Collective density wave excitations in two-leg   Sr_{14-x}Ca_xCu_{24}O_{41} ladders
  Raman measurements in the 1.5 - 20 cm-1 energy range were performed on single crystals of Sr_{14-x}Ca_xCu_{24}O_{41}. A quasielastic scattering peak (QEP) which softens with cooling is observed in the polarization parallel to the ladder direction for samples with x = 0, 8 and 12. The QEP is a Raman fingerprint of pinned collective density wave excitations screened by uncondensed carriers. Our results suggest that transport in metallic samples, which is similar to transport in underdoped high-T_c cuprates, is driven by a collective electronic response. 
A. Gozar, G. Blumberg, P. B. Littlewood, B.S. Dennis, N. Motoyama, H.   Eisaki, and S. Uchida