Charge density waves in Sr_14-xCa_xCu_24O_41: electron correlations vs.   structural effects
  We report inelastic light scattering experiments in the spin-chain and ladder system Sr_14-xCa_xCu_24O_41. A depletion of an electronic background is observed for 0 < x < 5 at low temperatures and with light polarization both parallel to the leg and rung directions. This points to the formation of 2D charge density wave in the ladders. Furthermore, the Ca-substitution dependence of the 305- and 249-cm-1 modes, sensitive to the lattice modulations of the respective chain and ladder system, suggests that the charge dynamics in the ladders is strongly influenced by the structural incommensurability. 
K.-Y. Choi, M. Grove, P. Lemmens, M. Fischer, G. G\"untherodt, U.   Ammerahl, B. B\"uchner, G. Dhalenne, A. Revcolevschi, an...