Charge-density wave formation in Sr14-xCaxCu24O41

The electrodynamic response of the spin-ladder compound Sr_14-xCa_xCu₂₄O₄₁(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm^-1 in Sr₁₄Cu₂₄O₄₁ for the radiation polarized along the chains/ladders (E∥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 and produces a phason mode pinned at 12 cm^-1. With increasing x 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.