinct from that for higher Ca region where the s uperconducting ground state is realized under high pressure. It is consider ed that the charge ordered state observed for s = 0 is another ground state in this system which competes with superconductivity, and that the Ca subs titution destabilizes the charge ordered state.