Charge-density wave formation in ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$

The electrodynamic response of the spin-ladder compound ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}\mathrm{}(x=0,3,9\mathrm{})\mathrm{}$ has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around $12{\mathrm{cm}}^{-1}$ in ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ for the radiation polarized along the chains/ladders $\left(\mathbf{E}\Vert \mathbf{c}\right).\mathrm{}$ 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 and produces a phason mode pinned at $12{\mathrm{cm}}^{-1}.$ With increasing x the mode shifts up in frequency and eventually disappears for $x=9\mathrm{}$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.