Charge-density wave formation in Sr14-xCaxCu24O41 -: art. no. 060508

The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(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 (Eparallel toc). 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.