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 m eV up to 12 eV. Our findings suggest a very anisotropic dynamics of the cha rge excitation spectrum when measuring along or perpendicular to the ladder s. Moreover? a metal-insulator transition develops at low temperatures, lea ding to a suppression of Drude spectral weight in the far and mid-infrared spectral range. We identify this behaviour as a consequence of the localiza tion effects for small Ca substitution, and of the possible formation of a charge density wave condensate for large Ca substitution.