Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41
  The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized by dc resistivity, low frequency dielectric and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping. 
T. Vuletic, T. Ivek, B. Korin-Hamzic, S. Tomic, B. Gorshunov, P. Haas,   M. Dressel, J. Akimitsu, T. Sasaki, T. Nagata