Phononic gaps in the charged incommensurate planes of Sr14Cu24O41
  The terahertz (THz) excitations in the quantum spin-ladder system Sr14Cu24O41 have been determined along the c-axis using THz time-domain, Raman and infrared spectroscopy. Low-frequency infrared and Raman active modes are observed above and below the charge-ordering temperature T_{co} ~ 200 K over a narrow interval of 1 - 2 meV . A new infrared mode around 1 meV develops below ~ 100 K. The temperature dependence of these modes shows that they are coupled to the charge- and spin-density-wave correlations in this system. These low-energy features are conjectured to originate in the gapped sliding-motion of the chain and ladder sub-systems, which are both incommensurate and charged. 
V. K. Thorsmolle, C. C. Homes, A. Gozar, G. Blumberg, J. L. M. van   Mechelen, A. B. Kuzmenko, S. Vanishri, C. Marin and H. ...
cond-mat.str-el cond-mat.supr-con