Temperature dependence of the anisotropic electrodynamics in the ladder compounds Sr14-xCaxCu24O41

The optical reflectivity as a function of temperature of the title compound for x = 0; 5 and 12 has been measured over a broad spectral range from 4 meV up to 12 eV. Our ndings suggest a very anisotropic dynamics of the charge excitation spectrum when measuring along or perpendicular to the ladders. Moreover, a metal-insulator transition develops at low temperatures, leading to a suppression of Drude spectral weight in the far and mid-infrared spectral range. We identify this behaviour as a consequence of the localization eects for small Ca substitution, and of the possible formation of a charge density wave condensate for large Ca substitution.