The electrodynamic response of the spin-ladder compound
Sr14−xCaxCu24O41 (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 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 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.Comment: One name added to author list 4 pages, 2 figures, email:
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