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 characterized by the strong suppression in the low-energy optical conductivity (sigma(omega)) with decreasing temperature, which is not observed for higher x. The decrease in sigma(omega) is responsible for the insulating behavior in these regions and may be associated with the ordering of hole carriers within the ladders. This possible charge ordering is suppressed by applying high pressure and the system becomes metallic for x similar to 0, although the metallic state is distinct from that for higher Ca region where the superconducting ground state is realized under high pressure. It is considered that the charge ordered state observed for s = 0 is another ground state in this system which competes with superconductivity, and that the Ca substitution destabilizes the charge ordered state.