Charge-density wave formation in Sr_{14}Cu_{24}O_{41}

The electrodynamic response of the spin-ladder compound Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$ ($x=0,3,9$) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm${}^{-1}$ in Sr${}_{14}$Cu${}_{24}$O${}_{41}$ for the radiation polarized along the chains/ladders ($\mathbf{E}\parallel \mathbf{c}$). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm${}^{-1}$. With increasing Ca doping the mode shifts up in frequency and eventually disappears for $x=9$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.